   Managing Wood Decay - July 15, 2009 Jeff Schalau, Associate Agent, Agriculture & Natural Resources University of Arizona Cooperative Extension, Yavapai County Wood is an excellent building material for outdoor fences, decks, benches, and planters. Here in the arid Southwest, wood lasts longer than it does in humid climates. However, when wood is in contact with soil and/or moisture, fungi or termites can attack it causing it to lose structural integrity. In March 1, 2004, I wrote a column called Termite Management, which is available on the Backyard Gardener Web Site. This week’s column will focus on wood destroying fungi and steps you can take to protect wood in your home and landscape. Dry wood does not rot. Wood must contain at least 20% moisture before it will support wood decay fungi. Lumber dried properly by air or kiln, contains only 6 to 16% moisture. Therefore, unless additional moisture is allowed to penetrate lumber, decay will not occur. As a rule of thumb, untreated wood should be at least 12 to 18 inches above soil to prevent decay. Remember, wood destroying fungal spores are everywhere: in the air, soil, and water. The good news: if wood decay is detected in time, repairs that eliminate the source of moisture and allow the wood to dry usually end destructive processes. Mold or mildews are often the first sign of favorable conditions for fungal growth. They are only present when wood is above 20% moisture content and humidity is high (at least 60%). Blue-stain fungi discolor the sap wood (the outside, lighter colored area on the tree) and cause no structural damage. Brown rot fungi are the principle cause of building decay in the U.S. The rotted pieces twist and crumble into small, cubical pieces as you see in the rotted portions of pecky cedar. Dry rots can infest dry wood. These rots are characterized by large, papery, mycelial mats shaped like fans. White rots give wood a bleached appearance and causes the wood to become spongy. To investigate wood structures for fungal decay, start by looking for fungal growths such as mushrooms or large mats of mycelium (masses of fungal strands). These growths tell you that the fungus has been growing for at least a year. If the wood feels soft and “punky”, then fungal decay may also be present. An ice pick is a great tool to inspect wood for rot. Insert it about ¼ inch into the grain perpendicular to the grain, then press sharply downward. If the wood is sound, then a long splinter will pull out of the grain. Decayed wood will produce a splinter that is brittle and will break off into short, across grain pieces. To treat decay fungi, first eliminate the source of moisture (unless its dry rot). A dilute bleach spray will kill molds and mildew. If decay is extensive, replace the decayed wood. Decay resistant heartwoods such as old growth redwood or red cedar can be used in areas that tend concentrate moisture. Pressure treated lumber is also a viable alternative. Wood preservatives are useful. These can be brushed on. However, if lumber is allowed to soak in the wood preservative, it works even better. Treating the end of boards with preservatives is also recommended. Arsenic and chromium were once used to pressure treat wood. Today, less toxic compounds are used to pressure treat lumber. These include copper or borate compounds. When lumber is pressure treated, the preservative is forced into the wood at least an inch or more. Use extreme caution when handling or cutting pressure treated lumber. Wear gloves, eye protection, and a dust mask when working with these products. For more information about preserved wood products, visit the American Wood Preservers Association at www.awpa.com. The University of Arizona Cooperative Extension has publications and information on gardening and pest control. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 or E-mail us at mgardener@verdeonline.com and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site: http://ag.arizona.edu/yavapai/anr/hort/byg/. |