Comments About Fall Pre-emergents
I’m often asked many different questions about the use of pre-emergent weedkillers in the fall – October and thereafter. Here are a few of my somewhat random thoughts.
• Even though it’s often hot and dry still in early fall, winter/early spring weeds may be really terrible if you don’t take action. That’s because many lawns have been challenged by summer drought and aren’t as thick and vigorous as usual. Weed seeds, by comparison, have been sitting out there just waiting their turn.
• All it takes to activate the weed seeds is rainfall or irrigation. Since you are probably trying to keep your lawn going at least in some order, you’ll probably be watering. Presto: weeds.
• Yes, you very well may want to apply one or both types of pre-emergent weedkillers, because there are both grassy weeds (annual bluegrass, rescuegrass and ryegrass) AND broadleafed, non-grassy weeds (henbit, chickweed, dandelions, clover and plantain).
• Types of pre-emergent weedkillers (herbicides you apply BEFORE weed seeds sprout): for grassy weeds, Dimension, Team, Halts. (Note that many grassy pre-emergent herbicides have the name “Crabgrass Preventer,” but this is NOT the time to apply them for crabgrass prevention. That time is spring. Use them now for the cool-season grassy weeds.) For broadleafed weeds, look for a Gallery product. They are currently offered mainly through local independent retail garden centers.
• Yes, if you want full weed prevention, you must make two passes over your lawn to put out the two types of weedkillers. DO NOT try to mix the materials in the fertilizer hopper. Their granules do not mix uniformly.
• These are PRE-emergent materials only. They do not offer any control for existing weeds, even recently sprouted ones. If you can see the weeds, basically, you’ve blown it for another year. They must be applied BEFORE THE WEED SEEDS SPROUT. (You do get a second chance with broadleafed weeds. If some still germinate, you can apply a broadleafed weedkiller spray in November to eliminate them. But, the pre-emergent is easier and more dependable. There is NO product that can be applied to kill grassy weeds once they start growing.)
• Timing varies with where you are in the state, but you might be able to use nighttime temperatures as a starting point. When it first drops below 70 degrees overnight, you need to be making quick plans to get the pre-emergents down. In North Texas, that usually translates to September 1-10. In Central Texas, from September 5-20. In South Texas, from September 15-25, and in the lower Rio Grande Valley, early October.
• If it stays hot unusually long, you can slide those dates back by a few days, but remember that it’s better to be a few days too early than one day too late in application of your pre-emergent.
• If you’re worried about sequence of feeding and applying pre-emergent, apply fertilizer first. Water the plant food deeply into the soil. Let the blades and runners dry before you apply the pre-emergent granules, either later that same day or 2 or 3 days later. Water lightly after you apply the pre-emergent, to wash it onto the soil’s surface.
• It’s best to keep pre-emergents out of flower and vegetable gardens.
• Read and follow all label directions for best results.
• Remember that these products are only effective on weeds that complete their entire life cycle in one year. They are not effective at eliminating weeds that return from fleshy storage roots. Weeds like nutsedge, dallisgrass and Johnsongrass will require other controls. They are discussed elsewhere in other Notes and at https://neilsperry.com/faq/