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Weather prompts questions about weeds

Posted: February 5, 2012 - 12:12am  |  Updated: February 5, 2012 - 5:34am

A few weeks back, I wrote about some of the winter weeds that are now present in lawns. The warm weather has gardeners thinking about summer weeds in the lawns, so I have been getting questions about when pre-emergent herbicides should be applied.

The best way to control summer weeds is to start early and use pre-emergent herbicides to kill the weed seed as they germinate. This is especially true of the grassy weeds, such as crabgrass and goosegrass.

Grassy weeds are very similar to the lawn grasses that we are trying to grow, and there are few herbicides that will control these weeds without hurting the lawn grass. So the best way to reduce the amount of weeds is to have a thick, healthy turf that reduces the amount of sunlight that reaches the soil surface.

The most common grassy weed is crabgrass. There are six types of crabgrass that will grow in this area: tropical crabgrass, smooth crabgrass, India crabgrass, large crabgrass, southern crabgrass and blanket crabgrass. Crabgrass will grow in any soil type and under any condition.

Crabgrass will start producing seeds in mid-summer and will produce seed until frost. These seeds will lay dormant until the soil temperature at 4 inches deep reaches 55 degrees, then they will start to germinate. Usually, this is during the first two weeks of March.

The University of Georgia College of Agriculture and Environmental Sciences has weather stations around the state. There are two stations close to our area. One is at Thurmond Lake and another is at McCorkles Nursery in Dearing. To look at these sites, go to www.georgiaweather.net.

Another common weed is goosegrass, an annual grass that occurs in areas with heavy traffic. These areas are usually compacted and will hold moisture. Also, goosegrass likes shallow, frequent irrigation schedules.

The amount of goose-grass can be reduced by aerating the lawn and changing the way the turf is watered. Lawns need to be watered for longer periods of time, but less often. Goosegrass seeds will start germinating when the soil temperature at 4 inches reaches 60 degrees.

Pre-emergent herbicides are one of the best ways to control these two grasses. Most pre-emergent herbicides available to homeowners come in a granular form. In order for these herbicides to work best, they must be watered in. The chemicals in them are not activated until water is applied.

Read the label for each product for rates and how to use the product. The rate on the label is the amount that gives the best control.

Make sure that the herbicide is getting on the grass and not in flower beds and on hard surfaces.

If you are planning on reseeding your lawn, you should not use a pre-emergent herbicide this spring. Otherwise, the herbicide will kill the grass seeds as they germinate.

When looking for a herbicide, check the chemical name on the label. Many of these herbicides can be found in more than one brand name.

This early in the season, it is not recommended to use a weed and feed product because it is too early to fertilize turf grass.

There are two ways that these herbicides can be applied. The first is all of the herbicide is applied in one application. However, research at UGA has shown that putting on a half rate of the herbicide now, then the other half six to eight weeks later, will be just as good.

Normally, there will be more cold weather and frost. The frost should kill any crabgrass or goose-grass that has germinated. So the pre-emergent herbicide should be applied at the normal time in mid-February.

 

Charles Phillips is a retired Columbia County Extension Service agent and operates Hort Consulting. He can be reached at cphillipshort@comcast.net, or at (706) 836-2152.

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