Department of Plant, Soil and Microbial Sciences Weeds

The more moderate weather conditions last winter led to extensive winter annual and perennial weed growth this spring. Winter annual weeds including chickweed, purple deadnettle, henbit, several mustard species and glyphosate-resistant (GR) horseweed (marestail) can form dense weed mats that can be difficult to control with spring burndown herbicides. These difficulties can arise from insufficient spray coverage, fluctuating spring temperatures, and timeliness of the burndown herbicide application due to uncooperative spring weather. Getting a start on next year’s weed control program with a fall herbicide application can help alleviate some of the weed management challenges that we have experienced over the past few springs.

There are a number of potential benefits from controlling winter annual and simple perennial weeds in the fall. Fall herbicide applications can prevent dense mats of winter annual weeds that can physically interfere with planting and tillage, reduces vegetation where insects may harbor, and allows for earlier planting due to increases in soil drying and warming. In addition, fall applications can prevent these weeds from producing seed, thereby decreasing the soil seed bank and help reduce future weed problems. Fall control of simple perennial weeds is much more effective than controlling these weeds in the spring. In the fall, food reserves in these perennials are being moved to the roots; when a systemic herbicide is applied, that herbicide moves with the food reserves to the roots and kills the plant. Additionally, higher rates of some translocated herbicides (i.e., 2,4-D and dicamba) can be used in the fall allowing for greater control of perennial weeds.

Which fields should I consider for fall herbicide applications?

No-till fields benefit the most from fall herbicide applications. However, in some cases reduced tillage (i.e., vertical tillage) fields may also profit from fall applications. Since thorough tillage is not a part of the management strategy for these fields, infestations of perennial weeds such as dandelion and white campion (cockle) can be an issue. Additionally, wild carrot (a biennial species) can also thrive in these environments. All of these species are best controlled when herbicides are applied in the fall. These applications will ultimately reduce populations of these weeds that can be problems for the following season’s crop.

Fields that have tremendous winter annual weed growth can also benefit from fall herbicide applications. This is especially true for earlier harvested fields where weeds are exposed to increased sunlight once the crop canopy is harvested. Controlling these weeds in the fall can also reduce issues with spring tillage, especially if tillage is delayed due to a cool wet spring.

When is the best time to make fall herbicide applications?

Fall herbicide applications are best made when weeds are still actively growing. This is when daytime temperatures are at least 50 degrees Fahrenheit. In Michigan, this is usually by mid-October. However, if daytime temperatures range from 40 to 60 F, herbicide applications can still be made but weed control maybe slower due to the cooler temperatures. Cooler temperatures slow plant growth and lower absorption and translocation of systemic herbicides, like glyphosate and 2,4-D, compared with warmer temperatures. Temperatures below 40 F for a prolonged period of time after herbicide application will reduce weed control. If a hard freeze occurs, evaluate the conditions of the weeds in your field prior to making a herbicide application.

Weeds that are damaged by frost will not be effectively managed. However, weeds vary in their sensitivity to frost; therefore, the application window is greater for some species than for others. For example, perennials such as Canada thistle and dandelion survive light frosts and can be controlled when the herbicides are applied through late-October and early-November. Additionally, some winter annuals can also tolerate frost and herbicide applications can be made once the weeds resume active growth. This is usually after multiple days with nighttime temperatures above 35 F followed by daytime temperatures of 50 F or higher.

What herbicide strategies are available for fall applications?

There are two basic strategies to fall herbicide applications: 1) apply a non-residual burndown herbicide such as glyphosate, 2,4-D, dicamba and/or Gramoxone to control emerged winter annual, biennial and perennial weeds while they are still actively growing, or 2) add a herbicide(s) with soil residual activity to the fall burndown herbicide to control additional winter annual weeds before they germinate. These applications may also suppress emergence of early spring germinating weeds. However, there are some pros and cons to each one of these strategies that are discussed below.

Non-residual fall burndown herbicides. Glyphosate is one of the most commonly used fall-applied herbicides for control of winter annual, biennial and perennial weeds. Glyphosate is often tank-mixed with other non-residual or residual herbicides. In general, glyphosate at 0.75 lb ae/A (20 fl oz/A of Roundup PowerMax 3) will consistently control most winter annual weed species with the exception of fall emerging glyphosate-resistant (GR) horseweed (marestail). Adding one of the other non-residual herbicides like 2,4-D or dicamba to glyphosate will control GR horseweed.

Sharpen is another herbicide that can be added to glyphosate to control horseweed. The inclusion of Sharpen at 1 or 2 fl oz/A + methylated seed oil will provide excellent GR horseweed control. The higher rate of Sharpen can provide some residual control. Higher rates of glyphosate (1.5 lb ae/A) are needed to control most biennial (wild carrot) and perennial weeds (dandelion and white campion). The addition of 2,4-D or dicamba to glyphosate can also enhance control of some perennial weeds, especially dandelion.

Since glyphosate and Gramoxone have no residual activity and dicamba and 2,4-D have very little, one of the potential benefits and to these applications is that there is very little concern with the potential for herbicide residues from fall applications to impact crops planted the following spring. This may also be viewed as a negative, since there will be no control of weeds that emerge after the fall herbicide application. If some residual weed control is desired the inclusion of herbicides that have soil residual activity can be included in the fall burndown application.

Herbicides with soil-residual activity applied alone or with non-residual fall burndown herbicides. Many herbicides that have soil residual activity also have some postemergence burndown activity. For example, herbicide premixtures that contain the active ingredient chlorimuron (Authority XL, Fierce XLT, Synchrony XP, etc.) can provide both foliar burndown activity and residual weed control. However, control of certain species can be missed even with the inclusion of an appropriate adjuvant. Therefore, these herbicides are often applied with one of the more common non-residual burndown herbicides like glyphosate. Additionally, herbicides that have residual activity often reduce the flexibility for which crops can be planted the following season. For example, if Authority XL is applied in the fall, only soybean can be planted the following spring.

It will be important to examine the crop rotation restrictions of fall-applied herbicides. Also, consider that with certain herbicides (i.e., Sharpen) the months that the ground is frozen does not count toward the months needed for the rotation restriction. Additionally, even if a residual herbicide is applied in the fall, there is no guarantee that another burndown application will not be needed in the spring and there are maximum amounts of herbicide active ingredients that can be applied to a crop that include both fall and spring applications.

For more information on what herbicides can be used in the fall and what weeds are controlled, refer to Tables 2A and 2F in the 2024 Weed Control for Field Crops from Michigan State University Extension. Additionally, refer to Table 12 and the label for more information on crop rotation restrictions.

This article was originally published in Michigan Soybean News Fall 2024 (Volume 16, Issue 3).