LSU AgCenter Horticulturist Offers Winter Weed Control Guidelines

Daniel Gill, Koske, Thomas J.

News You Can Use For December 2004

Winter weeds started germinating in October. Come late winter, retailers will be pushing weed and feed fertilizers to control them. Not an enthusiast for weed and feed, LSU AgCenter horticulturist Dr. Tom Koske advises caution.

"I don’t like to do my feeding and weeding at the same time," the horticulturist says.

Atrazine is the active ingredient in most weed and feeds. The herbicide is very active when timed properly on many winter broadleaves and annual bluegrass infesting lawns in Louisiana.

Weed and feed applications, however, rarely can be timed correctly to take full advantage of the herbicide. Unfortunately, manufacturers of weed and feed herbicides push applications in the fall and late winter (February and March).

"The problem with weed and feeds is the products typically contain high nitrogen fertilizer," Koske says, explaining, "High nitrogen applied in the fall and late winter encourages vigorous lawn growth and green-up, which can lead to frost injury and increased disease susceptibility."

He says another problem with weed and feed is in the application. If homeowners are not careful with granular broadcast applicators, atrazine will end up where it is not supposed to, like in flowerbeds and under trees.

"Again, there are a lot of problems associated with weed and feed for winter weed control, so I try to steer consumers away," the horticulturist emphasizes.

Timing is everything with winter weed control. Koske says the time to control winter weeds for homeowner lawns is right now, with liquid atrazine.

"This herbicide provides the most broad spectrum winter weed control available," he says, but notes, "The key word here is ‘available.’" Many of the best herbicides are not easily available to consumers.

At full professional strength (4 pounds per gallon active ingredient), atrazine nevertheless is available as a liquid in pint bottles at most discount centers, home improvement stores, feed and seed and garden centers across the state.

For homeowners, atrazine should be applied with a conventional pump-up sprayer with a flat fan nozzle calibrated to apply 1 gallon of water over 1,000 square feet.

"It is best to treat the entire lawn even if weeds have not emerged," Koske says. Atrazine works on both emerged and non-emerged weeds. If broadleaf weeds have already emerged in the lawn, a broadleaf weed killer can be tank-mixed with the atrazine.

"In fact, I like to include a broadleaf weed killer such as 2,4-D, Trimec, Weed B Gon, Bayer Broadleaf Weed Killer, Fertilome’s Weedfree Zone and so forth whenever I apply atrazine in November," the horticulturist says.

The addition of a broadleaf weed killer in the spray tank with atrazine will destroy emerged winter broadleaf weeds, and atrazine provides residual control. For best results, reapply this combination in late January or early February.

Koske offers a few tank-mix suggestions for winter weed control. (Note that 1 gallon of water should cover 1000 square feet.)

• Atrazine at1.5 oz. (3 tablespoons) + Weed B Gon at 2.5 oz. (5 tablespoons) in 1 gallon of water.

• Atrazine at 1.5 oz. (3 tablespoons) + Fertilome Weedfree Zone at 2 oz. (4 tablespoons) in 1 gallon of water.

• Atrazine at 1.5 oz. (3 tablespoons) + 2,4-D at 1/2 oz. (1 tablespoon) in 1 gallon of water.

What weeds will you control? Atrazine is well suited to control many problem winter weeds such as annual bluegrass and lawn burweed. The addition of some type of broadleaf killer as a tank-mix partner only enhances weed control. Weeds controlled include annual bluegrass, chickweeds, clovers, cudweed, dandelion, dichondra, Florida betony, henbit, lespedeza, dollarweed, mustards and thistles.

Koske says care must be taken to apply herbicides correctly regardless of the product that you are applying. Application of atrazine can cause serious injury to landscape plants and trees if the herbicide is applied near their root zones. The best advice is to stay away from the dripline of trees and shrubs with this herbicide. Establishing shrubs are particularly susceptible. Lawns with a lot of trees, shrubs, and flowerbeds are probably not good fits for atrazine.

As for turf injury, Koske says apply atrazine only to dormant bermudagrass. Actively growing green bermudagrass is somewhat sensitive to this herbicide. Expect some short-term injury to St. Augustine and centipede grasses when additional broadleaf weed killers are added.

Liquid atrazine provides the best available winter weed control for home lawns. "I try to apply it in my St. Augustinegrass lawn every year," Koske says, observing, "Whenever I miss a year and don’t apply the herbicide for winter weeds, I can surely tell."

For increased activity on emerged weeds, he recommends tank-mixing a broadleaf weed killer for winter weed knockdown. Two atrazine, plus broadleaf weed killer applications timings (in November and February) are necessary to maximize weed control.

"Do this, and no weed and feed will be necessary, and the lawn will be relatively free of winter weeds," Koske predicts, adding that most of the homeowner broadleaf weed killers, such as Weed B Gon, Bayer Broadleaf Weed Killer and many others, are very active on emerged winter broadleaf weeds with two applications.

"I had the opportunity to test most of these products last winter," the horticulturist says, noting, "The key is to apply early in the season to small actively growing weeds. Flowering weeds in March and April are much harder to kill."

For related topics look for Gardening and Get It Growing links in the Feature section of the LSU AgCenter Web site:


On the Internet: LSU AgCenter:
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Source: Tom Koske (225) 578-2222, or 

4/19/2005 10:28:36 PM
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