Help! We have a little weed popping up everywhere in our lawn, and it's running rampant through the whole neighborhood. It looks a little like clover but smaller, and the flowers are yellow. How do we tame this beast?
I think you have a weed that's pretty common in the Grand Valley called black medic. Black medic is a low-growing, trailing weed that forms dense mats in the lawn.
It has small, dark green, teardrop-shaped leaves in groups of three so it looks like a small clover (black medic is actually closely related to alfalfa). But, unlike clover, it has yellow flowers. The flowers are tiny and borne in small pom-pom shaped clusters from mid-spring into summer.
I expect we'll see more of this weed this summer because it thrives during cool, wet springs.
Black medic is usually an annual, but occasionally it can survive for several years as a short-lived perennial. It often invades lawns with poor soil — it's a good indicator of poor lawn conditions — in which lawn grasses have a hard time competing.
This is mostly lawns that haven't been fertilized adequately in the past or where the soil is compacted, too dry or waterlogged. Correcting the problem with the soil will likely minimize the weed and sometimes even eliminate it entirely.
If the soil is hard and compacted, you might consider core aerating the lawn. This can be done any time during the season and you might even consider doing it two or three times a year if the soil is especially hard. Regardless, plan on doing this every year.
Digging to check soil moisture and adjusting sprinkler coverage and watering schedule not only can help manage this weed, but can result in a stronger, better-looking lawn.
Another trick is to set your lawn mower to a higher setting. Since this weed doesn't do all that well in the shade, taller turf helps to out-compete the little devil. Another thing to think of is how the lawn if fertilized. Since black medic does well in sparse and nitrogen-poor soil — it fixes its own nitrogen, being a legume — consider regular fertilization or adding organic material to your lawn or garden to help encourage nitrogen and discourage weeds.
I especially like giving the lawn a late fall fertilization in October or November.
As for direct controls, there are several choices available. If you don't want to use chemical herbicides, hand weeding usually does a good job. I know it sounds like a lot of work, but since the plant grows from a central location and spreads from there, a big mat of black medic is often just a few plants, so it's not all that tough to clean out.
It also helps to do any pulling after the lawn has been watered so the taproot of the plant will pull out more readily.
Because it most often comes back from seed set the prior year, preemergent herbicides often will give good control. It won't eliminate any plants that manage to overwinter, but since most of them die over the winter, it should greatly reduce the number of weeds in your lawn.
Concentrate coverage in areas where you had problems with the weed the prior year. It's too late to do this now, but it's a good option for early next spring.
Right now, your best bet is to use a selective broadleaf weed killer. Fertilome Weed Out or Weed Free Zone should do a great job for you. Plan to spray one to two times, a week apart. You'll want to spray early in the morning when there is no wind.
I also like to spot spray. Just treat the areas where black medic is present. It doesn't make sense to me to do a general application over the entire lawn if the problem is in isolated patches. Not only do you waste the herbicide (and the money you spent on it) treating areas that don't need it, you risk damaging desirable trees, shrubs and flowers since they're all susceptible to the effects of these weed killers.
Finally, I will put a plug in for tolerating it as long as it doesn't go too far. I know this goes against the grain of the image of a perfectly manicured lawn, but black medic actually has its uses.
First, over enough time it will correct the lack of nitrogen in your soil by creating some itself.
Plus, it's a great forage plant for bees and other pollinators in a part of your yard that usually doesn't offer much to these vitally important workers.