OPTIMUM WEED REMOVAL TIMING IN DRILLED SALAD ONIONS AND TRANSPLANTED BULB ONIONS GROWN IN ORGANIC AND CONVENTIONAL SYSTEMS

In field experiments with drilled salad onions, a single weeding made at 4 weeks after 50% crop emergence was sufficient to prevent signific ant yield loss due to weeds. Weed removal 5 or 6 weeks after 50% crop emergence was generally unsuccessful in averting crop losses. It is we ll known that the relative timing of crop and weed emergence is import ant in determining the balance of crop-weed interactions. In the prese nt experiments, the pattern of crop and weed emergence varied between sites and years but the removal of early emerging weeds, at 4 weeks af ter 50% crop emergence, avoided weed interference consistently. In the transplanted bulb onion experiments, the crop began with an advantage over the weeds, and a single weeding made at 5, 6 or 7 weeks after pl anting was effective in preventing a reduction in yield. There was no difference in response between crops grown organically and those grown conventionally. Yield loss was due mainly to weed competition occurri ng prior to handweeding or mechanical injury to the crop following lat e or repeated weed removal. In the conventional crop, a single weeding that removed inter-row weeds but left within-row weeds in place gave yields little better than those of the unweeded crop.