Considering Herbicide Efficacy in Unfavorable Weather Conditions

Weather is an important factor to consider when making herbicide applications. Too little moisture means that herbicides won’t activate properly. Too much rain means you can’t get a sprayer out in the field.

Fortunately, there are ways you can manage your herbicide use while factoring in the impact of weather on herbicide efficacy.

Rainfall: A love-hate relationship

Soil-applied pre-emergence herbicides at the beginning of the growing season require rainfall for activation. This turns the herbicide into solutions that can be taken up by the weeds.

“We want conditions where the herbicide is activated and in the soil solution, but you don’t want too much water which causes the herbicide to get diluted or wash away,” says Bill Johnson, Ph.D., professor of weed science at Purdue University.

Water plays a part in post-emergence herbicide as well. Rainfall soon after application can wash some of the herbicide off the plant before it has time to be absorbed. Farmers will have to determine if another application will be necessary and at what rate.

“If a significant chance of rain is in the forecast, you may want to wait to apply the herbicide until a few days after the rain,” says Alan York, Ph.D., professor of crop science at North Carolina State University. “However, if no rain is in the forecast, you should check and see if the weeds appear to be growing. If the weeds are growing, treatment as soon as possible may be the best alternative.”

“Consult labels, but absorption is typically most rapid shortly after application and the rate of absorption decreases over time,” says York. “For example, if the label specifies a four-hour rain-free period and you receive rainfall two hours after application, chances are good that well over half of the total absorption has already occurred.”

Dew can also affect herbicide efficacy by diluting the herbicide or even causing the herbicide to run off the weeds, making the herbicide less effective.

Drought considerations

Weeds growing under drought stress often form a thicker cuticle (waxy layer) on the leaf surface in an effort to reduce their water loss. This can be a major barrier to herbicide absorption.

“If you spray drought-stressed weeds, good application becomes even more critical,” says York. “Put the full rate, include adjuvants as recommended on the label and get good spray coverage. An increase in spray volume may be in order. Try to spray in the morning hours while the weeds are still perked up.”

The spread of herbicide resistance makes careful and effective weed management strategies a must. Weather is often unpredictable, but keeping an eye on the weekly forecast can help avoid ineffective herbicide applications and costly subsequent re-applications. For more herbicide efficacy considerations and other weed management resources, visit