The effect of brush killers varies depending on the ingredients. Regardless of the components, most brush killers get rid of tender vines and plants, but not all of them will be effective on small trees and woody plants. To get rid of small trees, you should use products with triclopyr or glyphosate, which are considered powerful enough to deal with such vegetation. By applying the product to the leaves of plants, the poison will reach the roots. In many cases, it is necessary to apply a brush killer more than once after several weeks to kill a tree.