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Home HortScience Plants Recommended for Adding Diversity to Extensive Green Roofs
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On the roof, succulents come out on top

A study has determined the impact of gradual drying of extensive green roof substrate on the growth of 15 plant species and the relative water use for each species. Succulent species maintained viable foliage more than five times longer than the herbaceous species and revival rates of the succulent species were nearly double those of the herbaceous species. The study contains recommendations for succulents and herbaceous plants that can create more diverse green roof systems.

FORT COLLINS, CO—Green roofs are growing in popularity throughout the United States. These intricate systems provide environmental and aesthetic benefits in urban areas, helping to alleviate some of the downsides of urbanization. Creating effective green roofs starts with planting a healthy vegetation layer that can thrive in extreme rooftop conditions. Succulents—especially species of Sedum—have been the most studied and used plants for green roofs, but using Sedums alone can result in homogeneous, monotone roofs.

While incorporating native plants into extensive green roof systems has been investigated, studies have shown that very few plant species can match the growth and survival performance of nonnative Sedum species. Horticulturists and landscape architects are experimenting with new plants that they hope can revitalize the visual impact and health of green roofs. "Additional plant species need to be incorporated into the extensive green roof plant palette," noted Jennifer Bousselot, co-author of a study published in HortScience. "Therefore, knowledge about plant tolerance to severe moisture stress is essential, especially in a system such as an extensive green roof where moisture is typically a limiting factor for plant health."

Planting substrates used in extensive green roof systems are by necessity lightweight and well drained, making them prone to extreme fluctuations in moisture content. Because of these factors, plant species used in extensive green roofs must be able to adapt to periods of low moisture availability in their root zones. Any plant considered for use in extensive green roof systems must be evaluated for its response to gradual and long-term drying of the substrate. In semiarid regions this information is critical in determining the irrigation amounts and frequencies necessary to help green roofs thrive.

Bousselot and Colorado State University colleagues James Klett and Ronda Koski designed a research study to determine the impact of gradual drying of extensive green roof substrate on the growth of 15 plant species and to determine the relative water use for each of the species. The team performed two "dry down" trials in a greenhouse. Container plants were irrigated to saturation every 48 hours; this irrigation regimen was continued during the establishment period until 10 days before initiation of the dry-down period. Then, irrigation was tapered to every 72 hours between irrigation events for two irrigations and finally to 96 hours before the final irrigation just before dry-down initiation. Finally, all plants were irrigated and free drainage was allowed for at least 12 hours before taking the first substrate moisture measurement.

The general trend showed that succulent species retained more moisture in the substrate longer than herbaceous species. Despite differences in rate of dry down, the succulent species, as a group, maintained viable foliage for more than five times longer than the herbaceous species. After dieback and subsequent rewatering, the succulent plants were nearly twice as likely to revive as the herbaceous plants. "Not only are succulent species more likely to survive during periods of drought, but they have a better chance of surviving a period of drought once water is again made available," said Bousselot.

"Based on plant relative water use and revival of species, our study results suggest that succulents such as Sempervivum 'Royal Ruby' (Hens and chicks), Allium cernuum (Nodding onion), and Sedum lanceolatum (Lanceleaf stonecrop) would be ideal on an extensive green roof in a semiarid environment with limited moisture content," Bousselot concluded. "Additionally we found that herbaceous plants such as Antennaria parvifolia (Small-leaf pussytoes), Thymus pseudolanuginosus (Woolly thyme), and Buchloe dactyloides (Buffalo grass) are the most likely to survive low moisture content of the substrate and revive once is moisture available."

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The complete study and abstract are available on the ASHS HortScience electronic journal web site: http://hortsci.ashspublications.org/cgi/content/abstract/46/3/518

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 ashs.org


 

Original Article:

Moisture Content of Extensive Green Roof Substrate and Growth Response of 15 Temperate Plant Species during Dry Down
Jennifer M. Bousselot, James E. Klett, and Ronda D. Koski
HortScience 46:518–522. [Abstract] [Full Text] [PDF]

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