Herbicides for killing trees come in different formulations. First, accurately identify the tree species so you can select a product capable of killing it. Next, understand how the herbicide affects the surrounding environment. If the tree you intend to take out lives on a riverbank, for example, the chemical runoff could contaminate the water. Before tackling this task, talk to a nursery or cooperative extension staff member, and carefully follow the instructions on the product label.
Insert the ax blade into the tree trunk and move it downward to peel back the bark. Leave the bark piece attached at the bottom end of the cut. Move to the side and repeat this step. Continue to slice bark from the trunk until you have completed a circle around it. Treat the entire cut area with herbicide at the rate listed on the label. Apply the chemical right after you are finished with the ax. Use this elimination method on trees with trunk diameters wider than 5 inches, recommends the Washington State University Extension.
Spray the bottom 12 inches of the tree trunk, using an herbicide mixed with penetrant oil at the rate listed on the label. Use this technique on trunks thinner than 6 inches. Cover the entire circumference with the product. The dormant season is the best time for this treatment, according to the University of Florida IFAS Extension.
Apply herbicide to the foliage of trees shorter than 8 feet. Spray the leaves in early spring with triclpyr. To kill the plant in late summer or during fall, use a formula containing imazapyr or glyphosate. This method requires completely covering all leaves with herbicide without generating chemical runoff. Add a tracer dye to the sprayer's tank to mark treated areas so you don't saturate them while leaving others untouched.