Seasonal variations in tolerance of jack pine and white spruce to glyphosate and triclopyr were investigated in field and growthroom studies. Foliar injury was higher in both species following field applications of herbicides in July during periods of active growth than in September and October when the plants were not growing actively. Herbicides applied to trees in the growthroom 3 weeks after bud break caused greater injury than applications either before or 14 weeks after bud break. White spruce tolerated both glyphosate and triclopyr better than jack pine. Triclopyr injured both species more than glyphosate. Chemical analysis and electron micrographs showed that the cuticular wax deposition on both species was greater in field-grown trees than in growthroom-grown trees and was greater at 14 weeks after bud break than at 3 weeks. Increased wax deposition with time after bud break may reduce herbicide uptake and may account partially for the increased tolerance of both species to glyphosate and triclopyr.
