Fall herbicide applications have become a relatively common practice for farmers in many areas of Illinois. Much recent interest has focused on applying herbicides after harvest to control winter annual weed species, such as common chickweed, henbit, and various mustard species. If not controlled before the onset of winter, these and other winter annual species can create dense mats of vegetation prior to spring planting. Controlling these weeds and preventing them from producing seeds are important objectives of fall herbicide applications.

Before applying fall herbicide to control winter annual species, it might be worth considering some of the following aspects of fall application:

  • Which weeds are present? Scout fields before making any application to determine what weeds are present and whether their densities warrant treatment this fall. With the wide range in precipitation across the state, emergence of some winter annuals might be reduced in dry areas compared with areas where precipitation was higher in late summer and fall. Five common species are pictured below to help with identification. 
  • Which product(s) are you considering? Many herbicides used before or after crop planting and 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. Be sure to check the label of every product you are considering to determine if fall application is allowed.
  • When do you want to apply the herbicide(s)? Some farmers are close to completing harvest of the 2011 crop and may be considering applying a fall herbicide very soon. Keep in mind that some herbicides approved for fall application have timing restrictions. For example, the Dual II Magnum label indicates that fall applications should be made after October 31 and when the sustained soil temperature at the 4-inch depth is less than 55 degrees and falling. If you are considering a treatment without much soil-residual activity (for example, 2,4-D or glyphosate), time the application to be made after most winter annual species have emerged. Instead of early October, an application in mid- to late October might provide better results. If, on the other hand, your fall application will include a herbicide with soil-residual activity, the application could be made sooner.
  • Combinations of herbicides can broaden the weed control spectrum. This can be very important if winter annuals have 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 (discussed later). Be sure to include the appropriate spray additives with all applications.
  • Location in the state can influence fall herbicide applications, which seem to "fit" better in areas of central and southern Illinois, perhaps because of generally milder average winter temperatures the farther south one ventures (contributing to better winter survival of fall-emerged weeds), as well as earlier resumption of weed growth in the spring. Also, herbicide labels may indicate that fall applications can be made only in certain geographical regions of the state.
  • Fall applications that include soil-residual herbicides may not always result in a clean field by planting time. Delays in spring fieldwork may allow fields to green-up before the crop can be planted. We have occasionally observed that if the suite of winter annual weed species is successfully controlled, summer annual species (such as common lambsquarters and smartweed) have emerged sooner than if winter annuals were still present. (See below for an example.)
  • Fall herbicide application is not suggested as a way to provide residual control of summer annual weed species, such as waterhemp. Control of summer annual species is improved when applications of soil-residual herbicides are made closer to planting rather than several weeks (or months) before. If a soil-residual herbicide will be part of your fall application, we suggest using a rate that will control winter annuals through the remainder of 2011, and we recommend not increasing the rate in hopes of controlling summer annual species next spring.
  • Horseweed (Conyza canadensis) has become a challenging broadleaf weed in minimum-till and no-till cropping systems across much of the southern half of Illinois. Horseweed completes its life cycle in one year, but unlike many other annual species, it may exist as a winter or a summer annual. Populations of winter annual horseweed typically emerge in the fall, within a few days or weeks after seed is dispersed from the parent plant. In northern Illinois, most horseweed demonstrates a winter annual life cycle, whereas a substantially higher proportion of spring emergence occurs in areas south of (approximately) Interstate 70. Both winter and summer annual life cycles can be found across central Illinois. With the increasing prevalence of horseweed, 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 application in fall provides an opportunity to use higher application rates of products (such as 2,4-D) than are feasible in spring.

Fall months may offer a good opportunity to apply herbicides for improved control of certain biennial and perennial weed species as well as winter annual species. Biennial and perennial species often become established in reduced-till or no-till fields and can be difficult to control with herbicides once populations are established.

Biennials are species that complete their life cycle over two seasons. In the first year of growth, they form a rosette of leaves (a dense cluster growing close to the ground), whose size can vary greatly in diameter by species. The rosette represents the overwintering stage of the biennial. Sometime the following spring, the biennial plant produces a flowering stalk (it bolts) that branches and gives rise to flowers and seed production. Once bolting has initiated, biennial species can be increasingly difficult to control with herbicides. Control of biennial species that remain green into the fall months after their first season of growth, such as wild carrot and poison hemlock, can be substantially improved with fall herbicide applications as compared with spring applications. For the most effective control, consider using herbicides that translocate within the plant following absorption (such as glyphosate and 2,4-D).

Perennial weed species can be difficult to control because they store substantial food reserves in their root systems. Controlling the aboveground part of perennial species is usually not sufficient to achieve satisfactory, long-term control; the root system must be controlled as well. Translocated herbicides are usually the most effective chemical options to control perennial weed species, but the time of year they are applied can influence the control achieved.

In the spring, perennial species rely on stored food reserves to initiate new growth, so most food is moving upward from the roots to support new vegetative development. This movement means it is often difficult to get sufficient herbicide into the root when applications are made in the spring. Good control can be achieved when postemergence translocated herbicides are applied as food reserves are moving downward in the plant; this coincides with about the time that perennial broadleaf species begin to flower and during fall months, as day length shortens and temperatures cool.

Be sure to apply herbicides while the target perennial species still have ample viable leaf surface area. Warm-season species, such as hemp dogbane and common pokeweed, typically lose their leaves after the first frost; treat these types of perennials before the first fall frost. Cool-season species, such as dandelion and Canada thistle, often survive one or more frosts before losing their leaves; translocated herbicides can be more effective on these types of perennials if applied after a light frost. Before making any herbicide application, take the time to scout fields to determine which perennial species are present and to confirm that the plants are still actively growing.