CHANGES IN THE WEED SEEDBANK FOLLOWING DIFFERENT WEEDING TREATMENTS IN DRILLED SALAD ONION AND CARROT CROPS GROWN IN ORGANIC AND CONVENTIONAL SYSTEMS

Changes in the soil weed seedbank were monitored over a single growing season in field experiments with drilled salad onion and carrot crops following different weeding treatments. Viable weed seed numbers were determined in soil samples taken at crop sowing and at harvest. When the crops were left weedy until harvest, seed numbers in soil increase d by between two- and seventy-fold due to the influx of fresh seed. Th ere was much less change in weed seed numbers in the soil when crops w ere kept weed-free until harvest or weeded once at 4, 5 or 6 weeks aft er 50% crop emergence. An estimate of the seedbank at harvest in the w eeded treatments as a proportion of that of the unweeded control was d erived using a log-linear model that included a covariate adjustment f or seed numbers at sowing. There was little difference in the proporti on remaining when crops were weeded once or kept weed-free throughout. The results were similar in carrot and salad onion crops grown in org anic or conventional cropping systems. The findings highlight the risk of a rapid increase in the soil seedbank if weed control is neglected in a single crop. However. there was no indication that limiting weed control to one carefully-timed weeding would increase the potential w eed population in subsequent crops.