Rebuttals are always welcome - Sample:
In our experience, we too, found Sedum Hussein to be quite a public annoyance.  It has cost our government billions of dollars in irradiation methods, none of which seem to be completely successful.  Thank you for addressing such as serious horticultural issue!
 
Have you heard of the new cultivar S. 'Osama'?  Apparently it can live in caves without light for months!  - The MSU team.

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September 2007

Ask Ed introduces his Guest Ask Ed Contributor, "Ayehlet Cooper is our summer intern this year from Yale.  In addition to the more routine nursery work of propagation, weeding, watering and fertilizing she was asked to monitor the herbaceous green roof we installed in the spring of 2005.  She was to look at the survival strategies of the plants with an eye toward their ability to distribute viable seed across the roof.

We understand if we were to use this plant material commercially we would water occasionally, but this roof has always relied on rainfall.  2007 has been brutal for plants in our area, farm crops and green roofs alike so we though it an interesting time to see what this group of plants looked like.

I am proud to report that Ayehlet, like every intern to date, has found employment in the green roof world and will be starting her new job this fall."

Ed Snodgrass, Emory Knoll Farms/Green Roof Plants

Survivability, Aesthetic Value, and Seedling Dispersal of Hardy Herbaceous Plants on the Emory Knoll Farms Trial Green Roof, 2007

By Ayehlet Cooper
All Photos Courtesy Ask Ed


The trial herbaceous green roof established by Ed Snodgrass and Sarah Murphy in 2005 (see Ask Ed, January 2006) has run its intended two year course; this summer, I conducted a final evaluation of the roof, examining the plants in order to draw conclusions as to their worth to a person designing the planting of an extensive green roof.  I imagined by what criteria I would judge the plants individually and as a group if I were in such a designer’s place.  Then, towards the ends of reasonably evaluating the plants according to these criteria, I set up a number of rough numerical scales.  I used these scales to guide my evaluations of the plants on the trial roof, and then tempered these restricted numerical results with qualifications gleaned from observation.  From the set of qualitative results thus compiled, I drew conclusions which could serve as useful recommendations to any designer considering using this group of plants to extend their extensive green roof plant palette.  What follows is a summary of my methods, results, and conclusions.

The Emory Knoll Farms (EKF) trial roof is about 24 x 12 feet, raised approximately 6 feet off the ground. It holds five inches of media depth, and was filled with ‘Skyland’ green roof media.
 

The EKF trial roof is fairly sheltered from wind and faces south, receiving full sun throughout the day.  Only a tiny area of the northwest corner receives shade, from a building in the late afternoon.  The roof had been planted moving from west to east; three quarters of the way down, the trial plants were exhausted, so the final, eastern quarter of the roof was left effectively barren. At some point, a Sedum sexangulare plant was established in this area, providing the opportunity for useful comparisons between herbaceous and succulent materials.  The spring of 2007 was one of the driest on record, and the roof in summer of 2007 had survived for two years without any irrigation after initial establishment.

Because I intended to study seedling dispersal across the roof, I weeded the roof of all recognizable weeds starting in May, continuing lightly and sporadically for the month and a half during which I identified plants and their seedlings.

First, I set about identifying all the surviving mother plants on the roof, those plants remaining from the original planting two years ago. I used the original planting plan, marked how many out of the two original samples of each plant survived, and compared these numbers to Sarah Murphy’s records of plant survival.  I recorded both sets of numbers in a database of all the plants on the original planting list.  The roof itself was originally planted in a grid pattern, each square of the grid measuring one by one foot and containing two samples of one of the species being tested.  I re-divided the roof plan into thirty-two three by three foot zones.  For each zone, I recorded the seedlings which grew in it, generally identifying them by comparison to the mother plants.  For each of the seedling species I found in each zone, I assigned a number according to the following scale:

1   presence of seedlings of the species
2   a number of seedlings
3   significant coverage within area

Then, using a diagram of the existing roof, I plotted out the seedling dispersal by species: I used an indicator to mark the location of the mother plant on the plan, and then distributed colored paper slips numbered 1, 2, or 3 over the plan corresponding to the species’ assigned number in each of the thirty-two zones.  I was thus able to quickly create a visual representation of each species’ seedling distribution across the roof.  I numerically represented these findings according to the following two scales:

W-wide- how far from the mother plant the seedling distribution reaches
   0   within original location (‘mother’ zone)
   1   within immediate area (approximately meaning those zones directly abutting the ‘mother’ zone)
   2   within the greater area
   3   reaching across the roof (as far as possible)

N-number- density of the seedling distribution within the area reached
   1   appearance (presence of the species)
   2   a number of seedlings
   3   areas of significant coverage

Throughout the summer, until the disassembly of the roof in late July, I recorded with notes and photographs the process of wilting, browning, and revitalization of the species on the roof and recorded my observations of the roof as a whole.  I was especially diligent to observe the roof in periods after rains both brief and extensive.  At each date of observation, I recorded the recent weather trends, which plants were especially green, especially wilted, flowering, or otherwise notable.  At the end of my observation period, I was able to return to my notes and evaluate the aesthetic worth of the plant species based on the following two scales:

Rehydration -
   0   none
   1   slow (after prolonged rain)
   2   quick (more likely to reappear after brief wet-weather events)

Turgidity -
   1   quick to wilt and brown
   2   slow to wilt and brown

All of the above results were filed in a database of all the original species planted, and also recorded in a descriptive list in those cases where plants displayed similar behavior across genera.  The collection of results stopped abruptly in late July when the weed coverage of the roof, mainly that of prostrate spurge and horsetail, became so heavy that its germplasm posed a major threat to the surrounding areas of the nursery.  At that point, the entire roof was uprooted and the media removed.

A number of useful conclusions can be made from the results of this study which would be very helpful for a designer to consider.  Firstly, specific conclusions can be drawn about the behavior of certain species and genera of plants on comparable green roofs.

The following genera are especially notable:

Grasses:

Festuca glauca ‘Elijah Blue’: Self-sows more prolifically than the  others.

Allium:

Very hardy; keeps aesthetic worth through harsh conditions.  A. schoenoprasum self-sows in immediate area.

Campanula cochlearifolia:

Unlikely to survive, unlikely to flower, though some seedlings may appear in early spring in immediate area.  Quick to fail and unlikely to rehydrate.

Dianthus:

Very likely to survive - see database for exceptions.  Spreads prolifically both by growth of mother plants and by extensive seeding.  Will remain green and lush until it has flowered, when it becomes brown and ugly with the heat, resulting in large brown mats.  Seedlings often remain green much longer than mother plants, perhaps because they have self-selected their location.

D. knappii: Remains green very much longer than other dianthus and flowers later.  Does not self sow.
D. gratianopolitanus, D. deltoides, D. glacialis, D.arenarius:  Especially prolific self-sowers.
D. petreus: very hardy, remains green, does not self-sow.
D. myrtinervius: more delicate and quicker to wilt than many of the other Dianthus.

