Herbicide damage most often occurs when herbicide is accidentally applied to neighboring desirable plants during windy weather or on an excessively hot day. Before you attempt to fix herbicide damage, though, make sure that it is the root of your problem. If the herbicide persists in the soil, the damage can take a while to manifest and can mimic insect damage or a number of plant diseases. Check the resources section for a list of common herbicides and the types of damage that they can inflict.
Wash off any parts of the plant that have been accidentally sprayed with herbicide within 4 hours. Use a hose. If the herbicide is not washed away promptly, the damage will be irreversible.
Prune any browned foliage with a pair of sharp, disinfected pruning shears.
Dig up any soil that has been contaminated with herbicide with a trowel or shovel (depending on the size of the contaminated site) and replace it with gardening soil. Do not water after the accidental contamination. This will spread the herbicide deeper into the soil.
Prune the roots of any trees that grow in herbicide-contaminated soil. Excavate the root by digging out the soil around it. Then use an axe or a pair of lopping shears to cut the entire root. Do not prune roots that are close to the base of the tree or more than one-third of the tree's peripheral roots.
Fertilize your plant (most plants will take a 10-10-10 plant fertilizer applied according to the manufacturer's instructions, but research your plant's cultural requirements to be sure) and water it immediately after washing off the herbicide or changing its soil to help it recover quickly. Healthy plants are less susceptible to herbicide damage.