May 21, 2004 at 8:19 a.m.
Using treated landscape timbers in vegetable gardens has been a controversial subject for many years. Some of the concern comes from a preservative to treat pine. The chemical is chromate copper arsenate or CCA.
There hasn’t been a lot of research pointing to dangerous levels of CCA leaching from landscape timbers. Despite this lack of evidence, many gardeners have voiced concerns about growing vegetables near CCA treated wood.
The most common wood used is pine. If the wood is not treated, it breaks down after only two or three years of contact with moist soil. CCA treated timbers probably won’t begin to break down for 10 to 12 years or more. They are 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. One of the easiest materials to use is 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, they’re far more costly, they are still the choice for people who think the added cost is worth the peace of mind.
A study on treated wood was conducted over a two year period by the University of Minnesota. The study was under the auspices of the Department of Soil, Water and Climate. They looked at six raised-beds 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 once inch from the wood. They also tested the soil about five feet away from the bed for comparison. The study determined that the highest levels of arsenic were found within an inch of the wood. The concentration declined the farther away from the wood the soil was tested.
Arsenic levels found in the soil are only part of the story. Home gardeners are concerned how much arsenic their vegetables will absorb and whether those vegetables will be safe to eat. Soil was removed from the two sites with the highest levels of arsenic. One batch was dug within an inch from the wood and another five feet away. Then seeds were planted in both batches. After eight weeks in the green house, the plants were harvested and tested for arsenic levels. The vegetables tested included carrots, beans, spinach and buckwheat. Buckwheat was added because it is known to accumulate phosphate, a chemical analog of arsenate. However, it did not accumulate much arsenic. In all cases, the vegetables growing in soil taken closer to the treated wood than soil farther away. However, the edible parts of these vegetables consistently tested far lower than arsenic consumptions standards set by the U.S. Public Health Service which is 2,600 parts of per billion. It even fell well below the more stringent standards of 1,000 parts per billion set in Canada.
Some suggest that you shouldn’t plant vegetables within 15 inches of treated wood. Consider edging a bed with an ornamental flowering annual such as sweet alyssum or dwarf marigolds. Also, buy the best grade of CCA treated wood you can. Foundation-grade treated wood has an additional heat treatment which bends the chemicals more tightly in the wood. Don’t buy timbers that have areas with chemical deposits on the surface.
Ways to access information www.extension.umn.edu/county/chisago Check out the Hot Topics box in the middle of the page for current Chisago County Master Gardener news and events.
You can also click on Ask a Master Gardener next to the cute little flower on the right hand side of the page. Here you can search 1000s of answers from Master Gardeners around the state. If you don’t find your answer you can submit a question online or search for University publications, Bell Museum of Natural History. For information about snakes, skunks, raccoons or other wildlife around your yard, call the wildlife information line at (612) 624-1374 or www.bellmuseum.org