Seeds of dyers woad (Isatis tinctoria L.) readily germinated at temperatures from 3 to 25 C. Dyers woad seeds in their fruits, from which they do not normally dehisce, had greatly depressed germination compared to that of threshed seeds. The presence of dyers woad fruits in the same Petri dish markedly depressed germination and/or root elongation of threshed dyers woad seeds and the seeds of some competing broadleaf and grass species. The depression was especially evident on seeds of other species of Cruciferae. Similar reductions in germination or root elongation were obtained when water used to soak dyers woad fruits was used to wet germination substrates.