A few months ago we were contacted by Swarthmore College to help identify the cause of a disturbance on the roof. During a routine visit in August about 20% of a large, upper section of a green roof was completely torn up. The disturbance occurred in areas where low-growing Sedums, such as S. sexangulare, S. album, and S. spurium dominate. Areas where taller Sedums, such as S. takesimense, S. kamtschaticum grow were left undisturbed.
Further investigation located the likely perpetrators. Numerous white grubs were found in soil. The likely cause of the Sedums being torn up is from birds scavenging for a snack. The grubs feed off the roots of the Sedums, weakening their ability to resist the damage caused by the birds.
Our article about the damage inflicted on the green roof and ways to remediate the problem continues in our Autumn 2012 issue of Green Instance. The Scott Arboretum of Swarthmore College has also written a summary of the events, which can be seen here.