Gardening Articles: Landscaping :: Yard & Garden Planning

Wood Preservatives

by Alex Wilson

The four most common wood preservatives, left to right: CCA, CDDC, ACQ, and copper naphthenate.

For good reasons, wood is a highly popular material for outdoor projects. It's easy to cut. You can nail it. It's not too heavy to lug around. It comes in all sizes and shapes. And it looks good. The problem is that wood is also biodegradable. It's a food source for fungi, bacteria, insects, and other organisms. If we want to use wood outdoors--as fencing, sides for raised beds, retaining walls, or decking--we have to deal with these very real problems.

The most common option for controlling decay is to use wood treated with chemicals that make it more resistant. Most untreated wood in contact with the ground will last just one to four years, while preservatives can extend that life to several decades--or even longer--reducing pressure on our forest resources. It is estimated that use of pressure-treated wood saves a quarter of a million trees from harvest each year. Furthermore, pressure-treated wood comes from fast-growing trees on tree farms, not from very old or slow-growing trees.

This article addresses these preservative options. Other articles in this series will look at some alternatives to preservative treatment, including using woods naturally resistant to decay, and "lumber" made from recycled plastic.

Understanding preservatives

There's no getting around the fact that preservatives are designed to kill organisms. When we choose preservatives, we need to find the right balance between protecting wood from decay and protecting our own health and the environment from chemicals. The earliest wood preservatives--salts of lead and arsenic--were plenty toxic to the problem organisms, but failed miserably when it came to being safe for humans and the environment.

Creosote, patented in 1831, was the first wood preservative to successfully protect wood from ground contact and high moisture. It is distilled from coal tar (a by-product of making coke from bituminous coal) and is toxic to fungi and most other decay and wood-boring organisms. Because it is oil-based, it stays mostly in the wood, rather than leaching out. By the 1920s it had become the treatment of choice for railroad ties. Unfortunately, creosote is smelly, ugly, unpaintable, and toxic to some plants. Furthermore, it is now classed as a known carcinogen.

Pentachlorophenol (penta), developed in the 1930s, is another preservative equally out of favor. It was the first synthesized pesticide and was widely used until the 1980s. Like creosote, it is an oil-based preservative. The state of California now recognizes it as a carcinogen, and studies show that it becomes concentrated in organisms on the food chain such as fish and birds of prey. Since 1986, use of both creosote and penta has been restricted to certified applicators only.

Today, preservative manufacturers are pursuing two different strategies for making their products safer. First, they are finding chemicals that are highly specialized--targeted just toward fungi, for example. Most of the surface-applied preservatives sold today include copper or zinc compounds, which are quite toxic to fungi yet relatively safe for humans.

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