Winter lingered on as long as it could, but spring is in full force, and so are our summer gardens. I have seen the potting soil flying off the shelves of home garden departments, as well as the containers and the mulch. Here at the Extension Office we’ve answered several questions this week on raised-bed gardens and the use of treated wood in those gardens.
I believe what most gardeners are worried about is chromated copper arsenate (CCA). The majority of the wood sold for outdoor use in the U.S. between 1975 and 2003 was treated with CCA. Studies have shown that CCA-treated lumber can leach chromium, copper and arsenic into the surrounding soil. But the migration of these elements appears to be limited to a few centimeters adjacent to the lumber and research has not clearly shown a long-term negative impact upon plants.
Because CCA-treated lumber is not readily available to homeowners anymore, there really is no reason for concern. On Feb. 12, 2002, the Environmental Protection Agency announced a voluntary decision by the industry to move away from consumer use of a variety of pressure-treated wood in favor of new alternative wood preservatives. This transition affected virtually all residential uses of wood treated with CCA, including wood used in play-structures, decks, picnic tables, landscaping timbers, residential fencing, patios and walkways/boardwalks. The remaining stock of CCA-treated lumber was sold out in most stores by the fall of 2004.
If you still have some old CCA-treated lumber around and would like to use it, here are some things that can be done to minimize leaching:
1. Scrub the wood with detergent or power washing to remove surface residues.
2. Leave boards to weather several months before they are cut, drilled and assembled. Studies show the greatest amount of leaching takes place during the first rainy season.
3. Predrill holes for screws to prevent making cracks in the wood where leaching can initiate.
4. Line the inside of the bed with heavy-duty plastic before filling it to create a physical barrier to any CCA compounds moving into the soil.
5. Paint exposed wood surfaces with paint or water repellent finish to prevent skin exposure to CCA compounds.
6. Avoid growing root crops close to CCA-treated woods.
There are many types of less-toxic alternatives to CCA-treated wood such as plastic lumber, metal, wood that is naturally resistant to insects and decay and wood that is pressure treated with less toxic ingredients.
Plastic lumber is an increasingly popular building material. Most frequently composed of high-density polyethylene (HDPE), plastic lumber does not release hazardous materials into the ground. And since it is often manufactured with recycled plastic, plastic lumber conserves natural resources. Plus, it usually requires less maintenance.
Wood and plastic combined into one lumber product is called composite lumber. Wood/plastic composites generally exhibit low moisture absorption and high resistance to decay, insect and UV ray damage. The wood component provides the composite with greater dimensional stability than plastic lumber, but not as much as wooden lumber. Like plastic-only lumber, wood/plastic composite lumber is often made with recycled materials.
Naturally decay-resistant wood such as redwood, cedar, cypress and teak contain natural preservatives, which protect it from decay.
Lumber pressure treated with non-arsenic wood preservatives is available in the marketplace. Many of these wood preservatives are copper-based such as ACQ (ammoniacal copper quaternary) compound or CA (copper azole). For additional information about alternatives to CCA-treated lumber, visit epa.gov and search “CCA,” or contact your local hardware store or lumberyard.