The term ‘weed’ is both relative and subjective because it is defned according to context and in relation to humans. The most generally accepted defnition of weed is any plant growing where it is not wanted (Britannica, 2024). Weeds are usually fast-growing, opportunistic species that can cause economic problems and reduce yield by up to 100% in poorly managed cropping systems (Gu et al., 2021). The main effect of weeds in arable crop production relates to competition for space, light, water and nutrients between weeds and the crop (Riemens et al., 2022). Other threats include weeds as reservoirs of pests and diseases (Chauhan, 2020). Another form of negative crop–weed interaction is allelopathy where crops and/or weeds produce allelochemicals that interfere with crop and weed growth (Riemens et al., 2022). However, as we will see, not all allelopathic interactions are negative and competition is only one of the multiple biological interactions among species, including those within weed– crop systems. From this perspective, and considering grasslands are often multispecies systems, we could ask: what is a weed in a grassland?