Please note: The WHO considers glyphosate to be a probable human carcinogen. Its use is prohibited in some states and countries. Please check with your local laws and use caution if handling this chemical. Always spray on a clear, non-windy and dry day. Always wear protective gear when using an herbicide.
By cutting off light that the plants need to survive, this method will eventually kill the weeds. This method will probably also dry out the soil in the area, and may kill grass or other plants along with the onion weed. Plan it to take a year or so for the soil and area to return to normal.
Imazaquin A combo of 2,4-D, dicamba, and mecoprop (MCPP) in them. Glyphosate (useful only if you have a small number of wild onions to eliminate)
Spray in late fall and again in late winter or early spring. Mowing before you spray might help.
You can also try smothering wild onions under black plastic, in the same way as you would for onion weed.
