Naturally Rot-Resistant Woods
By: Alex Wilson
Twenty years ago, when I bought my present house in southern Vermont, one of the first things I did was put in raspberry beds and a sizable garden. To support the berry canes and fence the garden off from deer, I cut and split 8-foot posts of black locust, a local tree that I knew to be rot-resistant. In fact, old-timers told me that locust posts would last 60 years in the ground.
Although I'm only a third of the way into that 60-year life span, the posts are indeed holding up well. I had reason to pull up a few of them recently, and the heartwood is still rock-solid. By comparison, if I had used 4-by-4 southern pine posts pressure-treated with CCA (chromated copper arsenate), it is unlikely that they would have held up even this long.
Understanding Decay Resistance
Woods from different tree species vary greatly in their resistance to decay. Most offer relatively low resistance, while a few hold up quite well. What makes some woods able to resist decay?
As trees grow, they face a constant battle. At the same time as they interact with the soil to extract water and minerals, they also have to fight off microorganisms that consider those trees to be food. (Rot, or decay, is caused by molds and other organisms that feed on the wood.) As a defense, some trees have evolved complex chemical compounds--the industry term is extractives--that protect the wood against decay. That's the case with the black locust in my garden. The USDA Forest Products Laboratory (FPL) in Madison, Wisconsin, the country's premier wood laboratory, classifies black locust, along with three other domestic tree species (red mulberry, osage orange, and yew) as exceptionally decay-resistant.
Some tropical woods have similar characteristics. In fact, decay resistance is more common in tropical hardwoods, because the warmer temperatures and higher moisture levels in the tropics are more conducive to decay. Among exceptionally decay-resistant tropical woods are ipe, lignumvitae, purpleheart, and old-growth teak.
Not quite as resistant as these, but still defined as resistant or very resistant, according to the FPL, are more common woods that are widely sold for outdoor use: various species of cedar, cypress, redwood, and white oak. The following two sections list domestic and tropical tree species whose wood is exceptionally resistant, resistant or very resistant, and moderately resistant.
Exceptionally resistant: black locust, red mulberry, osage orange, and Pacific yew.
Resistant or very resistant: old-growth bald cypress, catalpa, cedar (either eastern or western red cedar), black cherry, chestnut, junipers, honey locust, white oak, old-growth redwood, sassafras, and black walnut.
Moderately Resistant: second-growth bald cypress, Douglas fir, eastern larch, western larch, old-growth eastern white pine, old-growth longleaf pine, old-growth slash pine, and second-growth redwood.