Originally created 02/08/02

Eliminate weeds before they appear



Is it going to be warm or cold? I wish I knew, and I know that plants wish they knew. But when it's February in Augusta, you never know.

With last week's burst of warm weather, you need to put out pre-emergence herbicides to prevent summer weeds from infesting your lawn.

Some summer weeds, including crab grass, begin germinating when the soil temperature reaches 55 degrees for a few days, and we reached 58 degrees on Feb. 1 at 2- and 4-inch depths. The good news is that any crab grass that started to germinate was probably killed by the freeze earlier this week.

Most people don't keep up with soil temperatures, so apply pre-emergence herbicides when air temperatures reach 65 to 70 degrees for four or five consecutive days. The air temperature is warm enough to promote weed germination when crab apple and forsythia bloom. Both were in bloom last week.

You can apply a pre-emergence herbicide into April, but the longer you wait, the more weeds you will miss. When weeds are growing in spring and summer, your options are digging them up or using post-emergence herbicides. The pre-emergence herbicides control the weeds by keeping the seeds from germinating.

A problem with post-emergence herbicides is that you cannot kill the weeds without killing your grass. The only alternative is to dig the weeds up.

Pre-emergence herbicides kill annual weeds but are ineffective on perennial weeds. Perennials will require hand digging or post-emergence herbicides.

There are several advantages to using pre-emergence herbicides:

  • Susceptible weeds never show up in the lawn since you applied the chemical before the seeds germinate.
  • You can use them in granular form, which tends to be easier to apply evenly than sprays.
  • Most ornamental trees, shrubbery and flowers will not be injured, unless the herbicide contains Atrazine. DCPA (Dacthal), oryzalin (Surflan), and benefin plus oryzalen (XL) are labeled for use on some ornamentals. Post-emergence herbicides can damage ornamentals unless you follow label precautions.
  • Established lawns are not noticeably injured, while most post-emergence herbicides cause temporary yellowing of turf grasses.
  • Never apply a pre-emergence herbicide if the lawn will be seeded, sprigged or sodded in early summer. These herbicides persist in the soil for two to four months and can interfere with turf grass establishment.

    Be sure to select one labeled for use on the turf grass species in your lawn.

    Pre-emergence herbicides will not control all weeds in the lawn, but they are particularly useful in preventing most of them. The best defense against weeds is to implement cultural practices that promote vigorous growth and development of turf grass. Weeds don't easily invade turf that is properly fertilized, watered and mowed at the correct height and frequency and is growing in the proper location.

    Sid Mullis is director of the University of Georgia Extension Service office for Richmond County. Call him at 821-2349, or send e-mail to smullis@uga.edu. The offices that serve Richmond and Columbia counties have a Web page at www.griffin.peachnet.edu/ga/columbia.