Wild onions grow from bulbs beneath the ground and have green grasslike shoots above the ground. Many people consider them weeds, because they interfere with the look of a pure grass lawn. They can also poison cattle and give crops an unwanted oniony taste. Unfortunately, wild onions can be difficult to kill, because herbicides do not affect their underground bulbs as much as other weeds.
Many gardeners have a hard time removing wild onions. Pulling out the entire plant along with its underground bulbs can prevent it from growing back, but it is tedious task and easy to miss at least a bulb or two. Additionally, pre-emergent herbicides do not prevent wild onions from growing. Only post-emergent herbicides affect wild onions, and they usually require multiple applications over multiple seasons because of the plants' glossy leaf surfaces.
To hand-pull wild onions, Clemson University Extension recommends using a thin trowel to dig the bulbs from the ground. Remove all of the bulbs or else the onions will grow back. This method works best to remove small amounts of wild onions.
Mowing wild onions will not kill them, but it can prevent their spread by preventing them from sending out new seeds. Mowing the plants often produces an oniony smell. Luckily, mowing weakens wild onions and creates surface wounds for herbicides to soak into more easily.
Herbicides containing imazaquin help control wild onions in lawns, but they can damage some kinds of grass, especially fescues and warm-season turfs. Purdue University and Clemson University Extension also recommend post-emergent broadleaf herbicides containing MCPP, dicamba, 2,4-D, triclopyr, clopyralid or some combination of these chemicals for lawns. To remove wild onions in wheat crops, Purdue University experts suggest an herbicide containing 2,4-D. Some other weed killers containing glyphosate, such as Roundup and Quick Kill Grass & Weed Killer, kill wild onions but also damage lawns and crops in the process.
Always apply herbicides in accordance with the manufacturer's instructions. Excessive use can pollute groundwater. Mow the lawn before spraying to weaken the onion plants. Purdue University recommends applying herbicides in mid-spring. The Roanoke Times also recommends another application in October or November. Since wild onions are perennial plants that survive herbicides somewhat better than many weeds, gardeners might have to spray them for more than one year to fully kill them.