February 15, 2013
A new University of Guelph study shows growers how they can help 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, says Professor Youbin Zheng.

Demand for hardy varieties of this succulent plant has increased as demand increases for 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.”

Zheng and research associate Mary Jane Clark planted sedum green roof mats at the Guelph 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.”

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, 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.”