How long should you wait to spray weeds after a frost?

How long should you wait to spray weeds after a frost?

The answer varies, depending on weed species, weed size and the herbicides used.

Iowa had cold temperatures earlier this week and last week. How long should you wait for weeds to recover from frost or cold weather before you spray a burndown herbicide in these situations? For farmers spraying herbicide treatments on emerged weeds and cover crops, that’s been a common question this spring.

WAIT TO SPRAY: These are frost damaged giant ragweed seedlings. A wise decision would be to wait for the true leaves to emerge before you spray herbicide on the weeds, says ISU’s Bob Hartzler.

When the weather forecast appears to be favorable for field activities, farmers and custom applicators are anxious to get into the field in late March and the first half of April even if the weather is colder than usual. A concern for many farmers and applicators this spring is the effect of a widespread freeze on the performance of burndown herbicides, notes Bob Hartzler, Iowa State University Extension weed management specialist. “Unfortunately, there is no simple blanket statement that can be made to offer guidelines in this situation, since the plant response will vary depending on weed species, weed size and the herbicides used,” he says.

How long should you wait to apply postemergence herbicides?
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,” says Hartzler. “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 these events.”

Performance of herbicides will be reduced if applied too soon following a frost. How long does it take for the weeds to recover? Again, no simple answer since it depends on the weed species, severity of the frost, and weather conditions that follow the freeze. “You need to closely monitor the weeds for evidence of new growth. That’s the best way to determine their recovery,” he advises.

<caption for photo> WAIT TO SPRAY: These are frost damaged giant ragweed seedlings. A wise decision would be to wait for the true leaves to emerge before you spray herbicide on the weeds, says ISU’s Bob Hartzler.

Herbicides vary in how environment affects their performance
Glyphosate relies on translocation within plants for good activity. “And herbicide movement within plants is greatly slowed during cool periods,” Hartzler explains. “The general recommendation is to avoid glyphosate applications when evening temperatures fall below 40 degrees F.”

What about spraying 2, 4-D? “The herbicide 2, 4-D is somewhat more consistent than glyphosate during cool periods, assuming you have sensitive weeds. Thus, addition of 2, 4-D LV ester can enhance burndown performance in certain situations,” he says. “Burndown herbicides that interfere with photosynthesis, herbicides such as paraquat, are affected both by temperature and the intensity of sunlight, during the day of application and during days following the application.”

Weather prior to and following herbicide applications
Weather conditions prior to and following application have a strong influence on 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.

Hartzler says it is best to avoid applications during periods of prolonged cool temperatures. That is, cooler than 40 degrees at night; and cooler than 55 degrees during the day. If applications must be made during marginal conditions, increasing the application rates of the herbicide and spray additives to maximum levels allowed on labels can enhance the consistency of performance. Adjusting the sprayer or spray volume to achieve more uniform coverage of the target can also reduce variability in activity, he notes.

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