Green Industry Resources
Hort Update- Seasonal Information for the Green Industry
Fall is Best for Control of Tough Weeds
Weeds are usually the most visible turfgrass pest and can be a major problem for homeowners. Weeds are opportunistic, taking advantage of thin areas in your lawn to grow and thrive. They compete with desirable grasses for space, light, water, and nutrients, and detract from the appearance and function of your home lawn.
For good control, it’s critical to target weeds at their most vulnerable stage, so its very important to make herbicide applications at the right time of year. For broadleaf, perennial weeds like violets, ground ivy, thistle, and dandelions, fall is an excellent time to get them under control.
Why Fall Weed Control?
Fall is the best time to control perennial weeds, because they begin moving carbohydrates produced by photosynthesis to the roots for winter storage instead of to upper plant parts for continued growth. If herbicides are applied at this time, the chemicals are transported to the roots along with the carbohydrates, killing the entire plant instead of just the parts above the soil surface.
This process flows the opposite direction in spring and summer, when carbohydrates produced are normally sent to the leaves and upper parts of the plant. As a result, herbicide applications made during spring and summer are often less effective than fall applications.
Only a Few Weeds?
When only a few weeds are present, they can often be pulled or dug up by hand. Herbicide spot treatment, or spraying individual weeds, can save money on chemicals when only a few weeds are scattered throughout the lawn. Ready-to-use liquid products provide better coverage for spot spraying. A garden center can help homeowners choose products to suit their weed control needs.
Identify Before You Spray
Before applying any herbicide, know what weeds you are trying to control. Get help from your local garden center or UNL 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 so control is unneccesary.
Kill the Roots
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 be more effective at translocating into the roots and killing them.
Don't expect 100% control with one herbicide application, when targeting difficult weeds. 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.
Herbicide spot treatment, or spraying individual weeds, saves money on chemicals when only a few weeds are scattered throughout the lawn. Ready-to-use liquid products provide better coverage for spot spraying.
Many products are available for broadleaf weed control, including and may contain 2, 4-D, 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.
Granular post-emergent herbicides may be applied with fertilizer application twice in the fall, usually once around Labor Day and again around Halloween. Apply granular herbicides to damp grass to ensure the herbicide will stick to dandelion leaves and be absorbed.
As with all chemical applications, be sure to read and follow all directions on labels and use personal protective equipment.
Winter Annual Weeds
Many winter annual weeds, like field pennycress, shepherdspurse, chickweed and henbit, should also be targeted for controlled in the fall. Winter annual weeds germinate in fall, grow into a small rosette then overwinter and resume growth in early spring. A fall broadleaf pre-emergent application, such as Preen (trifluralin), Surflan, Pendimethalin or Dimension, should be applied at the beginning of September to prevent weed seed germination, just as it does in the spring.
Before applying any herbicide, have the weeds identified. Fall control of annual weeds like crabgrass, foxtail, knotweed or purslane is unnecessary and wasteful. These weeds will die during the winter anyway and do not germinate from seed until spring, making fall control unneccesary.