Summary..Plants ofPolygonum lapathifoliumwere grown either in a glasshouse or outside, at a high or a low level of mineral nutrition, and either with or without competition from barley. The viability and percentage emergence of seedlings from germinated seeds (planted in sterile compost) were higher with seed from glasshouse‐grown plants. High nutrition increased plant weight, seed production and seed weight, and also increased seedling emergence. Competition reduced plant weight, but had only a small, non‐significant, effect on seed production. Seed from plants grown with competition had a greater percentage seedling emergence from germinated se