Gardening: all about landscape timbers

Using treated landscape timbers in vegetable gardens has been a controversial subject for many years. Some of the worry comes from the scary-sounding name of the chemical commonly used as a preservative to treat pine -- chromated copper arsenate, shortened to CCA in the lumber business.

There hasn't been a lot of research pointing to dangerous levels of the components of that preservative -- copper chromium or arsenic -- leaching from landscape timbers. Despite this lack of evidence, many gardeners have voiced concerns about growing vegetables near CCA-treated wood.

Why use treated timbers in the first place? Without preservative, pine breaks down after only two or three years' contact with moist soil. It's not practical to use untreated pine in the garden, even though it is inexpensive. CCA-treated timbers probably won't begin to break down for 10 to 12 years or more. They're widely available and reasonably affordable.

There are alternatives to CCA-treated pine. Bricks, stones or concrete blocks can be used to edge garden beds or build raised beds. From a do-it-yourself perspective, among the easiest materials to use are interlocking concrete landscape blocks. They are available in several soft landscape-friendly colors and come in various sizes and finishes. Once a block garden wall or raised bed is in place, it should last almost indefinitely.

If you prefer wood, untreated cedar and redwood both last years longer than untreated pine. While they do not last as long as CCA-treated timbers, and they're far more costly, they are still the choice for people who think the added cost is worth the peace of mind.

Before deciding whether to use CCA-treated wood in your garden, take a look at information gathered over the past two years at the University of Minnesota. The study, conducted under the auspices of the Department of Soil, Water and Climate, looked at six raised-bed planting sites across the metro area, each with treated landscape timbers at least 10 years old. The soil was tested for arsenic levels in several places at each site, starting about 1 inch from the wood. They also tested the soil about 5 feet away from the bed, for comparison.

The study determined that the highest levels of arsenic were found within an inch of the wood, with concentrations declining the farther away from the wood the soil was tested. By a distance of 14 or 15 inches, the amount dropped to approximately the same level as was found 5 feet away -- an amount that reflects the very small arsenic content found naturally in local soils.

As you might expect, different vegetables took in varying amounts of arsenic. The vegetables included in the test were 'Thumbelina' carrot, 'Indian Summer' spinach, 'Provider' green beans and common buckwheat. (Buckwheat was included because it is known to accumulate phosphate, a chemical analog of arsenate. However, it did not accumulate much arsenic.)

Arsenic levels were tested in the carrots before they were peeled as well as after peeling. The peels were also tested. Bean pods were tested, as were leaves and stems from the bean plants. In the case of spinach, the portion of the plant above ground -- basically the leaves -- was tested.

In all instances, the vegetables growing in soil taken 0 to 1 inch from the treated wood had higher levels of arsenic than those growing in soil taken 5 feet away, which confirms that some uptake does occur. The edible parts of these vegetables, however, consistently tested far lower than arsenic consumption standards set by the U.S. Public Health Service (2,600 parts per billion) and even fell well below the more stringent standards (1,000 parts per billion) set in Canada.

Carl Rosen, one of several soil scientists supervising this project, suggests that to be prudent, gardeners should grow vegetables about 15 inches away from CCA-treated timbers. (Consider edging a bed with an ornamental flowering annual such as sweet alyssum or dwarf marigolds, instead of growing vegetables right up to the wood.)

If you are growing edible plants with extensive root systems, Rosen suggests the plants would benefit from placing a plastic barrier 1 foot from the wood, down to a depth of 6 inches. This should keep roots from growing close to the wood, where arsenic levels are higher. Or you could line the insides of the treated wood with plastic when you build a new bed or replace the soil in an existing one.

For more information about this study, go to the Yard & Garden Web site at http://www.extension.umn.edu/projects/yardandgarden/. Scroll down the right side of the page, then click on Yard & Garden Line News archives for 2001. The study is in the June 1 issue.