Short of lighting fire to your yard, it seems almost impossible to eliminate unwanted wild onions from your garden or lawn. Wild onion plants are extremely difficult to control for two reasons. First, they grow from bulbs, so it’s difficult to remove an entire clump without leaving one of the bulbs or some of the roots behind. Secondly, herbicides have a hard time sticking to the leaves because of their thin, waxy nature; therefore herbicides can’t penetrate the wild onion as easily.
Let’s start with the soil itself. Wild onions (and wild garlic) prefer to grow in acidic soils that are low in organic matter. Try applying lime and compost to the soil. This increases the organic matter and changes the pH to levels that are inhospitable to wild alliums. (Be careful to not apply lime near acid-loving plants such as rhododendrons or azaleas.)
Moving onto removal…while this method of removal is most time consuming, it is best to attempt digging out each and every clump you have with a spade or trowel. (Simply tugging off the leaves will do nothing to eliminate them.) Once dug up, do NOT discard the clump in the woods; do NOT shake off the excess dirt; and do NOT place in your compost pile – throw them away! If you’ve missed any of the onion, you’ll know right away. It will re-sprout in a couple of days; repeat the process immediately.
Next, treat the area with either a non-selective herbicide (like Round-up) or boiling water. Remember this — both boiling water and non-selective herbicides will kill any plant it touches! And do not use chemicals if the wild onions are growing in your veggie garden.
If digging isn’t your thing, look for a post-emergent herbicide that can be applied to the wild onions. (Do not use a pre-emergent herbicide.) Before applying the herbicide mow the plants to rough up their foliage and increase their ability to absorb the herbicide. Once the herbicide has been applied, do not mow again for at least two weeks.
There is one last option; but it’s a tad drastic. No, it’s not setting fire to your lawn. With this option, you must go in knowing that you’d most likely have to replace your lawn. First, build a strong fence around the infested area and then bring in a PIG! Yep, a pig. They love wild onions and will root out and eat all of the bulbs.