Designed to prevent weeds before they occur, pre-emergence herbicides, have been an accepted weed-control strategy for decades. It takes a lot less effort (read: chemical applications) to maintain a weed-free site than to fight off an infestation.
As you plan your applications, remember that pre-emergents will not take weeds out after they germinate (although some of the herbicides do control newly emerged weeds). Don’t expect a pre-emergent to knock down weeds that are already established either. If there are only a few, consider hand-weeding before your application. If they are abundant, you might need a post-emergence herbicide, as well.
Applying a pre-emergent now will give you early-season control of summer annual weeds. Herbicide degradation slows down during cold weather, giving the pre-emergent the staying power needed to keep working through spring.
While warm temperatures combined with precipitation work to boost microbial activity, which breaks down herbicides, cool, dry conditions favor herbicide durability and maximize residual control. You’ll still need to make another application next year, but not until late spring or early summer, buying you some time at the beginning of the season.
You can apply pre-emergence herbicides anytime during the fall or winter, as long as the ground isn’t frozen and there’s no snow. While most pre-emergents that are labeled for turf will not injure ornamentals (some are, in fact, labeled also for use on ornamentals), always check the label, verify selectiveness and apply accordingly.
If you apply it before winter weeds germinate, pre-emergent can work toward controlling them, as well. In most of the United States, winter annuals (such as henbit and common chickweed) begin to germinate in early September, so it may be too late for this year, unless you’re in the lower South.
Fall is also the time when some perennial weeds germinate (such as dandelions). Your local extension agent can tell you when the most prevalent weeds in your area germinate so you can plan your applications accordingly. Pre-emergence herbicides can also battle many landscape contractors’ biggest nemesis: crabgrass. Several studies have shown that application of bensulide, dithiopyr and prodiamine between Oct. 15 and Nov. 15 each year provided crabgrass control greater than 90 percent.
And if you’re worried about how aeration will affect your herbicide application, don’t be. Research has shown that it has little effect, if any. However, if you’ve overseeded your clients’ lawns this year, you’ll probably want to hold off on applying a pre-emergence herbicide. This also holds true, if you’re planning on overseeding this spring. The herbicide will prevent the grass seeds from germinating in the same way it prevents the crabgrass from emerging.
