A green roof system was installed on an existing 35-year-old building. The purpose of the study was to evaluate the effect of three substrate depths on low-temperature injury of six herbaceous perennials: bugleweed (Ajuga reptans), sandwort (Arenaria verna `Aurea'), sea pink (Armeria maritima), whitlow grass (Draba aizoides), creeping baby's breath (Gypsophila repens), and stonecrop (Sedum xhybridum). Plants in 4-inch (9-cm) pots were transplanted into three substrate depths: 2, 4, and 6 inches (5, 10, and 15 cm) and evaluated over a 3-year period. The analysis of the results showed that the species have different winter hardiness, therefore some species were subject to more freezing injury than others. Stonecrop had significantly more damage at 2-inch than 4- or 6-inch depths during the two winters. Bugleweed and creeping baby's breath showed more damage at 2 inches in 1996-97, not in 1995-96. Substrate temperatures were measured from Oct. 1995 to May 1997. Low temperature injury was more pronounced at 2 inch than at 4 or 6 inch depths. Minimum daily temperature and temperature variations measured in fall and spring of these 2 years were also higher at 4- and 6-inch depths.