The selectivity of eight foliage-active herbicides (2,4-D, 2,4,5-T, silvex, dichlorprop, triclopyr, fosamine, glyphosate, and asulam) applied at different phenological stages of development to six coniferous species (ponderosa pine, Jeffrey pine, sugar pine, Douglas-fir, white fir, and red fir) was compared. Conifers were more tolerant to herbicide applications after fall dormancy. Herbicide treatments in spring or summer often resulted in substantial conifer injury and mortality. These periods corresponded to times when moisture stress was low and photo-synthesis was high. Pine species were more tolerant to fall applications of glyphosate. A relationship between conifer phenology, moisture stress, photosynthesis, and herbicide tolerance was observed. Forest Sci. 26:3-9.