Can Weeds Still Be Controlled This Late in the Season?

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. In the fall, foliar-applied herbicides can be effective as long as the plants are green and appear healthy. However, we are getting to a point in the season that the consistently cold temperatures are limiting active growth of weeds. Once this occurs, herbicides are not thoroughly taken into the plant for an effective kill. 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 that 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° 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, then a 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 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.

Also, we are getting more calls about roughstalk bluegrass in wheat. Most of the wheat is growing well but the bluegrass has broken dormancy and is developing alongside it. Once it is out of the ground, products such Osprey, Axial, and PowerFlex (and their related product lines) will provide control/suppression of it. Fall and early spring are the best times to apply effective herbicides. Osprey provides the most consistent control and generally works better on this weed in the fall, however, early spring applications have proven effective too. Keep in mind that these herbicides should be applied to roughstalk bluegrass during its 1-leaf to 2-tiller growth stage range. In the spray mixture, make sure to include all the required adjuvants that are specified on the herbicide label. However, due to cold weather, it is likely getting to be too late to apply these products for effective control of roughstalk bluegrass. In most cases, it would be better to wait until early spring to make a herbicide application to control this weed and others.