Borax is a mineral composed of sodium, boron, hydrogen and oxygen. 20 Mule Borax is a multipurpose cleaning product used to clean laundry and dishes. It is often recommended as a way to kill ants and silverfish in the home, and as a herbicide in the garden.
Boron is necessary for plant growth but only in minuscule quantities. An excessive amount is toxic to plants, and stays in the soil for a period of time. Applied repeatedly to an area or at too high a rate, it will render the area toxic; nothing will grow until the boron leaches out of the soil, which generally takes several years.
Borax May Kill Trees
According to the U.S. Forest Service, borax spilled on or applied to cropland or growing plants may kill or seriously hinder growth. Tree death occurs at high concentrations, especially during the growing phase. It is unlikely that once a tree is established, borax application will kill it, but the tree may die if borax is continually applied.
Borax Kills Tree Stumps
Borax is used to rid forests of fungus. Once an infected tree is cut down, borax is applied directly to the stump to eradicate the fungus and prevent it from infecting other trees, saving most of the forest stand.
Use of Borax in the Garden
A small amount of borax in your garden can be a useful pesticide, but keep in mind that continued application will decrease the quality of your soil and may prevent plant growth for several years.