The perennial dioecious weed, Rumex acetosa, possesses sex chromosomes (XX in females, XY1Y2 in males). Yet, the operational sex ratios are female-biased. Until now, sex ratio studies in R. acetosa, as in most plants, have relied on data obtained at sexual maturity. To resolve gender among the seeds and nonflowering plants of R. acetosa, a genetic method involving a DNA marker located on both Y chromosomes has now been developed and applied. The results suggest that the sex ratios are about 1 : 1 in the whole seed pool, but that a significant female bias develops by the time of flowering. Since the age of sexually mature plants is two years or more, the time frame during which the female bias present at sexual maturity develops can be several years. It appears that male seeds germinate at a lower rate and males suffer from a greater mortality during the years-long lifespan of R. acetosa. However, there are no considerable sex-related differences in vegetative vigour.