Pre-emergent herbicides are designed to stop the growth of annual weeds during germination. Pre-emergent herbicides are the cornerstone of effective lawn weed management. Pre-emergent chemicals will not work on any weeds that are already established in your lawn. It’s important to use these products before you have a weed problem; therefore, timing of application is extremely important. One important word of caution though, if you plan on re-seeding or re-sodding your lawn anytime in the next few months, you should avoid using pre-emergent herbicides until your lawn is fully established.
There are two seasons of annual weeds: summer and winter. Summer annual weeds are those that germinate in the spring, grow all summer long, flower, and then die in the fall after the first frost comes along. Examples of summer annual weeds include crabgrass, spotted spurge, pigweed, and yellow foxtail. Winter annual weeds are those that germinate in the fall, grow all winter long, flower, and then die as soon as it begins to warm up in the summer. Examples of common winter annual weeds include bluegrass (Poa), hop clover, henbit, chickweed, buttercup, and catchweed bedstraw. Late winter is the time of year that we get numerous calls about the little green clumps of bluegrass (Poa) popping up in dormant lawns.
The good news about annual weeds is that they won’t come back from the roots like perennial weeds such as dandelions. The bad news is that annual weeds are prolific seed producers and if they go to flower, the seeds they produce will come back with a vengeance. As an example, if you let annual bluegrass (Poa) go to seed, one plant can produce over 2,000 seeds. One crabgrass plant can produce 53,000 seeds! So, as you can see, it doesn’t take long for a weed problem to explode from one year to the next.
For lawn weeds, there are two windows for pre-emergent application to prevent most annual weeds in North Georgia: March 1-20 (for summer weed prevention) and September 1-15 (for winter weed prevention). The reason why these two windows are so important is that they are the average dates when soil temperatures are optimal for most weed seeds to germinate. If you miss these two windows for pre-emergent applications, then many annual weeds will have already germinated, limiting your control options.
For weeds that were missed by pre-emergent herbicides, you can use a selective herbicide (labeled for your lawn type) and spot spray as needed. Selective post-emergent herbicides can be used any time of year as long as weeds are actively growing. However, the best timing for a selective post-emergent herbicide is when the weeds are small and before they flower and set seeds. Weeds that have already flowered will come back next year from their seeds whether you spot-spray them now or not. For many weedy grasses such as Poa, there are very few selective herbicides options that provide decent control of actively growing grasses—which makes the timing of a pre-emergent even more important.
If you get into the routine of applying lawn pre-emergents at the proper time each year, you will find that it will eventually save you a lot of time, money, and in the long run, end up using fewer chemicals. When applying granular pre-emergents, be sure to split your application in half and apply in two different directions with your broadcast spreader across the lawn. This ensures a more uniform application and is more likely to avoid missing spots in your lawn where weeds could grow.
Another recommended practice is to change herbicides each year (using a different mode of action) so that you can avoid issues with weed resistance. Many lawn pre-emergents must be watered into the soil to activate the product. If the label instructs you to water-in the product, you should either irrigate your lawn immediately or time the application within a few days of rain being forecast. As with all herbicides, be sure to read and follow all labeled application rates and safety precautions.
