Sourced from the Living Architecture Monitor
Spring is in the air, and with that comes the important task of preparing green roofs for the season ahead. Good maintenance means good performance! While green roof maintenance plans vary according to the system, there are a few key things to know about facilitating the transition from dormant winter sedums to summer blooms. Our team maintained over 200 green roof sites in the 2018 season in the Greater Toronto area, and we will share a few trade secrets in this article.
In the Toronto region, April is the month to lay the groundwork for green roof health by assessing nutrient levels in the soil. Once frost melts and the growing media begins to thaw, a sample should be taken and sent to a green roof diagnostic lab for recommendations on soil nutrient deficiencies. This is more important on juvenile green roofs with less than three years of growth, but should be considered on established roofs if there are signs of chlorosis, stunted growth, or other deficiencies. Since we deal with many roofs of several manufacturers (ie. similar nutrient levels), we undertake soil tests on a representative sample of roofs. If it is a larger roof we will still do only one sample, but several samples could also be taken to account for variation across the roof. Based on nutrient levels in the soil, a fertilizer that compliments the nutrient deficiencies of the particular green roof should be selected. It may be useful to research fertilizer suppliers or speak with a local nursery or landscaper for suggestions. On sedum roofs we typically recommend an NPK ratio (nitrogen, phosphorus, potassium) of 5-2-6, which is lower than most commercial fertilizers. This is to avoid burning plant material by over fertilizing, and is based on the logic that one can always add more fertilizer if growth is not as desired. Typically, fertilizer is dispersed by hand or using a handheld applicator.
Meanwhile on the roof… seasonal variation in when sedums break dormancy is not uncommon, and can range from mid-April to mid-May. At this time of year, expect to see dormant, brittle and brown vegetation.