Allelopathy has the potential to promote invasion by exotic plants, but many studies produce inconclusive results. Jandová et al. examine the role of intraspecific variation by studying the root exudates of 41 maternal lines of Heracleum mantegazzianum (giant hogweed). Bioassays on germination of co-occurring Plantago lanceolata indicate that the highest variance in phytotoxic effects is within H. mantegazzianum populations, and the strength of inhibition of germination can be predicted by the metabolic profiles of the root exudates of particular maternal lines. The results suggest that intraspecific variability needs to be considered when allelopathic effects are being studied, and also indicate the potential for metabolic profiling as a tool for examining plant–plant interactions.