Canada thistle regrowth in a fallow field. (D. Lingenfelter, Penn State Extension)
When weather becomes cold, many often 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 the best activity, apply herbicides when daytime temperatures are above 50°F and night time temperatures are above 40°F for several days during the application time. Herbicides should not be applied 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 showed greater translocation of the herbicide after the first frost than before the frost. Plants having a prostrate growth habit such as the biennial musk or bull thistle will also 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.
If quackgrass and Canada thistle regrowth after harvest is 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° 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°) return for several days and the plants appear to be growing, an herbicide treatment may still be effective.
Fall is the best time to control dandelions, while both fall and early spring are the good times to control winter annuals. In fallow fields, a combination of glyphosate plus 2,4-D ester is somewhat 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 mixtures with other herbicides. If you have a pure stand cereal rye cover crop that has broadleaf winter annual and/or perennial weeds, 2,4-D + dicamba can be applied to control these weeds either now or in the early spring.
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. However, if you are applying systemic herbicides with these spray additives in a cereal rye cover, crop injury might occur.