Heat is the major stress factor on plant growth covering an underground electric transmission system. These effects were examined during two growing seasons on an Ontario Hydro air-insulated underground transmission prototype (OAMUT) with test thermal load of 210 watts per metre of duct length and soil root-zone maximum temperatures of approximately 35oC; winter temperature did not fall below 0oc. Fresh and dry weight and plant height data indicate that turf growth was stimulated over the heated ducts in early spring and late fall while shrub growth remained unchanged compared to unheated controls. Yields of agronomic crop species were reduced by approximately 40 to 50% compared to controls, indicating that normal agricultural productivity could not be achieved on such transmission rights-of-way. Germination of spring-sowed crops was reduced, probably due to soil drying and not a direct thermal effect. No major nutrient or pH changes were observed in the heated soil over a two-year period. It thus appears that suitable plant cover consisting of turf, shrubs and selected agronomic crops could be established over electric power transmission systems having thermal losses up to 210 watts per metre of duct buried at 1.1 metre depth.