Hieracium:

Strong plants that are likely to flower, retain turgidity into harsh weather, and are not too cautious to reappear in clement weather after they have died back.  Will produce seedlings within immediate area.  Will flower.  See database for those species most likely to survive.

H. umbellatum: Especially hardy, especially slow to die back, and especially quick to rehydrate. One of the most aesthetically dependable plants on the roof.  Will flower.

Papaver:

Will not survive.  Its only hope through the summers is to self-sow, and being unlikely to flower, disappears within two seasons or less.

Thymus:

Very prolific and aggressive spreader and self-sower within greater immediate area.  Produces large mats which, though they are attractive and will flower in nicer weather, become brown and unattractive in the heat and are slow to recover.  T. serpyllum is especially extensive.  T. vulgaris is more upright, bushy, and woody.  It does not spread like the other thymes, remaining contained, but dying back to bare sticks.

T. x citriodorus, T. pulegioides slightly less hardy (mother plants might die, therefore producing seedlings more slowly).

The following is a list of fifteen top picks for relatively dependable aesthetic appeal.  Certain of these can be counted upon to remain contained, others will predictably spread.

Hieracium umbellatum, Hernieria glabra, Dianthus petreus, Potentilla argentea, P. neumanniana, Alyssum argenteum, Allium, Artemisia chamaemelifolia, Aster alpinus, Salvia pratensis, Saponaria ocymoides, Silene uniflora ‘Compacta’, Satureja montana, Dianthus knappii, Dianthus gratianopolitanus.

Along with species-specific conclusions, general statements can be made about the behavior and viability of this group of plants as a whole.  For numerous reasons, designers are eager to include these and similar plants in their green roof planting designs.  However, they should proceed with caution and with awareness.  These plants do have amazing regenerative powers; during wet periods, plants which looked completely dead, brown, and retracted suddenly reappear.

 

Observationally, prolonged rains were more likely to produce recovery than heavy though short-term moisture. ( I did not measure the lengths of these periods of dry and wet nor inches of rainfall, because without years of data to average and a system of controls, I could not conclusively extrapolate such results as that x inches of rain over x days in x degree weather causes regrowth in x species.  Such would be a very different type of study, and would hardly be useful or reproducible in application.  The most valuable information which can be gleaned is observationally imprecise, subjective, and descriptive.)

However, though many of these plants have shown the ability to survive through prolonged drought, in many ways that is beside the point; designers must consider, when knowingly subjecting their plants to drought, what the plants’ response will be, and whether this response corresponds with the function those plants are supposed to fulfill.  Where sedums and other succulents use CAM as a drought response, the drought survival strategy of the group of plants studied is to wilt, brown, and retreat underground.  They are only doing what they are supposed to do, but the response can lead to problems on the roof.

 

Firstly, there is the aesthetic issue, which effectively ties in most closely with the issue of client satisfaction; aesthetically, it does not much matter if the plants are alive underground if the roof is visually dead and brown.  Secondly, and very importantly, retreating allows opportunistic weeds to take over the roof (hence the spurge takeover of the EKF roof).
 

Because large patches of the roof will be bare for long periods of the summer, the designer must include a weed control strategy.  Only with complete coverage can the need for weeding maintenance be reduced.  Thus, if the roof is designed and sold as an extensive, low maintenance roof, a designer must be very cautious in his use of these plants.  Finally, there is the question of the function of the plants on the green roof.  Wilted and dead plants will provide virtually no cooling benefits to the roof from shading or evapotranspiration.  Dormant plants are also more ineffective in preparing a roof to absorb water during the second of two consecutive heavy rainfall events.  If the functional aspects of the roof are not being fulfilled, it is very difficult to justify the cost of planting the green roof at all.

Throughout the period of observation, the one Sedum sexangulare plant on the roof did not once wilt, brown, or show any other signs of extreme stress.
 

 

It spread small seedlings around itself which also did well.  Though this could hardly be considered a controlled study, observationally it is safe to say that, if one wishes to plant an un-irrigated green roof in five inches (or less) of media, specifying hardy succulents will more dependably ensure coverage of the roof throughout the summer.

However, the EKF roof did provide certain clues as to how the designer might more successfully control the behavior of herbaceous plants on her roof.  One such clue comes from the Artemisia laxa plant, the only mother plant surviving at the time of my study to receive a few hours of late afternoon shade.  This plant remained consistently turgid and beautiful throughout the summer, even during periods when the rest of the roof exhibited signs of extreme stress.  A number of seedlings in the same area, in particular Alyssum wulfenianum and Dianthus, also seemed to benefit from the effects of the little bit of extra shade.  The fact that these few hours of late afternoon shade seemed to have such marked beneficial effects on plants, even throughout long periods of dry weather, seems to imply that heat rather than moisture might have the most significant impact on the state of the hardiest plants on a roof.  Further study must occur, but perhaps manipulation of shade or substrate heat might provide an environment in which these plants might be more viable options for extensive roofs.

The EKF roof also provided another clue for a designer: evidence as to the importance of consideration of the time at which the species she picks flower.  Certain more tender plants that will die back in the heat still may propagate and spread themselves if they flower early in the season while the weather is still cool.

 

 

Plants which on the ground might be aggressive spreaders may not behave in such a manner if they are summer flowerers; the heat stress on the roof may inhibit flowering, thus keeping a normally virulent self-sower more contained.  Also, if a designer is choosing her plants for their flowers rather than foliage, she must know that it is very likely that under the stressful conditions of the roof, these plants may not flower at all.  Certain other plants, such as the Aster alpinus and the Dianthus, may stay green until they flower, at which point they fade.  Choosing a species which flowers later, such as Dianthus knappii, may help ensure the roof remains greener longer.

 

Other plants, such as Papaver, can be treated as self-sowing annuals.  Papaver cannot survive through the summers, but if sufficient numbers are present, and they do flower, they may appear spring after spring.

A final consideration for the designer when choosing her plants is that the plants may not look on the roof as they do on the ground.  The surviving plants which were uprooted from the EKF roof were transplanted into a field, where they received consistent water.  Many of the plants soon began to look significantly different than they had on the roof.  For example, the Potentilla specimens grew much larger and darker leaves, and the Hieracium specimens acquired the appearance of a very different habit as they began to spread in a stoleniferous manner.  A designer must know that the plants she chooses on her roof will generally be smaller, slower growing, and otherwise exhibit physiological signs of stress even when at their most lush.

There are clearly methodological weaknesses to this study, and many sources of error which might be considered to weaken the conclusions.  When constructing the evaluatory scales used to reach many of these conclusions, great care was taken never to imply conclusions more precise than the many uncontrollable variables allowed.  However, there are ways in which the value of the scales may be considered compromised. The scales did could not take into account the different microclimates across the roof caused by shading, breezes, traffic, other plants on the roof and off, surrounding conditions, etc…

 

The scales also ignored the existence of collaboration, competition, or interaction of the plants on the roof.  For example, the large bare section on the west side of the roof provided a competition-free opportunity for seedlings.  Mother plants closer to this area might be considered to have had an advantage when seeding.  Certain species’ seedlings, such as Dracocephalum renattii, appeared in this area though nowhere else on the roof; on a fully planted roof, this species may not have appeared as self-sowing.  Furthermore, certain species which survived on the roof did so perhaps only by chance because of their proximity to another species which shaded them.

