SummaryA transplant experiment was performed to compare the survivorship ofRumex obtusifoliusL. andRumex crispusL., and also to determine the environmental characteristics of grasslands seriously colonized withR. obtusifolius.One‐month‐old seedlings of eachRumexspecies were transplanted into three old reseeded swards with different infestation levels ofR. obtusifoliusplants.Rumex crispuswas absent from all swards. Their survival was monitored at monthly intervals for 4 years (May 1982 to May 1986). The mortality ofR. obtusifoliuswas greater in the third and fourth years in all swards, less than 2% of the phytometers being alive in the fifth spring. The number ofR. crispustransplants declined more gradually in the course of the experiment and 4–24% of the phytometers still survived by the end of the experiment. This suggests thatR. obtusifoliusplants colonized in sown grasslands in the cool‐climate region have a comparatively short life expectation of 3–4 years. Mortality rate, growth in size and flowering percentage tended to vary significantly between sites: higher in more productive swards. In years 2 and 3 there were seven‐fold differences in mean dry matter weights of plants that produced an inflorescence and those that remained in a vegetative phase. The mortality of transplants was not size dependent in eitherRumexspecies. Phytometers died at random irrespective of the indiv