Q: My lawn is infested with onion weeds. I thought I had them beaten after pulling them up wherever possible, but this winter they returned in force. Do you have any suggestions for getting rid of them without also wiping out my lawn?
A: Onion weeds appear in late autumn, continue to grow through the spring, and then go dormant over the summer. Pulling them is only a temporary fix because often the bulbs remain in the ground, only to re-emerge later as stinky plants.
First, check with your local garden centre for advice about weed-control measures where you live. In many parts of the country where cool-climate grasses are the norm, you can try crowding out the onions with good lawn-cultivation practices. Reseed the lawn in late summer or early autumn, and apply either a fertiliser without herbicide, or a seed starter fertiliser. Keep the lawn moist to ensure that the grass seed germinates. Apply a second dose of fertiliser in the late autumn.
Next spring, apply a pre-emergent fertiliser (a fertiliser with herbicide that knocks out weeds before they sprout) just when the soil is warming. If you miss that window, apply post-emergent fertiliser. It will cause your lawn to grow like crazy and will choke out the wild onions. Here’s some expert advice for next time you mow the lawn: if you cut the grass too short, you are just asking for weed growth. Set the deck height of your mower to around 80 to 100 mm.