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How to defeat quackgrass and crabgrass in lawns and landscapes

"Growing Together" columnist Don Kinzler says different strategies are required to deal with the weeds.

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Quackgrass is easily identified by its pearly white undergrowth rhizomes.
David Samson / The Forum
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FARGO — Quackgrass and crabgrass have done quite well for themselves, as weeds go. Neither existed in North America until they were imported by well-intentioned people, and now the weedy grasses have spread uncontrollably throughout the continent.

Quackgrass was brought to North America by colonists in the 1600s, likely in mixtures of grass or grain seed. Crabgrass was introduced into the United States in 1849 by the U.S. Patent Office as a potential forage crop. Both grasses have spread like wildfire, much to the dismay of anyone with a lawn, landscape or garden.

Wide-bladed weedy grasses are all sometimes wrongly referred to as “crabgrass,” when many are not actually crabgrass. It’s important to identify whether the weedy grass invading a lawn is crabgrass or quackgrass, because the control is very different.

Crabgrass is an annual, meaning it grows each year from seed that sprouts in spring, grows during summer and dies over winter. Quackgrass is a perennial, meaning it grows back each spring from a winter-hardy root system.

Because crabgrass seed doesn’t germinate in spring until soil temperatures warm to about 55 degrees Fahrenheit, crabgrass doesn’t appear until after other lawn grass is green and growing. Quackgrass, though, greens up in early spring right along with the “good” lawn grass.

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A very easy way to identify quackgrass is to dig up a clump. Quackgrass spreads by pearly white rhizomes just below the soil surface. Quackgrass tends to be blue-green in color, while crabgrass is yellow-green.

Crabgrass produces seedheads in mid-to-late summer that are best described as looking like a goose’s foot. That’s the seed that creates a seedbank in the soil from which future year’s crabgrass grows.

Because crabgrass is an annual grass, it’s different enough from our perennial Kentucky bluegrass and fescue lawns that there are remedies. Garden centers and hardware stores sell products called crabgrass preventers, which are pre-emergent herbicides that are applied before the crabgrass seeds germinate. Most will not provide control after crabgrass germinates.

Crabgrass pre-emergent preventers must be applied before soil temperature reaches 55 degrees at a depth of 2 inches, meaning for much of the region it needs to be applied sometime between April 15 and May 1, depending greatly on the spring weather. Application must be followed by at least a half inch of rain to activate, and applying the product too late will have little or no effect.

There is a backup plan if crabgrass preventers aren’t applied in time, and tiny crabgrass seedlings are already evident. Herbicides called crabgrass killers, as opposed to crabgrass preventers, can kill tiny crabgrass seedlings. Follow the label directions carefully, as control depends on applying the product when seedlings are very young.

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Products like Grass-B-Gon and Grass Beater can remove actively growing grasses from many perennials and shrubs following label directions.
David Samson/The Forum

What about quackgrass? Crabgrass preventers and killers have no effect on quackgrass, which is why it’s important to accurately identify the weedy grass.

If quackgrass is invading a lawn, there are currently no herbicides that will selectively remove quackgrass from home lawns without damaging the desirable grasses. Areas of heavy infestation can be killed using glyphosate, smothering with cardboard or black plastic, or by carefully digging to remove all roots and rhizomes. Reseeding will then be necessary.

Some homeowners have successfully controlled quackgrass in their lawns by letting the quackgrass grow taller than the desirable grass, and then carefully treating with a wick-applicator that contacts only the quackgrass with glyphosate.

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Lawns sometimes also contain clumps of wide-bladed weedy grasses such as tall fescue, which are controlled much like quackgrass, with either digging or glyphosate application.

On a positive note, if quackgrass is infesting flower beds, landscapes and gardens, there are grass-killing herbicides that will selectively kill the quackgrass without harming perennial flowers and shrubs that are listed on the product label. Two such products are Ortho Grass-B-Gon and Bonide Grass Beater.

Both products can be applied over the top of actively growing plants such as peonies and asparagus, and even iris and daylilies. Follow label directions closely, and apply when quackgrass is 4 to 6 inches high. Grass-killing herbicides take time to work, so be patient.

Don Kinzler, a lifelong gardener, is the horticulturist with North Dakota State University Extension for Cass County. Readers can reach him at donald.kinzler@ndsu.edu.

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