To kill a tree using herbicides, first decide what the best application for your herbicide will be. A number of herbicides are on the market, and several different methods exist for applying them to a tree. Some of them can be applied to certain areas of a tree, thus minimizing environmental damage to surrounding soil and vegetation. Others require that the entire tree be sprayed. This can be a little more complex but both methods are equally effective.
If the tree is large in diameter, the cut-surface treatment will work well. To do this, take an axe and chop into the trunk at a downward angle toward the center of the tree. Make feathered cuts around the entire circumference of the tree and immediately apply the herbicide. Dicamba is a chemical that is used with this method; apply it from late summer to late winter. If the cuts are made in the spring, heavy sap flow from the wounds will prevent the absorption of the herbicide.
Spraying the foliage with herbicide is usually used on brush up to 15 feet tall. It is highly effective and there are a variety of chemicals to choose from such as Amitrole, Dichlorprop, Fosamine, Glyphosate Metsulfuron and Triclopyr. Read the label of the selected herbicide to ensure correct application. In most cases a complete saturation of the tree is not needed. Ensure enough space between the tree being sprayed and other vegetation and soil that may be nearby.
Some herbicides require that you cut down the tree first and apply the herbicide directly to the stump's freshly cut surface. If the tree's diameter is less than 3 inches, coat the entire surface of the stump with an herbicide such as Imazapyr. If the tree has a diameter larger than 3 inches, only apply the chemical to the outer 2 to 3 inches of the stump. For best results, apply directly after cutting the tree down. This will greatly improve the absorption rate of the herbicide.
A few herbicides can be applied directly to the soil surrounding the root structure of the tree. These solutions absorb into the soil after heavy rain or ample watering and attack the roots, thus killing the tree. This method is usually adopted when there are multiple trees to be destroyed. A technique called banding/lacing or streaking is where a concentrated herbicide solution is placed in lines in the soil, above the roots of the trees. These lines or bands are spaced 2 to 4 feet apart and around the trees that you wish to be killed. A good solution for soil treatment is Hexazinone.