Pennsylvania: Considerations for Nov. Weed Control

Canada thistle regrowth in a fallow field. Photo: Dwight Lingenfelter, Pennsylvania State University

Canada thistle regrowth in a fallow field. Photo: Dwight Lingenfelter, Pennsylvania State University

As temperatures get colder and even a few snowflakes start to appear, some people wonder if it is too late to control weeds, especially perennials. However, with milder temperature forecasted for the upcoming week, now would be a good time to consider spraying.

In the fall, foliar applied herbicides can be effective as long as the plants are green and appear healthy. For best activity, apply herbicides when daytime temperatures are above 50°F and nighttime temperatures are above 40°F for several days during application time (don’t apply herbicides immediately after a frost).

Some research from Iowa State and Ohio State indicates the following: Many perennial and biennial weeds can still be effectively killed after a few hard frosts.

Research with quackgrass and glyphosate actually found greater translocation of the herbicide after the first frost than before frost. Plants having a prostrate growth habit such as the biennial musk or bull thistle will be more tolerant of frost since they are protected somewhat by heat released by the soil. With most plants it is possible to determine whether the foliage has been severely affected by frosts, thus scouting the field prior to application is important to ensure that active foliage is still present.

Regarding quackgrass and Canada thistle regrowth after harvest, if these weeds are greater than 8 inches in height, then an application of glyphosate may provide good control of the above and below ground plant parts. If temperatures drop below 28 degrees at night for more than 4 hours, then these plants may die and a herbicide application may not be effective. Quackgrass can handle colder temperatures than Canada thistle. If warm temperatures (greater than 65 degrees) return for several days and the plants appear to be growing, then a herbicide treatment may still be effective.

Fall is the best time to control dandelions, while both fall and early spring are good times to control winter annuals. In fallow fields, a combination of glyphosate plus 2,4-D ester is fairly effective for control of most winter annual weeds and dandelion. Application of 2,4-D alone controls many winter annual weeds, but 2,4-D will not control chickweed and is less effective on dandelion than when in tank mixture with other herbicides.

Contact herbicides (e.g., Sharpen, Gramoxone) and systemic products (glyphosate, 2,4-D, dicamba) are much less active at low temperatures. Relatively speaking, 2,4-D is slightly more active than glyphosate in cooler temperatures (<40°F); whereas dicamba tends to be more impacted by cold weather, therefore, tank mixing them improves overall control.

As we move into late November, since foliar herbicide effectiveness decreases, the inclusion of a residual herbicide may be desirable in corn or soybean rotations. If you include a residual herbicide, research over the last several years has shown that any chlorimuron-containing product (Canopy EX, Blend, etc.) is at the top of the list if soybeans will be planted next spring and simazine is one of the better products for fields going into corn. Other products that have had some success include Valor for soybean and Basis Blend for corn.

In general, 2,4-D should be tank-mixed with any residual product. Also, when applying systemic herbicides this late in the year, make sure to include adjuvant such as AMS and/or crop oil concentrate or methylated seed oil to insure adequate uptake of the herbicide. Fall herbicide treatments should be kept in the cost range of $5-15/acre, if possible. Thus, glyphosate + 2 4-D (or dicamba) can be an initial low-cost option to consider that provides control of a relatively broad spectrum of weeds.

Cereal rye cover crop issues. 

Despite wet conditions delaying harvesting, some would still like to sow a fall cover crop such as cereal rye. Keep in mind that this late in the year timing for burndown and rye planting will be very tight so if possible, you may need to forego a burndown program and immediately plant after crop harvest. Burndown herbicides for no-till small grains include dicamba, Gramoxone, glyphosate, Harmony Extra, and Sharpen. Refer to the specific product label for more application information. The legitimate use of 2,4-D for burndown in wheat and other small grains is uncertain.

None of the 2,4-D ester or amine labels specify application just prior to small grain seeding or emergence. Some research suggests a minimum delay of 7-10 days after application at rates up to 1 pint/A 2,4-D ester. Since 2,4-D burndown in small grains is ambiguous at best, if injury occurs liability rests with the consultant or applicator.

Another situation to consider — you were able to plant a pure stand of cereal rye and it’s actively growing but has broadleaf winter annual and/or perennial weeds growing in it. In this case, 2,4-D ± dicamba can be applied to control these weeds either now or in the early spring. However, if you are applying systemic herbicides with spray additives in a cereal rye cover, crop injury might occur.

Herbicide shortages and burndown considerations. 

After providing suggestions about options to control weeds in the fall, the real question may be, “will there even be certain herbicides available to spray?” Herbicide shortages and significant price increases are real matters and will continue to plague us during the fall and well into next season. Many are wondering about herbicide alternatives if products such as glyphosate, Liberty, 2,4-D, clethodim, and several others are either limited or simply not available.

Don’t be surprised if you can’t obtain both glyphosate and Liberty (or generic versions) for your spray programs next season. Some general suggestions are to reduce rates and/or only use them at certain application timings and be creative with herbicide options that are available. To conserve product, it might be necessary to not apply glyphosate in the fall but hold it until spring burndown or even wait until the crop has emerged.

Other burndown product alternatives that could be used include, Harmony Extra, Canopy EX/DF, Basis Blend, Sharpen, Reviton, metribuzin, simazine, dicamba, Elevore, clethodim, Assure II/Targa, Aim, Scythe, and several others. But keep in mind, without glyphosate in the mix weed control in general will not be as robust, especially for grass weed control and cover crop termination.

A few of my weed science colleagues have written good articles about these topics, I suggest you refer to them for additional thoughts and suggestions. They can be found at Life in a time of glyphosate scarcity – part 2 – Fall Burndown | Agronomic Crops Network ( and Herbicide Shortage – How To Plan For The 2022 Growing Season | Purdue University Pest&Crop newsletter

As always, make sure to follow the labeled guidelines about which crops can be planted after applying burndown herbicides. If you don’t have an up-to-date Penn State Agronomy Guide, now might be a good time to purchase one since times are changing and herbicide programs and alternatives are becoming more complex. The simplified herbicide effectiveness tables and comments in it can help you navigate the various herbicide options and use considerations. To purchase an Agronomy Guide, refer to this webpage  .

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