 

For example, the Artemisia ludoviciana survived only in the shade of a neighboring grass; other species, if similarly positioned, might have done so as well.  Such microclimatic advantages are ignored in the scales.  The same scales would be a more accurate reflection of general behavior if many more specimens were present on the roof, allowing for a greater amount of data on each species in every microclimate.  Furthermore, because of their gross nature, the scales amplify differences in behavior.  The Artemisia ludoviciana did survive, but only as a small twiggy seedling in the shade of a grass, virtually useless to a designer.

 

However, this was enough to earn it a rating on the scale, something which other plants did not. Only in the descriptive results are such nuances suitably expressed.  The scales also did not take into account the idiosyncrasies of individual plants.  For example, certain plants which are generally more shy to flower and which did not appear as self-sowers might do so if the were present on the roof in greater numbers or with the correct pollinators.  Weather variations from year to year, such as milder winters or wetter springs, which might affect different species in different manners, were not able to be adequately accounted for by the method of data collection and evaluation.

A final and important source of error in the evaluation of the plants on the EKF roof was the fact that I did not witness the fall growing season.  I was unable to see how or if certain plants recovered, thus seeing how this summer’s especially harsh weather affected the roof.  I was unable to see which plants thrived or flowered in the fall, and how much the plants took advantage of the more clement weather to spread.  I was unable to see whether this summer’s especially harsh weather affected drastically affected survival.  Thus, the conclusions made about individual species must in some ways be taken as purely descriptive of the EKF roof from 2005 - mid summer 2007.

However, whether or not the past two to three years of weather, growth, and roof behavior have been representative of average conditions and their associated plant behavior in this area is somewhat beside the point.  For a green roof, success under average conditions is not enough. To know the level of maintenance specified for a roof, a designer must be assured of the hardiness or delicacy of her roof on a yearly basis, not only under average conditions.  One year of observation that weeds took over a parched roof is enough evidence to dictate design modifications.  It is most effectively by the accumulation of observation and description, not by controlled experiment, that the designer can usefully learn how to make smart choices. The plants that survived on the roof did so for three years without irrigation through harsh summers.  One could safely say that these species would probably survive on a green roof in this area of the country for at least three years and would exhibit similar behavior to that observed on the EKF roof.  A designer should feel confident using these plants in accordance with the conclusions drawn or perhaps more conservatively.

Based on my observations, I would not recommend a designer to exclusively use herbaceous plants on an unirrigated green roof of five inches or less media depth.

 

Certain of the plants, especially those which self-sow, such as Potentilla argentea, could potentially effectively be used as accent plants in a free design of sedums.  Other, hardier species with upright rather than spreading growing habits, such as Alyssum murale or Hieracium umbellatum, could be incorporated in a more structured manner into a design of mostly sedums.  Spreading plants such as Thymus serpyllum have the potential to leave large brown or bare patches when they die back, so should be carefully considered.  In all events, the designer must be aware of the potential for browning and death during summer months, and scatter the plants accordingly.

For all the sources of error present in the study, they ultimately cannot impact the greatest lesson that the roof provides for a designer, which is that of percent coverage and the implications thereof.  Because of these plants’ nature, the percent coverage of a roof with herbaceous plants can easily fall very low; on the EKF roof, the percent coverage, though about 70 percent in the spring, fell to almost zero in mid June.  This dieback leads to difficulties of aesthetics, maintenance, and functional value.  Therefore, great care must be taken by the designer when using these plants on an extensive green roof.

Ayehlet Cooper

 

Ayehlet Cooper grew up planning gardens and playing with worms in Connecticut and Massachusetts, and then went on to study music at Yale until she realized that rather than fiddling on roofs, she'd prefer to plant on them.  She then spent a year working around New Zealand in various horticultural and agricultural positions.  She was lucky enough to spend this summer working with Ed and the other wonderful staff at Emory Knoll Farms, and now looks forward to a new position as 'Plant Person' at Furbish Co. in Baltimore, Maryland.  She hopes one day soon to return to school to study landscape architecture or a related subject, and to use her schooling and experience to become an ecological designer, integrating plants with architecture and creating viable, healthy human ecosystems.

Ayehlet can be contacted at:  ayehlet@gmail.com

Send your questions or comments to: PlantEditor@greenroofs.com or phone Ed at:  410.452.5880.  Visit Ed's website:   www.greenroofplants.com

The opinions expressed by our Guest Feature writers and editors may not necessarily reflect the beliefs of Greenroofs.com, and are offered to our readers to simply present individual views and experiences and open a dialogue of further discussion, debate and research.  Enjoy, and if you have a particular comment, please contact the author or send us an email to:  comments@greenroofs.com.

 

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April 2007

Dear Ask Ed,

I recently discussed a green roof project with a landscaping contractor who was looking for sedums in one gallon pots for a green roof with a growth media depth of four inches.  I felt so inadequate because all I had to offer was 36 and 72 sized plugs.

I ran some numbers and determined that four square feet of four inch deep media equals about ten gallons.  The growth media specification was very specific about the blending ratio of mineral and organic ingredients.  With the specified planting density of one per square foot, it would appear that by introducing four gallons of highly organic nursery soil, we would significantly alter the blended ratio.  I’m also having a hard time smashing that seven inch tall mass into a four inch deep hole.  So once and for all Ed: Is bigger really better?

Signed,

Small Plugs
Dear Small Plugs,

Take heart and have faith, in the long run you will do just fine.  Specifying plants in sizes such as quarts and gallons are great for at-grade landscaping and planting plants in field soils; however, on extensive green roofs several factors work against those larger sizes.  Green roof media has been specially designed for optimizing the system of extensive green roofs, combining; porosity, % of organic matter, particle size just to mention of the elements of this engineered system.  Introducing substantial quantities of material outside the performance parameters of the green roof media to the roof system can, and mostly likely will lead to problems.

Educated clients will trade their desire for instant gratification for a healthy long lasting green roof.  The best time to transplant any plant into the landscape is during its juvenility.  That way as the plant grows, it can adapt to local conditions.

If larger plant material is used, care must be given to assure that it roots into the green roof media.  Usually those larger plants have been living in the nursery in that media for a while and those plants have been watered every day.  When that media dries, it can shrink and separate leaving a gap between the green roof media and the nursery media.   Plants must remain hydrated until new roots enter the green roof media.

I recently visited a job where most of the plants were dead, and all were planted as quarts.  I dug some of the plants up and there were no roots beyond the original line of the pot.  The few plants that were alive had come from cuttings falling from the top of the plant to establish on the top of the green roof media.
 

So, small plugs, bigger isn’t always better.  Understanding sound horticultural principles will help establish healthy green roofs so we all may benefit.

