Germination of winter weeds will vary from year to year, but on average the best time to apply a pre-emergence herbicide to the lawn is the last week of September through the first couple of weeks of October. An even better, more specific signal is when the night temperatures consistently drop between 55 and 60 degrees. This has not happened yet. Depending on which forecast you believe, it may begin next week.
There is no downside to applying the pre-emergence herbicide too early other than it wearing off sooner. The downside of putting it out too late is that you miss weeds that may have already germinated.
The predominant winter weeds in our lawns in the Augusta area are annual bluegrass (poa annua), henbit and chickweed. There are certainly several others.
For the most part, people are more willing to allow a variety of summer weeds to grow in their lawns because they at least sort of “blend in” with the grass because everything is green. But during the winter, the weeds stick out like a sore thumb because the grass is brown and the weeds are green. So it doesn’t look good to have all these green weeds scattered around in the lawn.
Keep in mind that pre-emergence herbicides, for the most part, only prevent annual weeds and not perennial weeds. Perennial weeds mainly come back vegetatively while annual weeds come back strictly from seed. Many perennial weeds require multiple applications of selective post-emergence weed killers, spot applications of Roundup, or will require hand digging.
Pre-emergence herbicides are a very useful tool for several reasons:
• You can use them in granular form which tends to be easier to apply evenly than liquid sprays.
• Most ornamental shrubbery, trees, and flowers will not be injured, except with herbicides that contain atrazine. In fact, some are even labeled for use on some ornamentals. Post-emergence herbicides can often damage ornamentals unless you follow label precautions.
• Established lawns are not noticeably injured, while most post-emergence herbicides cause slight injury to temporary yellowing of turf grasses.
Granular pre-emergence herbicides can be applied with a drop or rotary spreader. Divide the amount of herbicide needed into two equal parts. Apply in two directions at right angles to each other. This will help insure a more uniform distribution and help to prevent skips and excessive overlap.
Pre-emergence herbicides should only be applied on lawns that are well established. If newer sod is not well rooted, don’t use it.
Do not apply a pre-emergent if you are planning to overseed your lawn with ryegrass. This is a mistake I have seen a lot of people make over the years. Unfortunately, the herbicide cannot distinguish between weed seed and ryegrass seed.
Also, don’t use a Weed-N-Feed this time of year that contains a pre-emergent herbicide. The latest we should be fertilizing the lawn is about Sept. 15 so it is too late for that. For the most part, you would be wasting the fertilizer, but also fertilizing this time of year increases the potential of turf diseases.
After you apply herbicide, water it in or wait for it to rain. But don’t wait more than a week to 10 days because the herbicide will begin to lose effectiveness.
Pre-emergence herbicides for lawns are sold under a number of trade names. The most common ones available are benefin (Balan, Crabgrass Preventer, etc.), pendimethalin (Halts, Lesco Pre-M Plus), and dithiopyr (Dimension, StaGreen Crab Ex, Crabgrass Preventer, Vigoro Crabgrass Preventer). All of these are virtually the same in stopping weeds, so you have a lot from which to choose and all are safe to apply on all four of our warm season grasses.
Take note that most if not all pre-emergence herbicides are 0-0-7. That means they contain 7 percent potassium. Potassium helps in the prevention of diseases and promotes winter hardiness in the grass. Probably 95-97 percent of our lawns are deficient in potassium.
If you have been using the same product year after year, change to another one. Research has shown that some weeds can build up a resistance when used every year. This is particularly true with annual bluegrass.
If you put out a late summer Weed-N-Feed that contains a pre-emergent, for example, on Aug. 1, you can wait about three months, or about Nov. 1, before you apply the pre-emergent.
Remember, pre-emergence herbicides will not control all weeds in a lawn, but they are a useful tool in preventing most of them. You will still need to spot spray in fall and winter with post-emergence herbicides.