Onions are a noncompetitive crop with short vertical leaves that fail to form a canopy that suppresses weeds. Weeds are managed with selective herbicides combined with frequent cultivation.
Winter cover crops or crop rotations enhance both soil and weed management.
Plant in center of beds. Apply after planting to soil surface before weeds germinate and onions emerge. In eastern Oregon and Washington if overhead irrigation is not available, apply and shallowly incorporate herbicide with a nailboard. Rain after incorporation improves weed control. If overhead irrigation is available, results are improved by irrigating immediately following application.