No. 23 Article 2/October 7, 2011

Weed Control After Harvest

Several questions related to fall application of herbicides have been posed over the past couple of weeks. The following article appeared last year in the Bulletin, but it seemed beneficial to republish it to address some of the common questions.

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.

Winter annual weed species, such as common chickweed, can form dense populations prior to spring planting. Scout and identify which species have emerged before making a fall herbicide application.

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


Common chickweed.

Field pennycress.



The fall herbicide treatment applied to the plot on the right controlled most of the common chickweed, but common lambsquarters emerged much sooner when the common chickweed was controlled. The plot on the left was not treated in the fall.

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).

Biennial species, including poison hemlock, overwinter in a rosette growth stage.

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.--Aaron Hager

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