Winter annual weeds painted the landscape with brilliant purples and yellows last spring. While the colorful display provided a cheery exit from winter’s drab, the weedy patches were less than ideal for a clean spring start.
Ohio State University weed specialist Mark Loux says fall herbicide applications are a good idea for control of existing infestations of winter annuals or marestail and volunteer wheat. Biennials like wild carrot or poison hemlock or cool-season perennials such as dandelion, quack grass and Canada thistle are also controlled by fall herbicide applications.
Some of these weed species can become established in reduced or no-tillage fields and can be difficult to control with herbicides once the populations are established.
Loux says a late harvest in certain parts of the country may make fall herbicide applications tougher to accomplish this year. Mid-to-late October is typically ideal for fall burndown. “In our research, herbicides seem to be effective for control of winter annuals and biennials well into December,” Loux says. “The rate of plant death can slow considerably, but this is not a problem since weeds just have to die by early spring.
“Control of perennials typically declines in late November or early December, depending on weather.”
“The big picture on fall applications is that not every field needs one,” says Loux.
University of Illinois weed specialist Aaron Hager says fall herbicide application can save you time next spring. However, he agrees with Loux—every weed patch discovered during harvest won’t benefit from a fall application. For example spraying waterhemp that has already set seed won’t do a thing to help for next year, he notes.
However, fall herbicide application is considered an essential component of an effective marestail management program—although Loux says that’s not where the majority of the marestail control dollar should be spent. “Fall treatments should not substitute for a comprehensive, in-season herbicide program,” he says.
The volunteer corn currently sprouting isn’t a concern. “Winter is a great control for volunteer corn,” Hager adds. “It’s better for it to emerge now, than next spring.”
Before making a fall herbicide application, Hager offers a few reminders:
- Scout fields before making any application to determine what weeds are present and if their densities are high enough to warrant treatment.
- Many herbicides used prior to or after crop planting/emergence can be applied in the fall, but not all herbicides are labeled for fall application. Atrazine, for example, is widely used before and after corn emergence, but is not labeled for fall application. Check the label to determine if fall application is allowed.
- Some herbicides approved for fall application have application timing restrictions on their labels. If you are considering applying a treatment (such as glyphosate) that does not possess much soil-residual activity, the application should be timed to occur after the majority of winter annual species have emerged. Instead of applying such a treatment in early-October, a mid- to late October application timing might provide better results. If your fall application will include an herbicide with soil-residual activity, then the application could be made sooner, but check the product label.
- Combinations of one or more herbicides can broaden the weed control spectrum. This can be very important if winter annuals have already emerged before the application is made. Combining 2,4-D and/or glyphosate with soil-residual products can improve control of emerged species and help control biennial or perennial species. Include the appropriate spray additives with all applications.
- Location can influence fall herbicide applications. Fall herbicide applications seem to “fit” better in areas of central and southern Illinois. Check the label for geographical restrictions.
- Fall applications that include soil-residual herbicides may not always result in a clean field by planting time next spring. Delays in spring fieldwork may allow the fields to green up before the crop can be planted. Occasionally, if the suite of winter annual weed species is adequately controlled, the emergence of summer annual weed species may occur sooner than if winter annuals were still present.
- Do not utilize a fall herbicide application as an avenue to provide residual control of summer annual weed species. Control of summer annual species, such as waterhemp, is often improved when applications of soil-residual herbicides are made closer to planting compared with several weeks (or months) prior to planting. If a soil-residual herbicide will be part of a fall herbicide application, select an application rate that will provide control of winter annuals throughout the remainder of 2011, and do not increase the application rate in hopes of obtaining control of summer annual species next spring.
- With the increasing prevalence of horseweed (marestail), including glyphosate-resistant populations, fall herbicide applications may prove more efficacious than spring applications. Glyphosate alone may not provide adequate control when applied in either fall or spring, but a fall application timing provides an opportunity to utilize higher application rates of products (such as 2,4-D) than are feasible to use in spring.
-- Jennifer Shike contributed to this article.
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