Detrimental effects of road deicing salt on roadside vegetation are well known and have been examined in a number of different settings and species. Literature on the salt tolerance of typical green roof species is however limited. In cold climates, salt spread on walkways and salt spray from highways can be very detrimental to green roofs with public access and green walls near roadways. Three studies were conducted to compare the salt tolerance of five species of Sedum, two species of Allium and Poa pratensis (Kentuckey bluegrass). Two studies examined six levels of salinity (0, 10, 20, 30, 40 and 50 g/L NaCl) applied as either foliar spray or as liquid applications to the soil. The third study was a field experiment examining the effects of salt spray at three distances (18.8, 37.5, and 56.9 m) from a major highway in Michigan. Response variables measured included survival, a health score from 0 (dead) to 5 (no damage), and a growth index (height x width x width).

Results/Conclusions

Allium cernuum, A. senscens and S. ellecombianum were relatively tolerant of saline spray in terms of survival rate, mean health score, the percentage of healthy plants and growth index whereas S. reflexum was much less tolerant. Sedum reflexum also had the lowest growth index of all species in this study, which declined significantly at higher salinity concentrations. Sedum ellecombianum and A. senscens were relatively tolerant of soil inundation at high saline concentrations in terms of survival, mean health scores, percentage of healthy plants and mean growth index. Sedum reflexum was not very tolerant of soil inundation regardless of salinity levels. Sedum reflexum also performed significantly worse than the other species examined except for P. pratensis in mean health score and Sedum spurium and P. pratensis in percent healthy plants in the roadside study. These three species had significantly smaller mean growth indexes than all other plants examined at all distances from the road. Distance from the road had no effect on plant survival rates and only plants farthest from the road had higher mean health scores and a greater percentage of healthy plants.