With dropping temperatures, many wonder if it is too late to control weeds? Not necessarily; but read below for some comments about this question.
As temperatures get colder, some people wonder if it is too late to control weeds, especially perennials. In the fall, foliar applied herbicides can be effective if 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. Do not apply herbicides immediately after a frost.
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°F at night for more than 4 hours, then these plants may die and an herbicide application may not be effective. Quackgrass can handle colder temperatures than Canada thistle. If warm temperatures (greater than 65°F) return for several days and the plants appear to be growing, then an herbicide treatment may still be effective.
By late November, foliar herbicide effectiveness decreases, and 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 for soybeans and simazine is one of the better products for 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/methylated seed oil to insure adequate uptake of the herbicide. Note that if you are applying systemic herbicides with these spray additives in a cereal rye cover, crop injury may occur.