Regards, Ed Snodgrass, Emory Knoll Farms/Green Roof Plants

Publisher's Note:  Will you be in Minneapolis for the Greening Rooftops for Sustainable Communities Conference?  Come to Ed's book-signing of Green Roof Plants: A Resource and Planting Guide on Monday, April 30 from 1-3 PM in the bookstore area of the trade show. Also, Ed is an invited panelist in the Track 4 Networking Session, Session 4.1 "Greening the Green Roof System" on Monday, April 30, Room: Nicollet D, 1st Floor.
 

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December 2006

Dear Ask Ed,

I have an extensive greenroof and was under the impression I didn't have to do much maintenance or upkeep.  Can I just ignore my greenroof until the spring or is there something I should be doing before the snow really falls?

Lazy in Lansing
Dear Lazy in Lansing, here's some advice for all of us:

Winter on Your Green Roof
By Ed Snodgrass
All Photos Courtesy Ask Ed

Just when you thought it was time to forget about your green roof, well guess what?  Winter is a great time to do some botanizing on your roof.  Here are some things to look for while poking around.

Weeds:

Many spring blooming weeds set rosettes in the fall.  Weeds like cress’s and turnips that distribute tremendous amounts of seed early in the spring can be easily eliminated now.  Many of these have a small taproot, be sure and get the whole thing or you will be pulling in vain.  If there are any weeds will seed heads use a plastic bag to cover the seed head first and then pull the weed, thus preventing the distribution of weed seeds.  If you are going to more than one green roof remember to rinse the bottom of your shoes to assure you are not transferring one problem roof onto another.

Plant Health and Fertility:

Examine the plants for good color and health.  Look at the crown of the plant and see if it looks like healthy buds are set at the base for spring growth.  If there are grasses, you can cut them back now or if they are sturdy and erect you may choose to leave them until early spring for ornamental value.  Perennials can be dead headed and any seed can be distributed into bare spots on the roof.

If you are maintaining discreet borders relative to a design, it’s a good time to sharpen those lines by removing plant material that has crept across the line.  It is not a good time to fertilize as most of your nutrient will just be going down the drains; wait until soil temperatures reach at least 60 degrees F and the plants are active to fertilize.

Drains and Vegetation Free Zones:

Examine all drains to make sure water can freely flow over the winter and weed out any plants from the vegetation free zones to keep them clean.  These simple things can save you a lot of effort and grief later on.

Regards, Ed Snodgrass, Emory Knoll Farms/Green Roof Plants

 

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September 2006

Hello Heat and Drought Stress on Green Roofs
By Ed Snodgrass
All Photos Courtesy Emory Knoll Farms


In this summer of record heat, the proper green roof plant selection is readily evident.  Green roofs, gardens, and even lawns are showing the signs of the summer of 2006.  Green roofs that were well designed and properly established are holding up well under the stress and green roofs in which the plant material was improperly chosen are paying the price.  Let’s talk about heat and drought stress in plants.

The American Horticultural Society has produced a map of the U.S. by collecting data on the average number of days above 86°F or 30°C.  86 degrees is the temperature at which plants start to experience stress.  Plants need to evapotransporate to keep themselves cool under such conditions.  The AHS website has a wonderfully articulated explanation of this that I encourage readers to explore.
 

What does this have to do with green roofs?  One of the problems associated with green roofs is that the map of the heat zones may or may not apply to a particular site where a green roof exists.  Buildings create unique micro-climates and in writing the book on green roof plants (Green Roof Plants: A Resource and Planting Guide, by Edmund C. Snodgrass and Lucie L. Snodgrass, 2006 from Timber Press, Portland, OR. ), I felt I didn’t know enough to assign heat zone values to each plant.  However, I have been observing plants in out trial roofs here and have a couple of things to share.

Herbaceous Plants

Herbaceous plants have to evapotranspirate to cool themselves.  As they grow and have more expansive root systems they pull more moisture from the media and actually can reach a state where the water content of the leaves drops low enough that the turgidity is lost.  At that point it is important to re-hydrate the plant or, at the least it will lose ornamental value and, at the worst it will die.  While herbaceous plants are being established on green roof, it is critical they have enough water to remain out of this damaging state.

Green roof media is extremely well drained and what is normal watering to an experienced gardener may not be adequate on a green roof.  We are trialing over 280 varieties of herbaceous plant material in five inches of Skyland media from Laurel Valley Soils.  We water these plants for establishment and haven’t watered since.  They presented us with a wonderful spring show, but as the summer has heated up, the garden has wilted.  There hasn’t been too much mortality, but the garden has lost its ornamental value.  With each small rain, the plant spring back to life, but the stress has taken its toll.
 

Notice the shift in plant health from spring to summer in the accompanying top and bottom photos.  Designers and owners must be aware of this and make choices relative to these dynamics.
 

Succulents

Succulents are much less susceptible to heat and drought stress due to their CAM - Crassulacean acid metabolism.  This allows them to close their stomates and conserve water during the day and perform their gas exchange at night when conditions are more favorable.  The succulents typically used on green roofs can turn this process on and off as needed.  This is called facultative CAM and it lets this group of plants evapotranspirate when moisture is available and switch to CAM metabolism when it is not.

That being said, succulents do lose ornamental value under times of high heat stress - they go into dormancy and can turn yellow and red.  The difference between succulents and herbaceous plant material is that succulents will come quickly after water and cooler temperatures return.
 

High nighttime temperatures don’t give plants a chance to rest, and all green roof plants will welcome the cooler nights that fall will bring, as will we all.  While we open our windows and let the cool night air in, the plants will be recovering from one of the hottest summers on record having done their part to cool cities and give us some measure of relief.

Regards, Ed Snodgrass, Emory Knoll Farms/Green Roof Plants
 

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May 2006

Dear Ask Ed,

When estimating the overall roof loading on a greenroof, in addition to the saturated weight of the growing media, how does one calculate the weight of plugs and plants (including the soil of each container)? Is there a standard weight by size for say 3.5", one gallon, etc.? I know we are also expected to use the mature weight of each plant as well, right?

Clueless in Cleveland
Dear Clueless in Cleveland,

I wouldn’t worry about the beginning weight of plants, but the mature weight. For extensive systems most succulents such as sedums one would allow two pounds per square foot as a rule and around 3-5 pounds for perennials and grasses. These types of plants will be lighter in the winter because of dormancy when snow loads would be applicable so in temperate climates I think there would be plenty of loading to allow for these type of plants. It is, however, important to know your plants. Some xeric plants such as cactus add weight throughout their life and that would have to be accounted for.

For intensive roof plantings that use trees and shrubs, some calculation would have to be performed by species and it is probably wise to place trees over columns where concentrated loads are high to allow the greatest margin of error.

This question raises a separate but related issue, the selection of sizes of plant material for extensive plantings. I have seen specifications for gallon containers for 3” deep systems. This would require the bottom 3” of the root ball of the gallon to be cut away just to fit it into the profile of the media. The temptation to use larger sizes to obtain faster grow-in time is always there, but there is a balance. Plugs are the most often used size of material because they offer more stability than cuttings and more economy then 4” pots.

