What Are the Treatments for Wood Going Into the Ground?

How Treatments Work

• Wood-destroying organisms need warmth, oxygen, moisture and food from the cellulose and sugars naturally found in wood. Wood preservative chemicals poison the wood to kill insects and fungi that normally would use the wood for food and shelter. Certain wood types, such as the heartwood of redwood or red cedar, contain natural compounds that resist invasion by insects and fungi. But for wood that will be going in the ground, lumber properly treated with preservatives will perform as well or better than naturally resistant wood, and at much lower cost.

Pressure Treatment

• Most preserved wood rated for full ground contact has been pressure treated. Seasoned wood is placed in a large pressure chamber with the water-based preservative solution. The chamber is pressurized to force the preservative into the wood fibers. After treatment, the wood is air dried or kiln dried to a moisture content of less than 19 percent. The redrying also completes chemical reactions that bond the preservative chemical to the wood so it doesn’t leach out into the soil. Pressure treatment for full ground contact typically gives a service life of up to 30 years. Lumber rated for in-ground use will be labeled as such.

Chemicals Used

• Wood rated for full ground contact typically is treated with chromated copper arsenate. This water-based compound of elemental copper, chromium and arsenic is commonly abbreviated as CCA. This nonleachable compound kills insects and fungi and gives pressure-treated wood its characteristic greenish color. Wood rated for full ground contact contains 0.4 lbs. of CCA per cubic foot of lumber. Some lumberyards in the western U.S. and Canada use copper arsenate dissolved in ammonia, abbreviated as ACA. The ammonia evaporates quickly, leaving the copper arsenate bonded to the wood. CCA and ACA are not sold to the general public. You need to be a licensed pest control operator to buy them.

Other Treatments

• Two oil-based chemical treatments may be used on timbers destined for outdoor landscaping. They are not suitable for decks or other outdoor uses where wood will come into frequent human contact, and they aren’t sold to the general public. Creosote is an old-time preservative. It’s a brownish-black oil distilled from coal tar or crude petroleum. It turns wood brownish black and has a characteristic sweetly pungent smell. It is used to treat poles, pilings, railroad ties and other outdoor timbers that go on or in the ground. Another old-line oil-based treatment compound is pentachlorophenol, used because it leaves outdoor timbers with a pleasing medium-brown color and little smell. It is dissolved in organic solvents for application. These preservatives typically are applied by soaking the wood in solution for a day or two without pressurization.