Q: What can I do to rid my bermudagrass lawn of an infestation of wild onions? I’m a transplant from Illinois and have not faced this type of problem with my turf before.
A: As with any weed, you have two options: digging them out or spraying them with weed killer.
Onions are an interesting case because for each cluster of leaves you see aboveground there are many more bulblets under the soil.
In other words, less that half of the onion bulbs in your soil have green foliage on them right now. The other bulbs may sprout in several weeks or perhaps next winter. If you spray with a herbicide you’ll kill the onion plants that have leaves but might not hurt the dormant bulblets underground. The bulbs you miss this time will sprout eventually and you’ll have to spray again.
Broadleaf weed killer (click for sources) products are effective against onions. You can spot spray clumps when you see them. Remember that it may take repeat sprays to kill ALL of the bulblets.
A different chemical, imazaquin (click for sources) gives excellent control but it can not be used while your bermudagrass is greening up. Most years, you’ll have to wait until May at least.
Note that imazaquin can not be used on fescue lawns.
The effects of imazaquin will be slow to see on the onions but it will happen eventually.
The best season for spraying either chemical is in winter, while your grass is dormant.
Remember, though, there is a second option.
In my opinion, digging a cluster of onion bulbs with a dandelion fork is about as effective as spraying. You can probe with the forked end and flip up most of the bulbs, dormant or not. However, I realize that if you have lots of the fragrant weed, spraying will appeal to you more than digging.