SummarySize, germination and viability of seeds as well as growth of seedlings derived from three weed species were studied in a pot trial.Fallopin convolvulus(L.) A. Loeve,Galium. spuriumL. andThlaspi arvenseL. were treated with MCPA or tribenuron‐methyl at four doses and at five growth stages, from seedling stage to flowering, InG. spuriumsubnormal doses of tribenuron‐methyl, applied at intermediate growth stages, greatly reduced seed weight, gennination, viability, seedling shoot biomass and root biomass. Germination and viability, as well as the shoot biomass and root biomass of seedlings, were highly correlated with seed weight. In addition, but to a smaller extent, seed weight was reduced inF. convolvidusandT. arvenseby tribenuron‐methyl and inG. spuriumby MCPA. Germination was reduced inF. convolvulusby MCPA and inT. arvenseby tribenuron‐methyl. However, the effects varied greatly depending on the growth stage at appl