A: If you spray with something like atrazine on a cold day and it has been cold for a few days leading up to your application, the herbicide is not going to work very well, if at all. It needs to be a fairly seasonable day (at least the upper 50s). A day in the 60s would be even better.
This is one reason the label on atrazine tells you to use it on certain winter weeds in October and then again in February. The weather is warm enough in those months that weeds are actively growing and will absorb the chemical.
Many people ask me about using a Weed-N-Feed fertilizer to kill these weeds. This is the only way some herbicides come in granular form. But the problem with this is it is too early to be feeding your grass. With zoysia and bermuda grass, you would be wasting much of the fertilizer. And it can be especially damaging to centipede and St. Augustine, because freezes that can occur during green-up greatly increase the chances of winter kill, particularly with high-nitrogen fertilizers.
If you decide to use a Weed-N-Feed on zoysia and bermuda, wait until green-up. Don't apply on centipede and St. Augustine until late April or the first of May. By then you are concentrating as much on summer weeds as you are on winter weeds.
February is an excellent month to spot spray with a herbicide to eliminate unwanted winter weeds in your lawn. They are all very noticeable because the weeds are green while the grass is still brown and dormant.
It is also good to eliminate weeds now, because grasses will be breaking dormancy soon and winter weeds compete for light, nutrients and moisture while the lawn is greening up.
The product you use depends on the grass you have and the weeds you are trying to control.
Atrazine is one of the wintertime favorites for its ability to control a number of weeds, suh as Florida betony, annual bluegrass, henbit, chickweed, lawn burweed, and many more. Atrazine can be used on actively growing or dormant St. Augustine, centipede, zoysia. It should be used on bermuda grass only when it is dormant .
Also popular for winter weed control are the three-way mixtures of 2,4-D, MCPP and dicamba. These can be used on all four grasses during winter, but some at reduced rates for actively growing centipede and St. Augustine.
Numerous companies sell this product under various trade names, including Ortho Weed-B-Gon, Ace Lawn Weed Killer, Bayer Advanced Southern Lawn Weed Killer, Enforcer Weed Stop, Spectracide Weed Stop, Safer Weed Away, and Vigoro Lawn Weed Killer. The only difference is some slight variations for the three active ingredients.
2,4-D mixtures control only broadleaf weeds and most of the same ones that atrazine will, but it has no effect on annual bluegrass. It will give better control than atrazine on dandelions, another common winter weed.
One other option is spot spraying Roundup (glyphosate) for annual bluegrass control (plus any other winter weeds) in bermuda, but you should use it with caution. If the bermuda is 100 percent dormant there will not be any damage, but if there is a little green in it at the base, you may get some injury.
Regardless of what herbicide you use, always read and follow label directions. Make sure the product you are using is safe for the grass you are applying it to. And remember, there is never a need to increase the rate over what the label says. There is a reason for giving you a label rate -- it works!
Remember, too, that if you will use a pre-emergence herbicide during the fall, this will greatly reduce the need to apply post-emergence herbicides.
REACH SID MULLIS, THE DIRECTOR OF THE UNIVERSITY OF GEORGIA EXTENSION SERVICE OFFICE FOR RICHMOND COUNTY, AT (706) 821-2349 OR SMULLIS@UGA.EDU.