Discovering onion grass (Allium canadense), also called wild onion, on an alpine hike might be exciting for a budding botanist, but finding it in your lawn or taking over your garden is discouraging. While getting rid of this wild intruder can take several seasons, with vigilance your efforts will pay off. Onion grass, which is a perennial in U.S. Department of Agriculture plant hardiness zones 8a through 9b, has slender, upright, cylindrical leaves. Look for the leaves during the cool fall, winter and early spring months. In summer, the leaves disappear while the bulbs lie dormant under the soil.
Set the blade of a shovel 2 to 3 inches away from the base of the wild onion clump. Slide the shovel straight down and then pull back on the handle to lift the clump of onion grass out of the soil.
Inspect the hole for any small, white bulbs left behind. Remove any you find. Wild onions can repopulate an area from a few left-behind bulbs after digging.
Refill the hole with clean, fresh soil and tamp it down.
Keep an eye on the problem area for two to three years and dig out new onion grass plants as soon as they appear. Make sure to get the plants out of the ground before they flower and go to seed.
Stomp on the onion grass leaves to damage the shafts.
Spray the damaged leaves once in early spring as soon as they emerge and again in late fall. Use a ready-mixed, postemergent 2,4-D herbicide. Apply enough herbicide to moisten the leaves but stop before they are dripping wet.
Reapply the herbicide twice a year in late fall and early spring until the onion grass disappears.
Wait two weeks before mowing lawn areas where you recently treated onion grass.