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Home HortTechnology Expanding the Plant Palette on Extensive Green Roofs
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CORVALLIS, OR—Extensive green roofs are a growing trend among the environmentally conscious. Used to create attractive environments, absorb rainwater, and provide wildlife habitats, green roofs can offer many benefits but can create challenging environments for plants. Green roofs are often subject to extreme environmental conditions, including prolonged periods of drought, high temperatures, and intense wind. The shallow, lightweight growing substrates typically used on extensive green roofs do little to moderate temperature fluctuations, retain moisture, or allow plants to tap deep moisture reserves. Consequently, only a few drought-tolerant succulents such as stonecrop (Sedum) have been successfully used in extensive green roof applications.

Erin Schroll, John G. Lambrinos, and David Sandrock from the Department of Horticulture at Oregon State University published a study in HortTechnology that may help expand the "plant palette" for green roof enthusiasts. The team conducted a field trial on the campus of Oregon State University in which they evaluated the performance of eight plant species in 21 roof test beds. The plants were subjected to three different water management regimes post establishment: non-irrigated, irrigation based on green roof-specific water conservation guidelines for the region, or the minimum irrigation required to maintain good plant condition. Plants used in the trials, including succulents, shrubs, grasses, bulbs, and rhizomes, were chosen for their functional attributes such as habitat quality, aesthetic quality, and stormwater management proficiency.

The results showed that all eight species had "generally high survival" over the establishment year, although hardy iceplant and common woolly sunflower experienced some overwinter mortality. During the summer following establishment, irrigation regime proved to have significant effects on survival and growth, but effects varied across species. Irrigation had no effect on survival or the growth of succulents hardy iceplant and broadleaf stonecrop or the bulb small camas. For the remainder of the species, plant survival and growth generally decreased with decreasing irrigation and many species did not survive at all without irrigation.

"Several species, particularly the grass roemer's fescue and the shrub 'Lasithi' cretan rockrose, suffered aesthetically under low irrigation, partly reflecting adaptive responses to drought stress," the researchers noted. "Weed pressure was high on bare substrate and was enhanced by irrigation, but weed pressure was negligible following canopy closure across all water regimes. These results suggest that succulents, bulbs, and rhizotomous forbs have potential for use on extensive green roofs in seasonally dry climates even without supplemental irrigation."

The study confirmed that succulents commonly used in existing green roof applications have broad suitability for use on roofs in seasonally dry environments such as the Pacific northwestern United States with little or no supplemental irrigation. Bulbs and rhizome species that experience some form of drought dormancy also appeared to be good candidates, particularly when used as functional accents with other species. The scientists noted that grasses and shrubs that display drought tolerance in their native contexts may not be good choices on extensive green roofs with little or no supplemental irrigation.

"Designing extensive roofs composed of more diverse growth forms will likely require some amount of supplemental irrigation," noted the study's corresponding author Erin Schroll. "Our study highlights the need to design context-specific green roofs that match appropriate plant selections with explicit functional goals and management plans; this will improve function and reduce the overall costs associated with maintenance."


The complete study and abstract are available on the ASHS HortTechnology electronic journal web site:

Founded in 1903, the American Society for Horticultural Science (ASHS) is the largest organization dedicated to advancing all facets of horticultural research, education, and application. More information at


Original Article:

An Evaluation of Plant Selections and Irrigation Requirements for Extensive Green Roofs in the Pacific Northwestern United States
Erin Schroll, John G. Lambrinos, and David Sandrock
HortTechnology 21:314–322. [Abstract][Full Text][PDF]

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