Bioweed will control onion weed. It is important to get full coverage of the weed. This is because Onion weed is a bulbing plant and stores a lot of its energy underground. If you are able to target the crown or center of the plant, you will achieve a full kill. Best time to spray is when the plant is flowering, and the bulb is at its weakest. Repeat applications will be needed, coverage and timing are key in the control of onion weed.