BACKGROUND The common weeds Echinochloa crus‐galli L. and Setaria glauca L. were studied for their sensitivity to aqueous extracts or dry biomass of the following cover crops (CCs): Brassicaceae (Sinapis alba, Raphanus sativus var. Oleiformis, Camellina sativa), Fagopyrum esculentum and Guizotia abyssinica. RESULTS Treating E. crus‐galli with aqueous extracts of mixed CCs or individual brassica CC significantly reduced germination. Treating S. glauca with aqueous extracts of C. sativa or G. abyssinicia reduced germination. Aqueous extracts of all CCs significantly reduced radicle length of E. crus‐galli and S. glauca, with C. sativa and mixed CCs showing the greatest effect. Aqueous extracts of nearly all CCs delayed start and middle germination of E. crus‐galli and S. glauca, with S. alba and R. sativus showing the strongest effects. Aqueous extracts of Brassicaceae leaf and flower significantly reduced germination, coleoptile length, radicle length and seedling biomass of E. crus‐galli and S. glauca. Brassicaceae leaves and flowers contained higher phenolics than other tissues. Adding 4 or 8% S. alba and R. sativus dry powder to soil significantly reduced growth of E. crus‐galli and S. glauca; even concentrations of 1% measurably slowed growth of E. crus‐galli. CONCLUSIONS Brassicaceae may be allelopathic to S. glauca and E. crus‐galli. Aqueous extracts of leaves and flowers showed greater phytotoxic activity than other tissues and also contained more phenolics. Therefore Brassicaceae CCs may be most effective against S. glauca and E. crus‐galli if incorporated into soil during their flowering stage. © 2020 Society of Chemical Industry