In field experiments with drilled salad onions, a single weeding made at 4 weeks after 50% crop emergence was sufficient to prevent significant yield loss due to weeds. Weed removal 5 or 6 weeks after 50% crop emergence was generally unsuccessful in averting crop losses. It is well known that the relative timing of crop and weed emergence is important in determining the balance of crop-weed interactions. In the present experiments, the pattern of crop and weed emergence varied between sites and years but the removal of early emerging weeds, at 4 weeks after 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 planting 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 occurring prior to handweeding or mechanical injury to the crop following late 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.