Questions occasionally arise regarding using preserved wood in the construction of garden beds. Preserved wood is an excellent choice for many gardeners for raised and garden beds due to its value proposition and longevity. While redwood is an excellent untreated alternative, preserved wood is acceptable and approved for use in these applications. The following resources and information may be helpful for those trying to choose between redwood or another untreated natural wood species and preserved wood.
Garden Beds and Preserved Wood
Research Study from Oregon State University
Researchers at Oregon State University conducted a study to better understand impacts to the soil from using preserved wood in garden beds. Conducted in the Willamette Valley near the university, a western wood species, in this case Douglas-fir, was used in the study. Four home garden beds were built, two with untreated Douglas-fir and two with Douglas-fir lumber that was preservative treated with copper azole (CA-C) to ground contact specifications. The beds were filled with native garden soil amended with compost and irrigated as needed using drip irrigation. All the beds were planted with the same vegetables and the study has been conducted over two growing seasons to measure results.
The study results have found that garden beds constructed from preserved wood do increase soil copper concentrations, but only within 1-inch of the bed edge. The increase in copper was small and there was no excess copper found in other locations tested (3-4 inches from the edge or from the center of the bed) in either the treated or untreated beds. This means that the copper increases were limited to soils in direct contact with the wood. Further, the study showed no increase in copper concentration in the vegetables grown in the beds.
Gardeners wishing to use preserved wood in raised or garden beds should take care to only use lumber treated with preservatives approved for residential use, such as copper azole (CA-C), and to make sure the lumber is treated for “ground contact” applications to ensure good performance.
You may find additional information about the study at the Oregon State University Extension Service website, Pressure-treated wood for raised bed construction in the Willamette Valley .
Study Summary from the Western Wood Preservers Institute
The Western Wood Preservers Institute (WWPI) has summarized the same study from Oregon State University and provided a helpful handout, Safe garden boxes using preserved wood.
