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Herbicide Drift vs. Freezing Temperatures  
by
Bob Hartzler

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December 16, 2005 Many trees, shrubs and other landscape plants were damaged by widespread frosts and below freezing temperatures during the first week of May. There will likely be situations where people suspect this damage was caused by herbicides applied to adjacent corn and soybean fields. Frost injury normally appears as blackening or browning of newly emerged leaves or needles. Less severely damaged leaves may simply be malformed. (See images). Injury will usually be uniform around the entire plant. Susceptibility to frost varies widely among species and age of tissue, with expanding leaves more sensitive to damage than fully expanded leaves. Frost damage appears within a day or two of the frost event. Severely damaged leaves will quickly drop from the tree. Healthy trees will generate a new set of leaves to replace the frost damaged ones.

Symptoms of herbicide injury vary widely with the specific product applied. Glyphosate and 2,4-D will primarily affect leaves that emerge following exposure. Symptoms usually will not be visible for at least a week after application. Symptoms usually involve leaf chlorosis and malformation rather than rapid necrosis. Leaf malformations involve irregular veination, strapping or cupping of leaves. Drift of atrazine may cause rapid foliar chlorosis and necrosis of leaves present at application. Indidual lesions where spray droplets contact the foliage are often visible. Recent research in Illinois has shown that drift of acetamide herbicides onto oaks at the time of bud expansion can cause the damage known as oak tatters.

The pattern and timing of appearance of injury symptoms should allow determination of the source of injury. Plants in good condition should not suffer long-term damage from the freezing injury. The long-term impact of herbicide drift varies with herbicide, quantity of drift, and sensitivity of species. The majority of drift events involve a relatively low dose of herbicide and do not threaten the future health of the plant.

Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University

For more information contact:
ISU Extension Agronomy
2104 Agronomy Hall
Ames, Iowa 50011-1010
Voice: (515) 294-1923
Fax: (515) 294-9985
http://www.weeds.iastate.edu
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Common chemical and trade names are used in this publication. The use of trade names is for clarity by the reader. Inclusion of a trade name does not imply endorsement of that particular brand of herbicide and exclusion does not imply nonapproval.