Plant Nutrition Requirements for an Installed Sedum-Vegetated Green Roof Module System: Effects of Fertilizer Rate and Type on Plant Growth and Leachate Nutrient Content

The objectives of this study were to compare fertilizer rates and types to identify an optimum rate to maintain green roof vegetative coverage and encourage plant growth (i.e., plant performance) while minimizing the amount and concentration of nutrients leached from a green roof module system. Sedum-vegetated modules with no added fertilizer (control) were compared with modules fertilized with 5, 7.5, 10, 15, 20, 30, and 60 g.m(-2) nitrogen (N) of 16N-2.6P-10K POLYON (R) Homogenous NPK plus Minors, 5-6 month controlled-release fertilizer (CRF), 5 g.m(-2) N of a 2.9N-2.2P-2.3K fly-larvae processed chicken manure fertilizer (5-Sus), or 5 g.m(-2) N of 4N-4P-4K Gaia Green All Purpose organic fertilizer (5-OR). The total amount and concentration of aluminum (Al), calcium (Ca), cadmium (Cd), chromium (Cr), copper (Cu), iron (Fe), mercury (Hg), potassium (K), magnesium (Mg), sodium (Na), NH4+, nickel (Ni), NO3-, phosphorus (P), lead (Pb), sulfur (S), and zinc (Zn) in leachate as well as plant overall appearance, winter injury, vegetative coverage, shoot height, bloom duration, and leaf color of green roof modules were evaluated between July 2011 and Aug. 2012. A CRF fertilizer rate of 15 g.m(-2) N maximized vegetative coverage and overall plant appearance while maintaining leachate quality within Ontario and Canadian guidelines for most of the measured elements. The amount of Zn in the CRF appeared to be higher than plant demand and the high amount and concentration of P in leachate was likely the result of release from the growing substrate. The 5-Sus fertilizer resulted in increased coverage the first spring and increased greenness soon after application compared with the same rate of CRF. Overall, 15 g.m(-2) N of CRF was the best treatment based on vegetative coverage and plant growth in sedum-vegetated green roof modules.