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Source: Optigrün international AG, www.optigruen.de

The vegetation layer is the most vital and exciting part of the greenroof, and as such suitable and dependable plant material selection needs to be assessed on a per region basis.

Since 1995, the German professional association FLL - Forschungsgesell-schaft Landschaftsbau Landschaft-sentwicklung e.V. (The Landscaping and Landscape Development and Research Society) has highly researched various areas concerning greenroof design, including plant selection and includes Plant Compatibility, Seed Germination, Plant Regeneration, Plant Types, Care During Maturation and Subsequent Upkeep including Maintenance sections in their Guidelines for the Planning, Execution and Upkeep of Green-Roof Sites (English Release, 2002).

The American Society for the Testing of Materials (ASTM) Green Roof Task Force passed the ASTM standard E2400-06 - Standard Guide for Selection, Installation, and Maintenance of Plants for Green Roof Systems - in the Spring of 2006:

E2400-06 Standard Guide for Selection, Installation, and Maintenance of Plants for Green Roof Systems

This new standard E2400-06 provides guidance for the selection criteria for plants to be used on Green Roofs. Primary considerations of this standard include: Design Intent, Aesthetics, Climate (both Macroclimate and Microclimate), Plant Characteristics (including rate of establishment, longevity, disease and pest resistance), and growing media composition. The Standard also provides guidance for the installation of plants for Green Roofs. Methods include: Pre-cultivation or direct plantings such as seeds, root cuttings and plugs. Finally, the Standard provides guidance for the maintenance of plants for Green Roofs (read the former ASTM Updates column by Ralph Velasquez here).

Characteristics of landscaping typically used in extensive greenroof systems include shallow root systems, regenerative qualities, resistance to direct radiation, drought, frost and wind.  A much larger variety of plant selections is available for intensive roofscapes due to the greater possible soil depths (American Hydrotech, Inc. Product Literature, 1999).

Compatibility issues of greenroof type, anticipated use, temperature, humidity, rainfall, and sun/shade exposure are important elements for successful plantings of any kind.  Most importantly for the artificial environment of a greenroof, native and culturally adaptable plants need to be reviewed for heat and drought tolerance, as most systems are designed to be low maintenance.

When choosing plants for appropriate regions, it is always advisable to review the U.S. Department of Agriculture’s Hardiness Zone Map, which indicates a plant’s tolerance for cold.  Frost can be a particular problem for evergreens in dry periods.  The soil must be deep enough to protect the roots from frost damage (ZinCo, 1998).

Equally, if not more, important is a plant’s tolerance to the high heat variations found on a rooftop.  The American Horticultural Society has recently developed a Plant Heat-Zone Map which charts 12 color-coded regions of the U.S. according to the annual average number of days an area experiences “heat days,” or extreme temperatures above 86 degrees F or higher.  For example, a black-eyed Susan, or Rudbeckia spp., thrives in a wide range of heat zones, from 1 to 9.  It prefers full sun but can tolerate light shade (Danny C. Flanders, 1999).

The American Horticultural Society U.S. Plant Heat-Zone Map; Source:  AHS Website

Read a little more about heat and drought stress in the September 2006 Ask Ed Commentary, "Hello Heat and Drought Stress on Green Roofs" By Ed Snodgrass.

Photo Source: Optima© Planungs-Unterlage 9/97

The desired seasonal visual impact of a greenroof may also dictate the plant material.  The summer/ winter vegetative aspect changes just like it would in any landscape.  What happens to the greenroof plants in the dormant season?  Depending on the ratio of herbaceous to evergreen plants selected, the roofscape can have varying colors, heights and textures to create winter interest.

For example, a predominantly grass roof will appear beige and brown unless some evergreen and flowering species are included. During periods of drought, mosses could also appear beige, but will green up nicely after rainfall. To the left is a split screen of an extensive greenroof planted with sedums, mosses and grasses shown in early summer and winter (Optima, 1998).

Plant material can be applied to greenroofs by several means and, according to ZinCo, once a planting has been completed, a period of maintenance begins. Trees and large bushes on greenroofs create a lot of visual impact, and several critical points must be considered to ensure successful installation and problem free maintenance.

PLANT MATERIAL APPLICATION & MAINTENANCE

Plant material can be applied to greenroofs by several means:  pre-vegetated mats or blankets;  direct on-site planting of sedum cuttings and/or seed or root plants; hydro-planting; or any combination of these methods.  Hydro-planting applications require expensive custom equipment, and is more common as an option in Europe for very difficult to reach, high, and sloped surfaces.  Types of plantings selected dictate their planting times.  Root plants (or plugs) can be planted throughout the entire growing period if they are sufficiently watered, although avoiding the intense summer months should be avoided.

Plants used for extensive landscaping are cultivated in special flat-bottomed planting trays.  The sowing of sedum cuttings and/or seeds is restricted to spring and autumn (ZinCo, 1998).  Modular plantings have an advantage in that they can be pre-grown and hardened off site, and placed on a roof at almost any time if they have been previously acclimated to the local climate.

Photo Source: ZinCo International 3/98 Brochure

The ZinCo example above of an intensive greenroof garden at the Orthotech Dental Laboratory in Leipzig, Germany, shows how semi-mature shrubs were planted to give an instant garden effect.

This covers a set period of time at the end of which the ground should be covered 60 to 70%.  After the final inspection from the greenroof company, it is advisable to ensure regular maintenance of the greenroof (once or twice a year) by negotiating a maintenance contract. For example, Roofscapes, Inc. offers this service to their customers as well as certifying existing greenroofs that satisfy their demanding performance standards.  However, simple extensive greenroofs should not require much regular maintenance after the first two years.

In the same example as above, here you can see the newly planted greenroof on the left, and how it looks after one year on the right.
Photo Source: ZinCo International 3/98 Brochure

TREES AND LARGE BUSHES

Trees and large bushes on intensive greenroofs create a lot of visual impact, and several critical points must be considered to ensure successful installation and problem free maintenance.  Load weights must be calculated, as they cause a greater concentrated load due to the additional tipping movement under wind pressure.  Care must be given during and after placement.  Trees with smaller leaves and a relatively small crown offer the best wind resistance, and the minimum soil depth for small trees is approximately 500 mm or 20".  If the area on the main greenroof is shallower, retaining walls can be installed to create areas of deeper soil.

Photo Source: ZinCo International 3/98 Brochure

Correct irrigation is very important.  If the soil depth is greater than 500-800 mm or 20-31", irrigation from above is recommended as capillary action from below is reduced.  Due to possible damage to the waterproofing integrity, normal stake posts cannot be used to anchor the trees.  Thin, high-tension wires attached to the tree, see photo above left, is one possibility.  However, the best solution is to use a "spider" of steel wire, enveloping the root ball and anchored to a steel grafting beneath, see diagram at left (ZinCo, March 1998). 

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