Miscellaneous
Herbicide Injury
Forest Health Guide for Georgia Foresters
Written by Terry Price, Georgia Forestry Commission
Adapted for the web by the Bugwood Network
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Herbicide injury to trees often occurs along highway, railroad and utility rights-of-way. Applications of herbicides to control unwanted weedy vegetation in these areas are often wind blown over long distances. Damage may take the appearance of twisting of the foliage, stunting, curling, witches’ broom or blighting, depending upon the herbicide and concentration and tree species (Figure 150). Small trees, or very susceptible species, may be killed by
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Figure 150
photo by David J. Moorhead |
relatively small amounts, while larger trees, or more resistant species, may show only temporary effects from the chemical. In urban areas care should be exercised in the application of herbicides to lawn areas for the control of weeds.
Volatile fumes from herbicides may damage or kill many species of ornamental plants. Herbicides applied to lawn areas several times during the growing season, or in amounts greater than that specified on the label may be taken up in the roots of ornamental plants and trees causing injury or death. Many of these chemicals can also be absorbed through the bark of the tree. Label precautions should always be carefully observed.
Herbicide damage often causes concern to homeowners as it distorts leaves and causes necrotic areas in the tissues. Damage usually occurs where weed killers have been applied to lawns, and trees are injured by drift. Drift from herbicides may injure trees as much as a mile away. Other trees are injured when the herbicide is applied directly to poison ivy or kudzu climbing up the stem. In the latter case some trees may be killed outright. Herbicides should never be applied when the wind is over 5 miles per hour and never directly to a tree that is not to be killed.
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Herbicide drift to planted pine stands is quite common in agricultural areas particularly where cotton is being defoliated. Damage can be direct or can act as a precursor to insect damage. Pines weakened from cotton defoliants are often attacked by pine bark beetles. Another insect that appears to be attracted to pines affected by certain herbicides is the southern pine coneworm, Dioryctria amatella. Certain herbicides can increase resin production and resin flow in pines, which in turn attracts
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Figure 151
photo by Terry Price |
the coneworm. One documented case of herbicide induced coneworm infestation occurred when a ten-year-old loblolly pine plantation was accidentally contaminated by cotton defoliant. Approximately 30% of the trees were affected. Coneworm attacks had occurred at several branch stem junctions causing the branches to break away from the stems (Figure 151). Only the upper branches had been affected.
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