Farms utilize a lot of wood products on a daily basis. Proper selection and use of wood will help to insure its proper service life and reduce the necessity for replacement. Any wood product exposed to the weather should be properly treated to prevent insect infestation and decay. Pressure treated wood will provide the longest service life, but there are many local species of wood that will provide several years of good service.

Why does one need to use pressure treated or naturally decay resistant wood? Decay occurs when organisms such as fungi use wood as a food source. Fungi require several conditions to survive. They must have water, the right temperature, oxygen, and food. Common methods of reducing fungi are to keep wood dry; however, many applications require that the wood be exposed to the elements. In these situations, the best way avoid fungi infestation is to poison the food source. This can be done using treated wood, or wood that has extractives in the heartwood, which are toxic to fungi.

Pressure treated lumber is the best choice to use when wood is to be exposed to weather in use. When selecting pressure treated material it is important to use the proper treatment for the job. The most common preservative used in treated wood is Chromated Copper Arsenate (CCA). The amount of preservative in the wood after treatment is called the retention level. Wood that will be exposed to the elements, but is not in contact with the ground should have a 0.25 (lb/ft³) retention. Wood in ground contact will require a minimum retention of 0.40 (lb/ft³). The preservative retention levels are printed on the quality mark tag that is stapled to the end of the board.

Often trees grown around the farm can be used and although not treated, many of these species will provide several years of good service. There are still several species commonly available that provide excellent resistance to decay and should last several years. Listed below are some species that have heartwood resistance to decay.

What is heartwood? The list of decay resistant woods is for the heartwood of the species only. Trees contain both heartwood and sapwood. The heartwood is the darker center section of cells that are no longer living, but provide mechanical support to the tree. The heartwood contains extractives, which are toxic to most decay fungi and some insects. Extractives are made up of many different substances including polyphenols, waxes, oils, resins, gums, tannins, and aromatic and coloring agents. The extractives are what give the heartwood of certain species distinct color and odor. For example, the extractives in the heartwood of red cedar give it the aroma commonly desired in blanket chests and closets. Heartwood is located from the center of the tree outward. The decay resistance of the heartwood of a particular species may vary greatly depending on the growth site of the tree and the size and age of the tree.

The sapwood is located close to the bark of the tree and is a lighter colored tissue that transports water and nutrients in a living tree. The sapwood of all native tree species is susceptible to decay. When using decay resistant wood, care must be taken in selecting material with heartwood only.

Brian Bond

Assistant Professor