The effect of 5 winter and 2 post-winter defoliation frequencies on the morphology of medium-leafed and large-leafed white clover and perennial ryegrass plants in an irrigated perennial pasture in northern Victoria was investigated. Measurements included leaf appearance rates, axillary bud development and survival, stolon elongation and survival, and tiller production and survival. White clover leaf appearance rate was affected little by either defoliation frequency or cultivar, ranging from 0·4 leaves/week in July to 1·1 leaves/week in February. In contrast, perennial ryegrass leaf appearance rate in the same period ranged from 0·35 to 0·7 leaves/week. Axillary bud production on clover stolons was highest in winter and spring and lowest through summer, and was usually greater in the medium-leafed than in the large-leafed cultivars. When the swards were not defoliated through winter, the rate of bud production was reduced in late winter; this was followed by a high rate of bud production in early spring, resulting in little treatment difference in the number of buds per stolon by mid spring. Bud survival was greater when initiated in winter than in summer. The rate of stolon death in spring was higher in the swards not defoliated during winter and higher in the large-leafed than in the medium-leafed cultivar. These effects were associated with the stolon elongation rate in late winter; the medium-leafed cultivar was unaffected by defoliation frequency, whereas in the large-leafed cultivars the rate of stolon elongation increased as the defoliation interval lengthened and canopy shading increased. Perennial ryegrass tiller production was greater with frequent than infrequent winter or post-winter defoliation, but never exceeded 1·2 daughter tillers/tiller, with 44% of the reproductive tillers failing to replace themselves. The low leaf appearance rate and tiller production of the perennial ryegrass may contribute to its poor competitiveness and persistence in this environment.