Weeding salad onions at 4 weeks after 50% crop emergence prevents yield loss from weeds, while waiting until 5 or 6 weeks is less effective. Early weed removal is key to avoiding interference. For transplanted bulb onions, weeding at 5, 6, or 7 weeks after planting prevents yield reduction. Organic and conventional crops respond similarly to weeding. Yield loss is mainly due to weed competition before weeding or crop damage from late or repeated weeding. Removing inter-row weeds but leaving within-row weeds results in slightly better yields than leaving all weeds untouched.