Fall is the best time of year to control troublesome weeds in the landscape. At this time of year perennial weeds begin moving carbohydrates from the leaves down to the roots for winter storage. If herbicides are applied now, they are transported to the roots along with the carbohydrates, killing the entire plant instead of just the leaves. And even if the chemical doesn't completely kill the weed, the plant goes into winter in a weakened condition and is much more susceptible to winter kill. Finally, the potential for 2, 4-D and dicamba herbicide drift damage to non-target species is lessened in the fall.
Fall is also the best time to control winter annual weeds, such as Henbit, Marestail and the mustards (field pennycress, Shepherds purse, Tansy mustard, Blue Mustard, etc.). Winter annual weeds germinate in the fall, overwinter as a small rosette of foliage, and begin growing again in early spring. They complete their life cycle and go to seed in spring or early summer. In fall they can be killed as they germinate with pre-emergent herbicides or targeted as young plants with post emergent products.
Before applying any herbicide, know the weeds you are trying to control. Get help from your local garden center or Nebraska Extension office if you’re not sure. Fall control of annual weeds like crabgrass, foxtail, knotweed or purslane is unnecessary and wasteful. These weeds only live for one summer and naturally die in fall. In fall, it’s best to focus on winter annual weeds and tough perennial weeds including musk thistle, Canada thistle, field bindweed, poison ivy, curly dock and cattails.
Weeds are opportunistic, taking advantage of thin areas in a lawn to grow and thrive. So the best way to prevent future weed problems is to overseed and thicken lawn turfgrass stands. The best time for overseeding Kentucky bluegrass and tall fescue is Aug. 15 to Sept. 15. With good soil preparation, seed-soil contact and adequate water, young grass plants can grow well even while weeds are present. For more information on overseeding your lawn, refer to Establishing Lawns from Seed, http://turf.unl.edu/NebGuides/Establishingturffromseed2012l.pdf.
Be very careful when using herbicides on new grass seedings; choose herbicides carefully, read and follow label directions.
When only a few weeds are present, they can be pulled or dug up by hand, but if root sections remain for difficult weeds like dandelion or bindweed, the weed can regenerate and grow again next year. So with difficult weeds, herbicides can more effectively translocate into the roots killing them.
Many products are available for broadleaf weed control. Look for products containing the active ingredients 2,4-D, carfentrazone, sulfentrazone, quinclorac or triclopyr. These products are selective and won't damage grass, but use them with caution in landscape beds since accidental spraying or spray drift can damage shrubs and ornamental plants. Quinclorac is particularly effective at controlling wild violets.
Consider adding a spreader-sticker to your tank mix for even better control. Spreader-stickers are additives that help the chemical you apply spread over the weed leaf surface and adhere to it better. When you're trying to control weeds with waxy leaf surfaces, like wild violets, a spreader-sticker is particularly helpful. Look for products like Earl May's Turbo Spreader Sticker.
When targeting difficult weeds, don't expect 100% control with one herbicide application. Two or three herbicide applications, 2-3 weeks apart will usually be necessary to control them. After making the herbicide application, don't mow for 2-3 days to allow the plants to take in the chemical.
Winter annual weeds
If winter annual weeds, such as henbit, chickweed, and annual bluegrass, are a problem in your buffalograss lawn, a fall pre-emergent herbicide application should be made in early September.
Barricade (prodiamine), Dimension (dithiopyr) and Pendulum (pendimethalin) are commonly available home-use pre-emergent products labeled for use in turf or landscape plantings. Check the product’s active ingredient statement on the front of the label for these chemicals to make sure you are using the correct herbicide.
If you miss the application of pre-emergent, then a post emergent application in mid-October will kill many newly germinated winter annual weeds and prevent them from becoming a problem next spring.