Increasing nitrogen deposition and more frequent drought events are likely to change plant interactions in natural grasslands. Both factors may also influence the interactions between hemiparasitic plants, regarded as keystone species in many grasslands, and their host species.

We grew a combination of three suitable hosts, a grass, a forb and a legume, with and without the hemiparasite Rhinanthus alectorolophus at three levels of nitrogen (N) and two levels of water availability in a factorial design.

Biomass of parasite and host community increased with N‐level and was reduced by drought to a similar degree. Larger plants in fertilized pots started to wilt earlier, and the presence of a parasite further increased drought sensitivity. The parasite strongly reduced the biomass of the host community and overall productivity and affected the competitive balance among the host plants, because it particularly reduced biomass of the dominant grass. These effects were the opposite of those of high N. R. alectorolophus increased the root mass fraction of the hosts at all levels of N and water, indicating that the effect of the parasite on the hosts was mainly due to the loss of below‐ground resources.

Our results indicate that hemiparasites may not always respond more strongly to increased nitrogen availability and drought than autotrophic plants and that parasites can have similarly strong effects on grassland communities as soil fertility and drought. By preferentially attacking dominant species they may alleviate the negative effects of nutrient enrichment on grassland diversity.