On its list of the best ways to kill a tree, the Virginia Cooperative Extension ranks soaking the root area with herbicide No. 7. Although some herbicides are safe to use around trees, others can damage or even kill your tree. Taking reasonable precautions ensures that you target the weeds and leave the tree unharmed.
Herbicides are formulated to kill plants and, as the Oklahoma Cooperative Extension Service points out, they don't pick and choose between species the way an insect pest or disease will. Herbicides most often damage trees when they come into contact with and are absorbed by the roots. Trees affected by herbicides have yellow, spotted or deformed leaves and a spiraling pattern of dying tissue up the trunk.
Herbicide damage causes symptoms similar to pests and diseases, poor soil conditions, improper use of fertilizers, exposure to road salt and drought stress. The Purdue University Extension recommends ruling out those possibilities before considering herbicide injury. Look also at nearby plants in the landscape for damage, since herbicides rarely cause injury to a single plant while leaving others unharmed. Although special soil tests can determine the presence of herbicide, they are expensive, and the Oklahoma Cooperative Extension Service recommends taking a soil sample near the tree and trying to germinate easy-to-grow seeds, such as tomato seeds, in the soil. If seeds won't germinate or seedlings quickly die, herbicide damage is likely.
Typically, the first signs of herbicide damage appear within days of applying the chemical, although some symptoms may take a few weeks to appear. A notable exception occurs when tree roots grow into soil that has been treated with a sterilant. Because these chemicals can persist in the soil up to two years, tree damage may occur years after applying a sterilant or bringing in sterilant-contaminated soil.
Healthy trees can better resist herbicide damage, so if you conclude that your tree has been injured by herbicide, pamper it by providing fertilizer and plenty of water. The likelihood of recovery depends on the chemical used, the amount applied, and the time of year.
Preventing herbicide injury is far easier than correcting it. Always read the product label, and use herbicides only as indicated, paying particular attention to whether the product will harm trees. Tree roots extend well beyond the tree's canopy, so avoiding the area immediately under the canopy is not enough. Tree roots can also grow into soil treated with long-lasting sterilants. Apply herbicides during cool weather, and never spray during high winds that may cause the vapors to drift.