Plugs come in a number of sizes as well. They are most commonly identified by a number. This number is the number of plug cells in a 10” by 20” tray. There are 18s, 36s, 50s, 72s and 128s and these come in different depths to add more confusion. We here at Emory Knoll Farms grow in “deep” 72s. We find that using this size gives a deep root profile while still giving a good number of plants in a tray to help keep production and shipping costs down. All of these choices have to be understood to help designers and specifiers satisfy their client’s needs.

Dear Ask Ed,

I am planning to use a low-pitched to flat extensive green roof in the construction of a new house in rural New England.  My primary interest in this technique is aesthetics and the desire to blend the house into the landscape.  Most contemporary green roof systems, I've been able to find, use an engineered, lightweight planting medium and special plantings.  Assuming the structural concerns can be dealt with, could I just use a medium that is primarily native soil, say 6-12" deep and native grasses and wildflowers?  I find these type plantings more appealing for their more natural appearance.  Using such plantings, what depth of "unengineered" soil would be necessary?

Thanks, Al from Maine
Dear Al from Maine,

The short answer to your question is yes you can use native soils if you have the loading capacity.  Field soils weigh around 140 pounds per square foot at 12 inches deep when saturated.  That said, you must be careful to have adequate drainage and fertility for your plant choices.  Silt from native soils will plug the filter fabric so you either have to have low silt soils or a silt trap.  The consequence of plugging your filter fabric is that your roof becomes a pond when it rains.  I don’t have a lot of experience with this type of system and you may want to seek the advice of someone that deals with these issues regularly to obtain specific construction details.
 

Regards, Ed Snodgrass, Emory Knoll Farms/Green Roof Plants

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Mid March 2006

Dear Ask Ed,

I am a sustainable design and planning consultant and have a question regarding green roofs in the tropics. My office is in Panama, Central America. I have started to experiment with green roofs in this climate. We have high humidity, very heavy rainfall and lots of insects...house eating insects. My question is has anybody done any experiments with species of plants that are native to central America and appropriate for a green roof application. I am looking for something that is attractive to birds and butterflies and not attractive to ants or termites. We have leaf cutter ants here and they will find any new leaf and decimate any plant within hours. Many of the projects here are in or near National Parks and therefore introduction of non-native species of plant is bad. The plants also need to be able to withstand extreme downpours without being battered to bits and washed away.

I look forward hearing from you, Andrew

Dear Andrew,

I am afraid I don’t have much specific information to offer. I have been in Central America and seen many naturally occurring green roofs. Most seem to be populated by epiphytes. There are two potential problems with epiphytes in your climate. One is they provide a breeding area for mosquitoes and two, is they gain weight over their life and can add too much load to the roof. Living in the tropics always involves fighting back the jungle and therefore maintenance will be a requirement on a green roof in your part of the world. The only person I can think of for you to contact is Marco Schmidt from Germany, he is doing some green roof work in Brazil and may have insights for you. On a personal note, I love what you are doing and enjoyed your website very much. I regret I am not of more help, but I will keep looking and will be in touch as I find useful information.

Regards, Ed Snodgrass, Emory Knoll Farms/Green Roof Plants

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Late February 2006

Dear Ask Ed,

To my knowledge, contemporary green roofs are yet to come in dry New Mexico where I am. Can you suggest both plants and roof media and membranes for constructing green roofs here? Where can I get information about best ways to irrigate them? My immediate thought would be to run laser line across the roof off 1/2" black ply headers---is that what's done, and in what locales have roof irrigation systems been tried?

There was a tradition here on old style adobe structures made of vigas (large roof beams) above the latilla infill ( the ceiling above the vigas made of small poles in a herringbone pattern between the vigas), of piling dirt above that. So "green roofing has been done here, but I don't know what plants were used, perhaps seed was just allowed to blow in. Have you trialed any cactus as roof subjects? We have some tough desert grass species here, e.g. the grama grasses---what about them? I am going to trial some plants this summer in shallow trays--get them established then let them go on their own. Would like to know what's been tried in this climate. Any info on arid green roof plants for the inland high plains/ lower mountain reaches of the southwest would be appreciated!

As a landscape designer, I'd like to know if anyone's tried roof plantings with contoured or bermed soil---i.e., not just flat soil fill of a few inches thickness, but varied thick and thin soil layers up there.

Thanks for your work on a worthy subject.

Laurie in New Mexico

Dear Laurie,

I wish I could be of some help.  I do get a fair amount of interest from the south west, but I don't know of any successes without a real commitment to irrigation.  Keep in mind, without real persistent vegetative groundcover the media will be subject to wind erosion.  Be careful of grasses and their flammable nature during dormancy.
 

Regards, Ed Snodgrass, Emory Knoll Farms/Green Roof Plants

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January 2006

Emory Knoll Farms’ 2005 Green Roof Plant Trials
By Sarah Murphy and Ed Snodgrass
All Photos Courtesy Emory Knoll Farms

Sarah Murphy, Emory Knoll Farms summer intern from Virginia Tech, was tasked with evaluating herbaceous plant material for extensive systems during the summer of 2005.  The following is a summary of her work.  The trials were conducted at Emory Knoll Farms, located in Street, Maryland, which is in a temperate climate and USDA hardiness zone 6.  The trial green roof has remained planted through the winter of 2005-2006 and we will be looking at winter hardiness for at least two years before replanting the roof.

Abstract:

To determine which herbaceous plants will survive on a green roof, 284 varieties of herbaceous plants were started from seed and transplanted to 3 inch and 5 inch pots filled with green roof media as well as a green roof with 5 inches of green roof media.  They were monitored for their germination, drought tolerance and general survivability on a green roof.  In total, 284 plants were herbaceous perennials, with three grasses and 13 succulents used largely as a control group.

Methods:

The seeds were planted in 72-cell flats during the last week of February 2005.  They were kept in a heated green house, where their germination date was recorded, and they were regularly watered.  After the threat of frost had subsided the flats of plants were transferred to benches outside.

Their germination dates were continually recorded, and during the week of May 23, those plants that had germinated were planted into both 3 inch and 5 inch depth pots.  They were planted in a commonly used expanded shale media, which consists of 80% shale and 20% organic matter. These potted plants were placed in a green house were they were watered as needed and fertilized with Nutricote 14-14-14.  The plants in the study were fully rooted and in great health before any drought stress was applied.

During the week of the June 6, two of each variety was planted onto a trial green roof with 5 inches of media.  The three grasses and 13 succulents were planted separately from the 284 herbaceous species, to determine how, in large, the three groups vary in survivability on a roof environment.   The trial green roof was watered initially once a day for one week to allow the plants to establish themselves, but then left to local rainfall to provide water.  A Roof-Lite media was used which is known for its water holding capacity, and approved by FLL guidelines.

On the week of July 4, after the potted plants had well-established roots, a stimulated drought period was enforced in the green house containing the 3-inch and 5-inch potted plants.  The plants were monitored for time that elapsed before symptoms of water deprivation appeared.  The plant was labeled dead after it had either turned brown, or the leaves had lost all turgidity and become brittle.  A few plants were re-hydrated to see if they would come back from their crown, very few did and even those were too stressed to survive another drought of any duration.

