Q: How do I tell the difference between wild onion and wild garlic?
A: Like edible onions and garlic, wild onion, Allium canadense, and wild garlic, Allium vineale, are members of the onion family. While both have thin, green, waxy leaves, those of wild garlic are round and hollow, while those of wild onion are flat and solid. Leaves of wild garlic are hollow and branch off the main stem. Leaves of wild onion are flat, not hollow, and emerge from the base of the plant. Wild garlic flowers may be green or purple; wild onion flowers are generally white or pink.
Imazaquin (click for sources) controls both plants. The best time to treat is November and again in early spring. Read label precautions before applying. This product should not be used on fescue and should not be applied to warm season turf during green up in spring. Wait at least 1-1/2 months after treatment before reseeding, winter overseeding or plugging lawns.
One gardener reports good results from spot spraying onion clumps with imazaquin and then dousing the spot with two quarts of water to hasten the chemical going to the roots.
You could also use a 3-way broadleaf weed killer (click for sources) but you won’t kill all of the underground bulbs . It might take three years to eliminate a clump using this method.
For a non-chemical option, try using a water-powered weeder. Jetting water into the soil beside the bulbs allows you to easily pull up most, but not all of them.