Effects of frequency of the stale seedbed after autumn or spring ploughing on weed density and composition in sown lemon balm

Abstract Weed management in organic farming needs to integrate a wide range of preventive tools. Here we tested the stale seed bed technique in field experiments with organic lemon balm. In a two‐factorial trial, four intensities of stale seedbed after either autumn or spring ploughing were tested in 2019/2020 and 2020/2021 on alluvial loams of the research farm Wiesengut in the Rhineland, Germany. Increasing the number of stale seedbed passes did not promote weed emergence before sowing in 2020 due to serious spring drought; in 2021 only the two last stale seedbed passes in later spring caused extra weed flushes, while cold weather conditions in early spring prevented weed response to the earlier passes. Despite the differences in the number of germinating weeds in 2021 before sowing, intensification of the stale seedbed had no decreasing effect on weed density in lemon balm after sowing. However, weed species‐specific periods were identified in which the stale seedbed selectively promoted emergence. Autumn ploughing followed by a superficial incorporation of the catch crop in spring resulted in a lower weed emergence than complete residue incorporation by spring ploughing. Seedbank size differed greatly between fields, probably due to crop rotation. The overall weed reducing effect of the tested tillage treatments was low, probably due to high weed pressure on the organic farm. To increase the efficacy of the stale seed bed technique, sufficient soil moisture as well as temperature requirements of the dominating weed flora need to be considered.