Cool Weather and Burn Down Herbicides

By John Wilson, Extension Educator

The weather seems like it can’t make up its mind whether it’s early March or late May. But finally the extended weather forecast appears to suggest more consistent springlike conditions. One concern I’ve visited with several local farmers about is how the freezing temperatures will affect the performance of burndown herbicides. Unfortunately, there is no simple answer that will fit all situations because the plant response will vary depending on the weed species, the size of the weed, and the herbicides used.

Burndown herbicides will reduce weed competition with emerging crops later this spring. But another reason to control certain weeds commonly found in fields this time of year might not be quite as obvious. Pennycress and henbit are both winter annuals, meaning they started growing last fall and will complete their life cycle early this spring. Both of these weeds are alternate hosts and allow SCN reproduction to occur on their roots. By controlling these weeds, you can reduce the potential for SCN levels to increase in SCN-infested fields.

The following was taken from a news release by Bob Hartzler, extension weed specialist at Iowa State University. A statement found on most postemergence herbicide labels is ‘Apply when weeds are actively growing.’ This is by far the most important consideration in determining whether to apply a postemergence product. Most weeds that emerge in March are adapted to sub-freezing temperatures and will not be killed by frost. However, it takes time for them to recover from a frost.

A good example would be some pennycress I noticed growing along the side of my driveway. It was laying flat on the ground after the hard freezes last week. However, less than a week later, they are showing signs of recovery and new growth. I knew I couldn’t be so lucky as to have them be killed by that frost.

Performance of herbicides will be reduced if applied too soon following a frost. How long does it take to recover? Again, there is no simple answer since it depends on the weed species, severity of the frost, and weather conditions that follow the freeze. Closely monitoring weeds for evidence of new growth, such as newly emerged leaves above those damaged by a frost, is the best way to determine recovery.

Herbicides will vary in how the weather affects their performance. Roundup, or any glyphosate-based product, relies on translocation within the plant for good activity, and herbicide movement within plants is greatly slowed during cool periods. The general recommendation is to avoid glyphosate-based herbicide applications when evening temperatures fall below 40°F.

2,4-D is somewhat more consistent than glyphosate during cool periods when applied to sensitive, broadleaf weeds. The addition of 2,4-D can enhance the burndown performance in certain situations. Burndown herbicides that interfere with photosynthesis such as Paraquat are affected both by temperature and the intensity of sunlight the day of and days following application.

Weather conditions prior to and following a burndown application will have a strong influence on the performance of early spring herbicide applications. In some situations, the result will simply be a slower kill of target plants. But in other situations control failures may occur. It is best to avoid applications during periods of prolonged cool temperatures, when temperatures drop below 40°F at night or don’t get above 55°F during the day.

If applications must be made during marginal conditions, increase the rate of the herbicide and spray additives to the maximum levels allowed on the labels. This can enhance performance consistency. Adjusting the sprayer or spray volume to achieve more uniform coverage of the target plant can also reduce variability in the effectiveness of burndown herbicide applications.

For more information on spring weed control, contact you local Nebraska Extension office.

John Wilson