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April 2010
guest feature
article
Perennial Peanut:
Gold for Green Roofs
in the Humid Tropics
By Patrick J. O’Connor
All Photos Courtesy of Patrick O'Connor
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Newly
installed green roof in Costa Rica using perennial peanut,
Arachis pintoi. |
I
was recently challenged to come up with a plant spec for a region which
is relatively new to the world of green roofs: the humid tropics of
Central America and the Caribbean.
Here in North
America and in Europe, sedums and other hardy succulent-type plants are
the unquestionable plant choice for extensive green roofs. They
are tough, handsome, fast-growing creepers which root almost
spontaneously when sown by hand broadcasting clippings over moist
growing medium. In short order, they set up a uniform mat of
durable green. Sedum is to green roofs in temperate and
subtropical North America what Ice plant is to the California freeway
median.
In Mexico, as
far south as Guadalajara and Mexico City, sedum and other succulents,
including aloes, agaves, and echeverias also do well in green roofs.
But further south, well inside the Tropic of Cancer, the story is quite
different. Nighttime temperatures seldom fall below 65 degrees F,
and the difference between “seasons” is marked only by the amount of
daily precipitation. There is the dry season (which is bone dry),
and there is the rainy season (bring a kayak).
Sedums don’t particularly care for these balmy, wet conditions.
And when I began work on a green roof project at an “eco” home
development in Costa Rica, I was chagrined to find--after exhaustive
research and field investigation--that no sedum, nor any such succulent
counterpart in this region--stacked up to the wish list we needed on the
roof.
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Author
inspects newly installed perennial peanut, Arachis pintoi
in green roof project in Costa Rica. The groundcover may be
the plant analogy
for sedum in tropical green roofs. |
Enter perennial
peanut, Arachis pintoi. Not the dry-roasted Planter’s kind,
but a close leguminous relative in the Fabacae family, which is more
akin to clover. Clouds of yellow, sweet pea-Iike flowers are borne
above dense, emerald-green, pinnate foliage of this low, spreading
groundcover which I began noticing all over the place on my initial
trips to Costa Rica. Then I noticed it on trips to other places in
the region as well: in Puerto Rico, Nicaragua, the Dominican Republic,
and Brazil--the plant’s origin.
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Arachis pintoi (foreground right), Tulbaghia violacea
(left), and Agave attenuata
(upper right) on the roof in Costa Rica. |
Where I
noticed it was what got my attention: planted in full-exposure on the
banks of near-vertical road cuts, close to the beach in sand, or under
the cool shade of trees and other vegetation, it seemed like a tenacious
(yet non-aggressive) ground cover which tolerated a slew of extremes.
I inquired about this little peanut plant, referred to by locals as
either “maní’’ or “manicillo,” Spanish for “peanut.”
Our project was
an extensive-type green roof system: only 4” of growing medium over a
moisture retention/drain core, over a 40 mil-thick root barrier, on top
of a 60 mil TPO (thermoplastic polyolefin) waterproofing membrane.
Individual roof gardens on a dozen model homes ranged in area from 2,500
sq feet to 5,000 sq ft. Roof slopes were from 2% all the way up to
12%, and included an automated, subsurface drip irrigation system.
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Sloped
green roofs planted with perennial peanut (Arachis pintoi)
on new homes;
these living roofs have a 12% slope. Right: Initial
watering. |
I produced a
large crop of the “mani” for the project from stem cuttings, near the
jobsite itself, rooted in trays of milled coconut coir. It was
ready to install within 4 weeks. I was banking on the plant’s
performance based on its apparent durability, ease of propagation, and
survivability during the region’s epic dry season, which lasts without a
single drop of rain from November to May. Dry--but only from a
precipitation stand-point: humidity remains in the sticky 90-percentile
range and temperatures average 98 degrees F by day, and 88 F by night.
In the green
roof setting, the plant took immediately. It did not bulk up or
spread quite as quickly as its groundling counterpart, but still, it
looked great, covered great, and was doing its part to put the word
“green” in the green roofs after only a month.
It was during the middle of the rainy season when we planted it, and it
got a great head-start with plenty of water from the heavens alone.
And then, three months later, as “mud” season tapered off into dusty
dry, something went wrong. Our peanut had its first real test by
fire.
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Newly
planted perennial peanut, Arachis pintoi in Costa
Rica. |
As a result of
plumbing issues at the jobsite, the irrigation supply went completely
dry along with the weather, and remained unserviceable for nearly 2
months. After frantic reports from the developer that the green
roofs were “failing,” I made haste to the site. I arrived to find
what once was green, looking rather bare and brown. The peanut had
failed me, I thought, succumbing to some factor I hadn’t predicted or
considered. The growing medium was dry as a bone, too. Large
barren areas of the green roofs had been subject to wind-scour.
The scene looked more like a lunar landscape than “eco-friendly.”
But upon closer
inspection, I realized that the once-luxuriant crowns of foliage had
only just died back. At their base, hidden under the growth media,
there were small shoots of green, tightly furled and seeming to lay in
wait. A tug revealed that the plants had firmly rooted into the
moisture retention mat below—a good sign. It seemed they had just
gone dormant and were actually not doornail dead after all. With
the plumbing back on at this point, we got the medium good and damp, and
continued spot watering for a few days. In two weeks, the
situation was completely turned back around. The peanut was
resurrected and on its way back to green.
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On the
roof: Checking the sprinkler system.
Perennial peanut, Arachis pintoi (foreground). |
Although the
green roofs on this project were predominantly planted with the
Arachis pintoi for coverage, it served as a test-bed for other
cultivated species. Among the success stories, though in varying
degrees, were Tulbaghia violacea (society garlic), Asparagus
myerii (asparagus fern), Wedelia trilobata, Agave
attenuata, Sanseveria trifasciata (mother-in-law tongue) and
Rhoeo spathacea (Moses-in the-cradle). These seemed to
thrive under the harsh, dry-tropical conditions.
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Arachis
pintoi in the foreground with Sanseveria trifasciata
and Cuphea hyssopifolia. |
Conversely, we
were less impressed with the results of other hopefuls. Lantana
montevidensis, Plumbago auriculata and Cuphea hyssopifolia
all got established, then quickly petered-out, after seeming to lose
vigor when their deeper roots hit bottom in the shallow system.
The results
from this experiment might earn Arachis pintoi a suitable place
in green roofs in semi-tropical, frost-free regions in the continental
U.S. such as Florida, Gulf-coast Texas and perhaps southern bayou
states--where green roofs are still a relatively nascent building
feature, and where sedum may not be an effective planting option.
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One of
twelve new tropical green roofs in the eco home project in
Costa Rica. The Arachis pintoi groundcover may
be the plant analogy for sedum in tropical green roofs.
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Patrick J. O'Connor
Patrick J. O’Connor is the green roof
manager for ICS Group, Inc., Los Angeles, representative for Carlisle
Construction Materials -- roofing, waterproofing, green roof systems and
solar energy in Latin America and the Caribbean. A SCASLA (Student
Chapter of the American Society of Landscape Architects)
member, he continues his studies in the landscape architecture program
at UCLA Extension. His industry “roots” include two decades of
commercial horticulture and independent landscape design.
Contact Patrick at
p.oconnor@carlisle-lac.com.
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