Secondary plant metabolites may influence plant–plant interactions and plantinvasions. Distinguishing such chemicals requires integrating varyingchemical ecology approaches, information on the amounts and persistence ofspecific chemicals in nature, and measures of effects (e.g., phytotoxicityassays) to judge the importance of the chemicals (e.g., allelochemicals).The invasive plant croftonweed has caused substantial ecological andeconomic losses in China. We examined contents and degradation ofcroftonweed chemicals in the soil and their potential phytotoxic effects onconspecific and five allospecific plant species. Soils in which croftonweedwas grown had four phytotoxins: DEHP, DBP, DTD, and HHO. All chemicals weredetected in croftonweed-invaded soil, with contents ranging from 0.013 (forDEHP) to 0.353 (for DTD) µg g−1 of soil. All four compounds weredegraded rapidly in 1 wk. Combinations of the chemicals inhibited seedgermination or seedling growth of four of the six plants, includingcroftonweed itself, at mean contents found in the soil. The putativeallelochemicals degraded rapidly in the soil, and the low levels ofallelochemicals observed in the soil may be sufficient to affect seedgermination and plant growth.