Spring Yard 'n Garden: Treating Crabgrass 2/15/01

MANHATTAN, Kan. -- "Too Much, Too Little, Too Late" can be spring’s theme song for lawn owners with crabgrass problems.

Often, they apply a pre-emergence herbicide just once (too little). They also get around to applying the chemical preventer after annual weeds -- including crabgrass -- start to emerge. So, the herbicide can’t do its job (too late), said Matt Fagerness, horticulturist at Kansas State University.

Preventing Crabgrass
Old Rules May No Longer Apply

Many Kansans learned at a grandparent’s knee to apply crabgrass preventer when the redbuds bloom.

That was good advice when the state was mostly rural, says K-State Research and Extension horticulturist Matt Fagerness. But, it can lead to problems now, because the rule of thumb applies only with native redbuds, growing wild.

"Few of us live close enough to wild redbuds to have any confidence in using their bloom time as a clue for our yard’s microclimates," he said. "And, if our landscape includes redbuds, they’re usually hybrids developed for characteristics other than blooming at the exact time that Kansas natives do."

"You can apply crabgrass preventer as soon as an established lawn greens up and starts growing well. Your deadline should be no more than a couple of weeks after that," he said. "In fact, despite what Kansans may have learned from folklore and traditional guides, I’d recommend they consider April Fools Day as the absolute latest for their first application." (See box at left)

* Pre-emergence herbicides can harm young, shallow turf roots, as well as prevent crabgrass. New lawns and newly overseeded sections are safe only after they’ve grown enough to merit mowing three to four times.

"If you seeded so early last fall that you were mowing the new turf before winter hit, those mowings count as part of your three or four," said Fagerness, who is K-State Research and Extension’s turfgrass specialist.

* Tall fescue, Kentucky bluegrass and other cool-season turfs treated with a fertilizer-Barricade combination last fall still have residual crabgrass protection. Applying another preventer in spring could not only waste time and money but also "burn" the lawn.

"Barricade is one of two long-lasting herbicides homeowners can buy now in fertilizer mixes," he said. "The other product, Dimension, seems to be less practical under Kansas conditions, unless you’re growing bermudagrass or some other warm-season turf.

"We’re still conducting trials here at K-State. But Dimension was designed for spring use. And logic suggests that effective timing for its herbicide won’t match the recommended timing for fertilizing the state’s more popular lawn grasses."

The proper rate for applying a traditional, shorter-lasting crabgrass preventer comes down to two factors: (1) what the herbicide’s label says and (2) whether you plan two applications.

"Your lawn can only handle the amount the label specifies. So, if you’re treating twice, you should apply half the recommended amount each time," the horticulturist said.

Because the Midwest typically has hot, dry summers, a second herbicide treatment eight weeks after the first is "advisable," particularly on cool-season lawn turfs, Fagerness said.

"When fescue and bluegrass lawns start greening up and filling out in July, what you’re seeing is usually crabgrass -- crabgrass that germinated after the active ingredient in your first application’s herbicide broke down," he said. "Your cool-season turf can’t fight off the attack. It’s busy coping with the stress of summer weather.

"The turf won’t have another growth surge until things cool off in fall. And, by then, summer’s lush crabgrass will already have deposited the seeds for next year’s infestation."

Whether applied once or twice, pre-emergence herbicides can provide good control only if applied thoroughly and evenly. Fagerness advised taking these steps:

1. Check the applicator manual (or with someone who knows the equipment well). Make sure the applicator is calibrated to release the correct amount of herbicide -- when operated at your walking pace.

2. Take at least a rough measure of your lawn’s square surface area, to help determine the amount of herbicide to use.

3. If permitted by the herbicide label, lightly water the crabgrass preventer in, after applying, to help it spread out more evenly.

K-State Research and Extension is a short name for the Kansas State University Agricultural Experiment Station and Cooperative Extension Service, a program designed to generate and distribute useful knowledge for the well-being of Kansans. Supported by county, state, federal and private funds, the program has county Extension offices, experiment fields, area Extension offices and regional research centers statewide. Its headquarters is on the K-State campus, Manhattan.