There have been many studies of the diverse impacts of invasions by alien plants but only a few have assessed impacts on water resources. In a new study published in AoB PLANTS, Le Maitre et al. review the impacts of invasions by alien plant species on the quantity of rainwater that reaches rivers and streams. Some studies have found that there is a large impact and others have found little or no impact. These conflicting conclusions can be explained largely by differences in the structure (e.g. height, depth of root systems) and the physiology (e.g. evergreen, deciduous, water stress tolerance) between alien and indigenous plant species. The greater the differences between the two, the greater the impact is likely to be.