During the simulated drought period, noticeable plant stress did not occur until about day 7, with the median of the plants dying around day 17.  The last of the herbaceous plants survived until approximately day 31.  Most of the succulents were alive at day 70 when the experiment was terminated.

There are many factors that affect each plant’s survivability.  Since this was a controlled drought, an obvious factor was drought tolerance.  In addition, heat tolerance, and plant physiology also affected the outcome.   Physiology was a large factor; those plants with leaves adapted to conserve water outlived the plants with large-surfaces leaves.  Plants with hairs, or long, cylindrical leaves lived longer, for the most part.  Another factor was the size of the plant, or rather, the amount of vegetative matter it had produced.  Smaller plants are better adapted to conserve water, and the results showed that is was indeed the smaller plants that lived longer.

The 21 herbaceous plants that survived with no water for at least 24 days were:

Aethionema grandiflorum, Anacyclus pyrethrum var depressus, Antennaria dioica, Corynephorus canescens, Dianthus petraeus, Dianthus pinifolius, Dianthus subacaulis, Dracocephalum ruyschiana, Erigeron linearis, Lavendula angustifolia ‘Hidcote Superior’, Lavendula angustifolia Hidcote Blue-Strain, Liatris cylindracea, Poa chaixii, Santolina chameacyparissus, Santolina chameacyparissus ssp. Tomentosa, Seslaria caerulea, Silene waldsteinii, Silene zawaldkii, Senecia cineraria ‘Silverdust’, Euphorbia myrsintes, and Allium schnoprasum.

The 13 control study succulents follow – this list includes both hardy and tender species:

Sedum kamtschaticum, Delosperma basuticum, Delosperma sutherlandii 'Peach Star', Delosperma spurium ‘Fuldaglut’, Delosperma 'Kelaides', Delosperma dyeri, Chasmatophyllum ssp. Af. Musculinum, Rabia albipunctata, Titanopsis calcarea, Aloinopsis spathulata, Sedum Pachyphyllum, Sedum rubrotinctum, and Sedum comixtum.

Regards, Ed Snodgrass, Emory Knoll Farms/Green Roof Plants

Ed Snodgrass will be publishing a book on Green Roof Plants in the fall of 2006, by Timber Press - look for updates soon!

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Late October/November 2005

Dear Ask Ed,

Hi, I am proposing green roofs for a student landscape architecture project in the city of Sweetwater, FL. This is a small city in South Florida, not wealthy, very hot, floods regularly, and currently hosts mostly one and two story rectangular concrete and stucco buildings. Would you please let me know what plants might be useful for an extensive green roof or a series of extensive green roofs that can survive high temperatures, sometimes heavy loads of water, and might be a real possibility to propose to the city. I have read about sedums, but am not sure which ones in particular would be best for this climate. If possible- it would also be great to have hurricane proof roofs. Any ideas to attract birds and butterflies? Any suggestions will be very helpful.

Thanks for your help, Sara

Dear Sara,

I wish I knew all the answers to your questions. Florida and other subtropical environments do not have many green roofs. The first thing I would say to someone is be prepared to experiment and possibly replant some areas. There are two schools of thought for the subtropics. One is to find a set of plants that can live through the dry periods and tolerate the wet periods. Both offer considerable challenges, but together it is brutal for plants, especially in thin media (3-6 inches). Many of the Sedums that are typically used in green roofs need a cold period to be viable and many of the Sedums from Mexico can not take the constant humidity, being adapted to more desert conditions they cannot evapotransporate enough water to survive. Many of the local natives have a dormant period that may not be acceptable aesthetically or will need deeper media to establish. Maybe Portulca or Delosperma, Malephora, Drosanthemum could work for a non-irrigated system once established.

On to the other school of thought, find plants that transpire a lot of water, capture all the storm water and pump it on the roof for the plants to put back in the atmosphere.  Dr. Marty Wanielista is trying that very method at the University of Central Florida.

Hope that helps, Ed

Dear Ask Ed,

Hi there, we live in southern California, or basically on the sun! But we would love to incorporate a living roof on our new addition and to the rest of our home. Are there any plants that will work in desert type conditions? I am an interior designer, but the more I find out about green design and eco design the more I want to create for the environment and less for me.

Thanks, Nicole

Hi Nicole,

The plants for areas like your depend not only on normal green roof considerations like roof loading capacity and light conditions but also on your ability to irrigate the roof. There may be real concerns with dormant plants on a roof during the fire season in Southern California. There may be more earth friendly ways to irrigate depending on your local laws. You could use water from your washing machine or capture rain water for later use to use if permitted. There are a number of nice Delosperma in the trade now and there are Sedums native to Mexico that could be used as well as Echeveria and some small barrel cactus. You might start with a trip to your local garden center and plant out some succulents in containers and after establishment stop watering them and see which are good candidates.


Regards, Ed Snodgrass, Emory Knoll Farms/Green Roof Plants

"Ask Ed" Commentary by Ed Snodgrass

As yet another hurricane moved through the Gulf of Mexico bringing with it enormous storm water challenges, it makes me think that this summer was the summer of storm water.  We in central Maryland just had 12 inches of rain from Hurricane Tammy on the 7th and 8th of October. At the same time we in the Chesapeake Bay water shed are adding 100 acres of impervious area a day.  It doesn’t really matter if the increase in storms is caused by a natural cycle or it is caused by global warming.  If we systematically decrease our environment’s ability to manage storm water, we will see more and more increases of sewage discharges, fish kills, property damage, and other storm water related issues.

I am not naïve enough to think green roofs can solve all these problems, but I do think we should turn back to nature for our solutions.  Nature has been managing storm water for a long time.  We need to look at our wetlands, forests and open spaces as machines that can help lower the cost of these events.  Maybe we should be calculating the value of land differently, bringing into the equation the service that these areas provide.  I think there is recognition in urban planning more and more to combine man made and natural systems to deliver just such value.  Green roofs are one such hybrid.  As we at Emory Knoll Farms pass the 25 acre point in green roofs supplied, we are joyful at the milestone, while sobered that we have so far to go.


Ed Snodgrass, Emory Knoll Farms/Green Roof Plants

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August 2005

Dear Ask Ed,

I am thinking about a green roof on a building that will be quite heavily shaded. I was going to use sedum but am not sure it will cope with the shade.  The building is close to and shaded by a mature yew, above and to the side of that there is an ash. It doesn’t feel all that dark to me but then I don’t have to depend on sunlight to make a living.  I am in the west of the UK, not far from the Welsh hills. Rainfall is about 20 inches a year.  I was wondering if I could put moss in to any patches where sedum failed to thrive.  If you have any advice I would be very grateful.

