EAST LANSING, MI - One way to maximize the eco-friendly factor
of a structure is to include a green roof - and this doesn't refer
to the paint color. "Greening" a roof, or covering a roof with
vegetation, is gaining popularity in North America, where the
number of green roofs increased 30% from 2006 to 2007. Benefits of
green roofs include improved storm water management, energy
conservation, reduced noise and air pollution, improved
biodiversity, and even a better return on investment than
traditional roofing.
But a healthy roof requires the selection of a species that can
survive extreme climates and propagate easily to reduce erosion and
weed growth. Kristin L. Getter of Michigan State University's
Department of Horticulture led a study to determine the effect of
the growing medium's depth on the success of green roofs. The
research study, published in a recent issue of HortScience,
focused on Sedum, a variety of succulent known for its drought
tolerance.
Plots were constructed using the drainage mats and waterproofing
systems typical of green roofs, but the growing material varied in
depth from 4 cm, 7 cm, and 10 cm. Twelve species of Sedum were
planted, fertilized, and watered once. The moisture of the growing
material was measured at random times each week. Measurements of
chlorophyll fluorescence were taken to monitor the health of the
plants during a variety of environmental conditions.
Plants were monitored over the course of four years. Since the
average lifespan of the inorganic components of a green roof is
about 45 years, the researchers determined that it was important to
study the longevity of the plants. The study found that the
shallowest plot had the lowest moisture levels on average and dried
the fastest after a rain. At the 4-cm depth, four species failed to
exhibit significant growth over the 4-year period.
Five species showed no or little growth at the 7-cm depth, and
six species showed no or little growth at a depth of 10 cm. Some
species declined over the 4-year period at the varying depths. The
remaining plants that flourished were the same species for all
three depths (S. floriferum, S. sexangulare, S. spurium 'John
Creech', and S. stefco). The 4-cm depth also included two other
species (S. hispanicum and S. reflexum 'Blue Spruce').
Furthermore, the results indicate that, for the surviving and
most-abundant species, there is no benefit to depths greater than 7
cm, which would appear to be good news considering shallow depths
are more desirable because they make for lighter roof loads.
"However, at deeper depths, these plants would likely be healthier,
contain greater biomass, and be less susceptible to adverse
environmental conditions. This study shows the importance of
growing medium depth for plant performance and demonstrates the
need for long-term evaluation of species for use in this green
practice", concluded the researchers.
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