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Home HortTechnology Irrigation Key to Plant Diversity, Survival on Green Roofs
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Irrigation Key to Plant Diversity, Survival on Green RoofsBIRMINGHAM, AL--Green roofs are becoming very popular in the United States and around the globe. The economical and environmental benefits of roof gardens are often touted, but the reality of finding plants that can survive the shallow substrates and moisture-limited environments of green roofs can be a challenge for planners and enthusiasts. New research shows there are ways to add some color to the currently limited green roof palate.

According to Julie Guckenberger Price, who led a team of researchers from the University of Alabama at Birmingham and Auburn University, research on plant species suitable for green roofs has most frequently been performed in climates unlike Alabama's. For example, many studies come from researchers in northern states such as Michigan. "The average temperature in July in Lansing, Michigan, is 70 °F and average annual precipitation is 32 inches," Price said. "In Birmingham, Alabama, the average temperature in July is 83.5 °F and average annual precipitation is 55 inches." Price and colleagues set their experiments in Birmingham to help identify plants capable of surviving on green roofs in the region’s challenging weather conditions.

Another unique aspect of the study was the introduction of irrigation to the experimental green roofs. "A few studies have evaluated the effects of irrigation on plant selection, and have concluded that succulents are best at tolerating nonirrigated conditions," Price said, adding that irrigation is not commonly used on extensive green roofs because of the additional expense of installing and maintaining an irrigation system.

The research team studied 19 species and cultivars under seasonal irrigated (half of the mini-roofs in the study were equipped with subsurface capillary irrigation) and non-irrigated conditions on experimental green roofs. Succulents, grasses, and forbs were planted in late October 2009. The plants were then evaluated for survival and increase in substrate coverage during establishment, after overwintering, and after the first growing season. A complete report of the experiments was published in HortTechnology.

The winter of the study was colder than normal, and some plants that are considered to be cold-hardy in the region (such as ice plants) did not survive through the winter. The experiments proved that irrigation moderated substrate moisture and temperature extremes; the irrigated experimental green roofs had significantly lower summer substrate temperatures (up to 20 °F lower) and were shown to be conducive to plant survival.

"Our study illustrated that plant selection options are increased with irrigation," Price noted. "Essentially, nothing survived without irrigation except some stonecrops and limestone fameflower. However, plants such as pussytoes, mouse-ear tickseed, eastern bottlebrush grass, glade cleft phlox, and eggleston's violet survived both winter and summer under the irrigated conditions."

The authors recommended irrigation systems that employ non-potable water, such as those that capture roof stormwater runoff in a cistern and pump the water back to the roof for irrigation. "These systems are good options for decreasing the environmental impact of irrigation," they said.


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:

Irrigation Lowers Substrate Temperature and Enhances Survival of Plants on Green Roofs in the Southeastern United States
Julie Guckenberger Price, Stephen A. Watts, Amy N. Wright, Robert W. Peters, and Jason T. Kirby
HortTechnology 21:586–592. [Abstract][Full Text][PDF]

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