Climate models predict decreasing amounts of precipitation for future summers in Switzerland. Since grasslands cover about one quarter of the area, severe consequences might be expected for Swiss agriculture, ranging from loss of grassland productivity to changes in vegetation composition. Since stressed ecosystems are also more susceptible to invasion, future drier conditions might favour the emergence of weeds. However, the response of temperate grasslands to drought has not been investigated in great detail so far. Using transparent rain shelters, we simulated extreme summer drought conditions in intensively managed temperate grassland in the Swiss lowlands at 400 m (Chamau, located near the city of Lucerne) and studied the drought response of Rumex obtusifolius, one of the most troublesome weeds for forage production. We quantified above-ground biomass and assessed the resource use in terms of carbon, nitrogen and water. R. obtusifolius increased its above-ground biomass production in response to drought, comprising up to 80% of the total community biomass in 2006. Within the drought plots, highest pre-dawn leaf water potentials, high values for midday leaf water potentials, stomatal conductance and assimilation clearly indicated that R. obtusifolius was much less affected by drought than other plant species. In general, no significant differences were found for these variables between drought and control R. obtusifolius plants, in contrast to the other plant species. Higher water use efficiency together with a change in N acquisition patterns resulted in a competitive advantage of R. obtusifolius over other species, favouring the spread of this weed. Thus, our results suggest a potential increase of weed pressure by R. obtusifolius under future climatic conditions, demanding additional management measures to limit its success. © 2009 Elsevier B.V. All rights reserved.