Best, Patrick

Dear Patrick,

I am in the U.S. and don’t know your climate first hand, but I have been there several times on holiday. I would suspect your options are wider than you would think. Sedums tolerate an amazing amount of shade for an otherwise sun loving plant. That being said, there are some good choices for Sedums in the shade: Sedum ternatum, Sedum stoloniferum, Sedum hybridum, and Sedum sexangulare are all good choices. I would be very aware of tree seedlings on the roof as they can be very aggressive and can damage roofing material. I don’t think you will have to plant moss; it will arrive if the conditions are favorable.
 

Regards, Ed Snodgrass, Emory Knoll Farms/Green Roof Plants

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July 2005

"Ask Ed" Commentary by Ed Snodgrass

A recent trip to Colorado was inspirational. First a visit to my friend and fellow green roof plant enthusiast, Dr. John White. Dr. White has spent years in the high dry country around Durango collecting and trialing possible green roof plants and hiking in the mountains looking for suitable candidates for green roofs. We talked green roofs and green roof plants, I marveled at his Columbine collection, and we argued the merits of irrigation on green roofs. I felt very fortunate to spend time with the Whites.

On to Denver to visit the Denver Botanical garden and its renowned plants man, Patayoti Kelaidis. Patayoti has brought back xeric plants from some of the most challenging of the world’s ecosystems, most notably many South African succulents. For those of you who are in Denver don’t miss the DBG - great plant collections combined with fabulous design and education work. The plants of Lewis and Clark are highlighted for the 200th anniversary of the Lewis and Clark Expedition. The West is full of possible green roof plant additions and I came home with many new plants to trial here in the east. Stay tuned for results.

Back on the home front, our Emory Knoll Farms summer intern, Sarah Murphy, is trialing over 200 varieties of herbaceous perennials for drought tolerance from the Jelitto seed catalog. Thanks to Jelitto Seeds at www.jelitto.com for their generous contribution to this effort as we try to expand the plant selection for non-irrigated roofs.
 

Ed's Feature Summer Plant ~ Talinum calycinum

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May 2005

Dear Ask Ed,

This may be a little outside your normal questions, but I thought I would ask: I have seen a picture of a Grasspave driveway planted with what appears to be a succulent. Do you know of a succulent that could handle being driven on regularly?

Also, I am working on a design and the Architect would like a green roof/ wall that rises out of the ground at a 45-degree angle can you point me in the direction of a planting system that could support soil and irrigation on this type of structure. I am currently looking at Grasspave, and a pour in place system out of England.

Sam in Southern California

Dear Sam,

I don’t know of any succulent that can handle traffic of that kind. In fact, because succulents store water, they can become very slippery in applications like that.

As far as the system question - I don’t know that I am qualified to answer that. Better to talk to an installer.

Publisher's Note:  Greenroofs.com and our contributing editors don't recommend specific service providers or products, although we can be hired as consultants for our individual opinions.  Greenroofs.com does suggest you search The Greenroof Directory.

Dear Ask Ed,

I'm an architecture student at the University of Minnesota and I'm currently designing a building with a green roof, and I'm curious if you know anything regarding the possibilities of retaining stormwater that has been filtered through a green roof in order to use it for other utilities in the building, such as toilets and potentially drinking water. Any information or input is welcome along with any known related projects where this might have already been tried or tested.

Jonathan

Dear Jonathan,

I know of two green roof projects here in Maryland that have creatively used storm water. At Montgomery Park Business Center in Baltimore they collect storm water to flush toilets and at Joppa Hall at Harford Community College they are using storm water for cooling the building in the summer. Both entities are easy to find on the web.

Publisher's Note: This was originally sent to Christine Thuring, the Student Editor, in early April who commented "Reusing green roof runoff for greywater needs within buildings is quite common in Germany, and is an excellent design element!" We then passed along to Ed (and we also alerted Jonathan about our current May Student Guest feature Article which addresses this question.)

Dear Ask Ed,

Recently I was in Pittsburgh doing a small green roof for the Children’s Museum. Some visitors from Carnegie Mellon University asked me about using slag, one of the byproducts of steel manufacturing, as the mineral aggregate for green roof growth media. Have you heard of anyone using slag or having tested slag for chemicals that might make it unsuitable for growth media? Evidently there are huge stockpiles of this stuff and people in Pittsburgh are always looking for new uses. The idea of getting away from expensive kiln fired aggregates peaks my interest. How about yours?

Searching for Greener Media

Dear Searching for Greener Media,

I don’t know of anyone who has used slag. I would guess the questions would be, is it toxic? How much does it weigh? Does it hold its structure through freeze thaw cycles?

Regards, Ed Snodgrass, Emory Knoll Farms/Green Roof Plants

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april 2005

Ed's Feature Spring Plant ~ Sedum acre 'Aureum'

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March 2005

Dear Ask Ed,

I am trying to find out if I can put plants in my rain gutters. In researching this on the web I find much information on how to plant green roofs but nothing on this. Will it destroy the rain gutters? I thought it would be an efficient way to distribute the rainwater, rather than letting it all go to the sewer. Thanks for any direction you can give me.

Jason in Sacramento, CA

Dear Jason,

I see a couple problems with planting gutters.  One is that there is not enough area to act as a water storage device and another is that when gutters are full during peak rainfall events the excess water can back up on the roof and possibly go under flashings or shingles and create leaks.  This is why cleaning the gutters becomes part of good roof maintenance.  I think you are on the right track, but move your idea down stream just a little and build a rain garden at the bottom of the gutter or use a rain barrel to capture water for use later.  These two practices help a great deal to reduce storm water flow into the sewer.

Regards, Ed Snodgrass, Emory Knoll Farms/Green Roof Plants

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February 2005

Dear Ask Ed,

Once we have designed a green roof or two, are there elements of plant specifications that can be templated for future green roof projects?

Question from a fictitious designer (amalgam of several letters)

Dear Amalgam Designer,

First, I am a nurseryman, not a designer.  I will say that we certainly see many of the same plants being successful on a wide range of green roofs both geographically and relative to site considerations.  Certainly a plant such as Sedum album and its cultivars is successful on almost any temperate extensive green roof, but there are many project specific considerations.  I think a reasonable approach would be to develop, as time goes on, a template for a starting point separated into two main plant types, groundcovers and accents.

Groundcovers are plants that give fast and reliable coverage on green roofs throughout the year and provide a stable root mass to mechanically bind the media. Accents are just what you might think, plants that can provide value on a more limited scale.  This could range from something as simple as a flash of red from Dianthus deltoides on an extensive roof to Asclepias tuberosa on an intensive system which is a larval food for Monarch butterflies.  Each accent is timed and place for maximum effect which is the essence of design I guess. I think the groundcover template would be more reliably built over time and a wide range of accent plants will come into play as experience is gained.

Dear Ask Ed,

An increasing number of projects we consider for green roofs have areas that are shaded by other building sections or neighboring buildings.  The sedums we have been working with require substantial direct sunlight.  Do you have any plant suggestions for these shaded areas?  If so, are there shade tolerant plants for various hardiness zones in the United States?

Kelly in the Midwest

Dear Kelly in the Midwest,

Surprisingly, we have found many of the successful Sedum species are more shade tolerant than you would assume.  So don’t give up on them without a try; remember many of the same plant stresses exist in shade, too, and that will eliminate many of the at grade shade species.

