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Garden Help Desk: Managing crabgrass and other grassy weeds

By USU Extension - Special to the Daily Herald | Mar 9, 2024
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Some perennial grasses spread by rhizomes, underground stems or runners that send up new plants as they move along under the soil surface.
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Some perennial grassy weeds grow in clumps, putting up new shoots at their edges instead of sending out runners. They're often referred to as bunch grasses.
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Some grassy weeds, especially annual species, spread by seeds. Spot treatment with a grass killer, keeping the weeds mowed down or using preemergent herbicides to prevent the establishment of the germinating seeds can reduce problems with these kinds of grasses.
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The light green seedheads of annual bluegrass show up in mid- to late spring. Because this grass is an annual, a preemergent herbicide to prevent seedling establishment can help with control.
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Any kind of weed, including grassy weeds, can easily get a foothold in thinning lawn. Good lawn care that maintains a dense, healthy lawn is an important part of weed control.

I had some crabgrass show up in my front lawn halfway through March. Just small sections of crabgrass dispersed throughout the Kentucky bluegrass. I have heard I can treat with a preemergent. Is it too late? What do you recommend?

Every time I mow my lawn there are patches of grass that stick up above the rest of the lawn a few days later. What’s going on and how can stop it? Will a spray work?

It’s certainly frustrating when you do your best to have a beautiful lawn and then notice problems like this. The quest for a perfect lawn is a constant battle with Mother Nature. Lawns are monocultures: a large area with just one or a few closely related species. They aren’t “natural” or sustainable. Just as she likes to fill empty soil spaces with plants, Mother Nature also wants to disrupt monocultures like your lawn by including other plant species, creating a healthier plant community. This means extra work for you.

It’s true that crabgrass can be controlled with a preemergent herbicide. Just like annual bluegrass, crabgrass is an annual grassy weed that comes back from seed every year, but once you see these grassy weeds, it’s too late for a preemergent herbicide. If you were already seeing this grassy weed in March, though, it’s not crabgrass. Crabgrass is a warm-season grass and needs at least a few days of soil temperatures above 55 degrees to germinate. Also, crabgrass is usually found at the edges of lawns, not scattered throughout a lawn.

You’re probably dealing with a perennial grassy weed. There aren’t any chemicals that will effectively kill these grassy weeds without also damaging or killing your cool-season lawn. There are a couple of options for dealing with weeds like this, though.

You can use glyphosate (KillzAll, Ultra-Kill, Roundup, etc.) to kill out the infested area and then resod. You need to kill a wider area than just where you’re seeing the grassy weeds because rhizomes (underground stems or runners) may be sending up small shoots that you just can’t see yet. Then you should wait long enough to make sure the grassy weeds are dead (it sometimes takes more than one application to kill some of the tougher grassy weeds). If the grassy weeds are scattered over a wide area, it may not be practical to try spot-treating.

Some perennial grasses grow in tight clumps and spread by sending up shoots at the outer edges of the clumps. You can spray these clumps, spraying a few inches beyond the outer edges, or you can try digging out the clumps. You’ll need to dig out the grass a few inches beyond the edges to make sure you’re getting all the weedy grass. Digging out the unwanted grasses isn’t an effective option for grasses that spread by rhizomes.

You can also opt for my favorite method and choose to ignore the grassy weeds. Mowing the lawn a little more frequently and mowing shortly before outdoor activities will help to hide the faster growing grassy weeds and give your lawn a more uniform appearance.

Annual grassy weeds like crabgrass or annual bluegrass can be managed with preemergent herbicides. These herbicides interfere with the establishment of germinating weed seeds and should be applied about the time you see forsythia blooming in your neighborhood but no later than when lilac flower buds are first seen. If you miss this window, there are crabgrass killers you can apply to the lawn to control any crabgrass that you find growing in your lawn in late April or May. Annual bluegrass can also germinate in the early fall, so late summer and fall applications of preemergent herbicide may be needed.

Maintaining a thick, healthy lawn is one of the best ways to discourage grassy weeds. Adequate nitrogen (enough is good, more is not better), deep and infrequent watering, and a mowing height of 3 inches or more will help your lawn keep the weeds at bay.


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