Yes, but likely not every year. The need to apply fertilizer will depend upon a number of factors, including plant type, climate, weather patterns, irrigation water, and more. Thus, LiveRoof recommends that each spring, before spring growth flush, a sample of green roof soil is sent in for testing to a lab qualified to test green roof soils (as compared to a typical field soil test).
If the Nitrogren (N) levels are low, the green roof will benefit from an application of controlled release fertilizer:
If Phosphorus (P) tests are low and the municipality allows for a fertilizer containing Phosphorus – we recommend Lesco 14-14-14. The application rate can vary based on need, and typically ranges from 1.5 to 3.0 pounds of actual Nitrogren (N) per 1000 square feet which is appx. 10 to 20 points of actual fertilizer per 1000 square feet. It comes in 40 pound bags, so a bag would cover 2000 to 4000 square feet of green roof.
If P tests are low and the municipality does not allow P in the fertilizer – we recommend Lesco 21-0-21. The application rate is typically 1.5 to 3.0 pounds of actual N per 1000 square feet, but in this case calculates to 7 to 14 pounds of actual fertilizer per 1000 square feet. It comes in 50 pound bags, so a bag would cover 3500 to 7000 square feet of green roof.