The contribution of yellow devil hawkweed (H. floribundum) to the total standing crop of an abandoned pasture in southern Ontario increased from approximately 8% to 70% asH. floribundumcolonized the pasture by forming patches containing as many as 3400 rosettes/m2. Within the hawkweed population, there was a significant (P < 0.05) shift in the distribution of standing crop from reproductive to vegetative structures and within the vegetative structures from leaves to roots and rhizome during the process of patch formation. These shifts presumably occurred in response to changing environmental conditions (i.e., increasing light availability, reduced soil moisture levels, greater intraspecific competition). The mechanism by which the allocation pattern changed was more likely phenotypic plasticity than genetic change sinceH. floribundumis an apomict and vegetative reproduction accounted for most recruitment into the population.The frequency distribution of population members in dry weight classes became more negatively skewed during the process of patch formation, but the distribution did not approach log-normality as predicted in some studies. Significant (P < 0.05) differences were observed in the allocation patterns of individuals belonging to different weight classes in the population. Only the largest plants in the population allocated dry weight to reproductive structures.