Also it depends what is shading the roof - is it trees or buildings?  Many times if it's buildings during the growing months of May, June and July, the sun is very high in the sky and there will be more sun than you think.  Make sure you observe the site during the summer before labeling it shade. If it’s trees, you will have deeper shade and more than likely more tree seedlings to contend with, so make sure your groundcover species are tight to minimize seed bed conditions and let it dry down in the summer to help kill off the tree seedlings.  As to specific recommendations, I would need to know what kind of shade, media depth, annual rainfall, geography and other site specific requirements.

Regards, Ed Snodgrass, Emory Knoll Farms/Green Roof Plants

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December 2004

Dear Ask Ed,

I have followed the development of green roofs with interests and have seen some successfully established ones on the coast of Denmark. Now I am located on the Outer Banks of North Carolina and I am interested in applying some of the concepts down here.

What I am specifically looking for are small hardy types of plants resistant to salt spray and periodic draught and saltwater flooding.

Thanks, Sedum Plants for Outer Banks?

Dear Sedum Plants for Outer Banks?,

Delospermas would be great as they are salt tolerant and climatically appropriate for the Carolinas.

Dear Ask Ed,

Wise sage...We're very familiar with the low-growing species like Sedums and Sempervivums that are widely adaptable for Green Roofs. And we're trying to refine the list, further, for best species for regional use in different parts of North America. But, I wonder... how well do extremely drought resistant herbaceous species like Asclepias, Baptisias, Callirhoe and Oenothera - all with deep tap roots - do on green roofs?

Another question...not so self-serving...How do you deal with winter weeds like chick weed and bitter cress on a green roof?

Allen

Dear Allen,

The truthful answer is not enough is known. Generally speaking, drought tolerant tap rooted plants survive periods of drought by drawing energy from those very tap roots. Two things are unclear, one is how well can a tap root develop in green roof media conditions if at all and how well will this time tested survival strategy work in green roof conditions. One thing is clear from Brad Rowe at Michigan State and that is that herbaceous species cannot take the dry down that is common on green roofs without some kind of support strategy.
 
As to your second question, there are two ways of dealing with those weeds, one is let the roof eradicate them the next year when the media dries down or just make sure they are weeded before they set seed in the spring. Luckily, the nature of green roof media makes weed pulling very easy.

Regards, Ed Snodgrass, Emory Knoll Farms/Green Roof Plants

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November 2004

Ed's Feature Fall Plant ~ Sedum bohmeri

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October 2004

Dear Ask Ed,

I live in Central Florida, and have been considering putting a greenroof on my warehouse. But, with all these hurricanes threatening, do you think it's really a good idea? Or, are there ways to minimize wind damage by using certain types of plants, or increasing the growth media or weight of the greenroof overall?

Thanks, "Weary in Wet Florida"

Dear "Weary in Wet Florida,"

I am not a structural engineer or a roofing consultant, but common sense would tell me if you have additional ballast in the form of plants and media and a way to secure that ballast (roots) then the uplift from a storm would have to be greater to damage the roof than a traditional roof. No one to my knowledge has calculated what that range is, there will always be winds in a category 4 or 5 storm that will destroy almost any structure.

Dear Ask Ed,

Everyone talks about the necessity and ability of greenroof plants to retain water for the extremely dry periods of occasional droughts. But what kinds of plants would be useful in a humid, tropical or semi-tropical climate where humidity might be an issue? Can I still use the various Sedums being promoted as the perfect greenroof plant?

Thanks, Danny G.

Dear Danny,

Most temperate Sedums will not work in a tropical setting. Temperate plants need a frost cycle to be viable in the short and long run. Portulacas and Delospermas are showing some promise but this is an area that needs much more study. There are tropics which stay wet all year and tropics which get very dry part of the year. I would imagine there would be two different sets of plants to suit those two different settings.

Dear Ask Ed,

Hello, I live in a 7 story concrete co-op in Vancouver British Columbia (that’s in Canada, eh). We are researching green roofing right now as we are eligible to receive up to a $50,000 grant from a local credit union towards a green project. We are planning on doing it anyway; we just want to win the grant so we can do a lot more.

Part of the application asks for how we are going to measure our success. Since I have a business background and I grew up with hippy parents I am interested in building the business case for greening our roof. I’m not exactly sure how to put some numbers to great benefits like:

CO2 consumption;
Reduced heating and air conditioning;
Reduced maintenance and capital costs (if any);
Reduced trips to the grocery store to buy vegetables.

Our deadline is October 1st. I have stumbled into the LEED program but the next available intro course in my area is not until Oct 7th so I am scrounging for clues else where. Any hints? Thanks!  Justin

Dear Justin,

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September 2004

Dear Ask Ed,

I recently discovered some bluish/greenish scaly bugs on the underside of Sedum Spurium leafs & on the 'stems' of Sedum Reflexum. These plants are growing in ground level containers right now and their growth is much less vigorous than plants which are not affected. Is there a low cost-low impact solution to this problem?

Thanks, "Got Bugs?" Andy

Dear Got Bugs,

First, identify which insect is troubling you. Use a magnifying glass to see better, then you can use the appropriate insecticide. Safer (trade name, not description) soaps and oils work on most sucking insects. Ladybug introductions also work well.

Dear Ask Ed,

I was recently in Austin, Texas in the company of a landscape architect and a businessman who sold compost. They spent the entire lunch selling me on the benefits of organic fertilizers and they had nothing but bad things to say about Osmocote. Our grower firmly believes in Osmocote and all of our Green Roof Blocks are sprinkled with a handful at each plant when they leave us for their new home on the roof. There are obviously differing opinions regarding chemical fertilizers. However, I’m not sure these parties are completely unbiased. After all here in St. Louis we have Monsanto and in Texas they have cows. What is your take on this subject? Is chemical proliferation going to be the ruin of us all, or does the unique environment of the rooftop present at least one situation where a slow controlled release of engineered nutrients is just what the doctor ordered?

Thanks, Kelly in St. Louis

Dear Kelly,

We use both products here at EKF. We do have to think about where these green roofs are and what impact any possible nutrient would have on the local watersheds. It is important to sequester nutrients in the media and to minimize leeching. Osmocote is one of several encapsulated time release fertilizers; its chief benefit is that it slowly releases it nutrient over a period of time. You can choose different release time up 100 days, although be aware that all of those calculations re based on air temperatures around 75 degrees. When it gets hotter it releases faster.

Compost can come from, lawn waste, animal waste, sewage treatment plants, worm castings, spent mushroom substrate, and a number of other sources. I am no soil expert, but in my opinion, the benefit of compost is that it delivers more than just nutrients; it can provide microbiological communities that are beneficial to the media, and can help to build a healthy media. It can also contain weed seeds, residual herbicides and other nasties.

The bottom line is, understand the products and their qualities relative to your design needs. As with most things, there are no simple answers.

Regards, Ed Snodgrass, Emory Knoll Farms/Green Roof Plants

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Inaugural "Ask Ed" Column, August 2004