SummaryAfter cultivation in early April of a sandy loam soil with 9500 apparently viable seeds/m2in 0–10 cm, 295 seedlings/m2emerged of which about half survived to maturity in July. Seeds were dispersed from mid‐June to November and 136,460/m2were returned to the soil, representing a 14‐fold increase in the seed bank. Application of soil‐acting herbicides reduced the numbers of weeds and the total seed output, but that of tolerant species was increased. Maximum numbers of seeds were 59,980/m2forChenopodium album, 39,430/m2forSleltaria mediaand 37,580/m2forVeronica