U of G Research May Help Boost Winter Survival of Green Roofs

December 13, 2012 - News Release

A new University of Guelph study shows growers how they can help increasingly popular “green roof” plants survive southern Ontario winters without unnecessary fertilizer costs.

Finding ways to make Sedum (“stonecrop”) plants more cold-resistant would help growers eager to install green roofs in fall as well as in spring and summer, said Prof. Youbin Zheng, School of Environmental Sciences (SES).

Demand for hardy varieties of this succulent plant has increased as growers learn more about the environmental, economic and esthetic benefits of green roofs.

“Green roof plant installations are typically exposed to more extreme weather conditions than ground-level plantings,” said Zheng. “Particularly in northern climates, a late fall installation of green roof plants can result in cold damage or low plant survival.”

He and SES research associate Mary Jane Clark planted Sedum green roof mats on campus in October.

“The effect of multiple rates and types of fertilizer were compared,” said Zheng. “We also evaluated the survival and growth of an unfertilized versus a fertilized Sedum-vegetated green roof.”

Fertilizers contain three key elements. Nitrogen (N) helps foliage grow quickly and strong. Phosphorus (P) allows plants to withstand stress and to develop roots and flowers. Potassium (K) helps with photosynthesis needed for overall plant health.

Clark and Zheng compared effects of balanced NPK fertilizers with those of added-P or added-K fertilizers on Sedum.

They found that controlled-release fertilizers with balanced amounts of N, P and K can improve winter survival of low-maintenance Sedum plants in green roof systems installed in fall. Using fertilizer with added P or K made no significant difference.

“Plants in all the mats survived the first winter, and added P or K did not boost the Sedum plant performance compared to the plants treated with a balanced NPK controlled-release fertilizer,” Zheng said.

Clark added: “For climates similar to Guelph’s, our study showed Sedum-vegetated green roof mats can survive without extra P or K fertilization when planted in late fall. Reducing the amount of fertilizer applied at installation makes green roofs more cost-effective to install.”

The study appears in the December issue of HortScience.

Green Roof

Prof. Youbin Zheng
School of Environmental Sciences
University of Guelph
519-824-4120, Ext. 52741

For media questions, contact Communications and Public Affairs: Lori Bona Hunt, 519-824-4120, Ext. 53338, or lhunt@uoguelph.ca, or Shiona Mackenzie, Ext. 56982, or shiona@uoguelph.ca

University of Guelph
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Guelph, Ontario, N1G 2W1