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.
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.
Exceptionally resistant: angelique, azobe, balata, goncalo alves, greenheart, ipe (iapacho), jarrah, lignumvitae, purpleheart, and old-growth teak.
Resistant or very resistant: aftomosia (kokrodua), apamate (roble), balau, Spanish cedar, courbaril, determa, iroko, kapur, karri, kempas, American mahogany, manni, secupira, and wallaba.
Moderately Resistant: andiroba, avodire, benge, bubinga, ehie, ekop, keruing, African mahogany, dark red meranti, mersawa, sapele, second-growth teak, and tornillo.
Why is it that some pieces of redwood will last very well on an outdoor deck, while others have a much shorter life? There are two reasons. First, the sapwood (wood closest to the bark that tends to be lighter in color) of most trees, even highly decay-resistant species, offers very little decay resistance. If a piece of lumber has much sapwood, it won't last as long outdoors. As an illustration of this, when I pulled up some of those locust posts around my garden after 15 or 16 years in the ground, all of the sapwood up to ground level was totally gone--eaten away by decay--while the heartwood remained.
Second, the amount of extractives deposited in the heartwood depends on how the tree was grown--the more rapid the growth, the fewer extractives in the heartwood and the lower the decay resistance. The FPL defines old-growth bald cypress and old-growth redwood as resistant or very resistant, for example, while second-growth lumber from those species is defined as only moderately resistant.
Unfortunately, the most resistant woods are not widely available. Hardly anybody raises black locust as a timber crop, and some sawmills are loathe to mill it, fearing it will dull the saw blades. While black locust trees often grow straight and tall, lumber from this species tends to twist and check as it dries. It is not an easy wood to work with. The other domestic tree species that offer exceptional resistance tend to be small, twisted trees that offer very little commercial lumber potential--though cutting fence posts from these trees is still possible.
As for domestic woods that can be milled easily into lumber, old-growth trees are the best, but old-growth forests are almost gone. Commercially available old-growth redwood is quickly disappearing; most harvesting today is done in second-growth forests. Old-growth western red cedar is still being harvested extensively, but once this supply is gone, in another few decades or so, clear western red cedar will be much harder to come by: it does not regenerate as readily as redwood, and the species is rarely planted. Virtually all cypress harvested today is second-growth, and there hasn't been any old-growth longleaf yellow pine--a moderately resistant species--for close to a hundred years.
For many species of wood today, the only way to get highly resistant, old-growth wood is to salvage it from structures that are being torn down or from rivers and lakes where it sank a century ago. (Underwater, where there is very little oxygen, wood lasts for decades, or even centuries, with little decay.) Dozens of companies specialize in salvaged wood: redwood from wine vats in California, longleaf yellow pine from bridges, cypress from timber buildings in the Southeast. Wood is even being salvaged from a timber-crib dam currently being dismantled in Maine. You can find a list of companies that supply salvaged wood at the Web site of Environmental Building News: www.ebuild.com.
If salvaged wood isn't an option, you can get new wood from a well-managed forest by insisting on certification from the Forest Stewardship Council (FSC), an international organization that has developed standards for responsible forest management. FSC accredits companies or nonprofit organizations, which then carry out the actual forest certification and "chain-of-custody" certification, so you can be sure the wood you buy as certified actually came from a certified forest. Choosing only FSC-certified wood is especially important with tropical timber, such as teak and ipe, because many tropical forests are poorly managed. The Certified Forest Products Council, can provide you with names of companies that supply FSC-certified wood.
Using naturally resistant woods for outdoor applications makes a lot of sense. It allows us to avoid, for example, environmental risks from CCA-treated wood. Even with highly resistant wood, however, we can often take measures to increase durability. For fences and decking, for example, the USDA's FPL recommends treating the lumber with a clear, low-toxicity preservative, such as copper naphthenate or IPBC, then coating the wood with an exterior-grade, water-based finish such as paint or opaque stain. This finish forms a film that sheds water but allows water vapor to escape.
Alternatively, you can avoid wood altogether, choosing instead one of the new recycled-plastic or wood-plastic composite products on the market.
Alex Wilson is editor and publisher of Environmental Building News, a monthly newsletter about environmentally responsible design and construction. He lives in Brattleboro, Vermont.