Raised garden beds are a popular choice for many gardeners. Raised beds can eliminate problems from poor-quality native soils, help with drainage issues and increase accessibility. The most popular choice of building material for framed raised beds is wood. It is low cost (compared to permanent building materials like concrete block) and is easily purchased at home improvement stores. However, wood is prone to decay because it is biodegradable. Raised beds often stay wet year-round due to irrigation and rain, which speeds up wood decay.
The type of wood used for raised beds makes a difference. Redwood and cedar are more expensive but resist decay, so they last longer. Pine and fir will break down more quickly but are available at a fraction of the cost. Pressure-treated wood is also an economical option for improving the life of raised garden beds. However, some gardeners have expressed concerns that the chemicals used to preserve the wood could potentially contaminate the soil and plants.
What is pressure-treated wood?
Pressure-treated wood has gone through a process in which a liquid chemical preservative is forced into the lumber and between its cells using vacuum and pressure. Pressure-treated wood has a different color and appearance from untreated wood due to the addition of the chemical preservative, most often copper-based. In some cases, especially in the western United States, a finish is applied to the outside of the wood that distinguishes these products from untreated lumber.
Wood used for industrial uses, like railroad ties or utility poles, will receive a different treatment than lumber used for home building.
All pressure-treated wood has a label or end tag that indicates its intended end use. The specific preservative used depends on the end use of the lumber. For example, wood used for industrial uses, like railroad ties or utility poles, will receive a different treatment than lumber used for home building. The most common wood preservatives used for lumber are copper azole (CA-C), micronized copper azole (MCA) and alkaline copper quaternary (ACQ). CA-C and ACQ are more commonly used in western states such as Oregon to treat Douglas-fir, whereas MCA is commonly used to treat southern pine lumber in the eastern United States.
What about arsenic warnings?
When researching this topic, you may come across warnings not to use pressure-treated wood due to the presence of arsenic. It has been over 20 years since the arsenic-containing wood preservative chromated copper arsenate (CCA) was used for wood available to the general public.
The industrial preservatives used in railroad ties are not intended for garden or residential use.
Reusing or salvaging lumber that is over 20 years old may risk introducing CCA-treated lumber into your garden. While there is no evidence that the use of CCA-treated lumber increases the risk of arsenic accumulation in vegetables, manufacturers do not recommend it for this application. Using old railroad ties for garden boxes or beds is not recommended. The industrial preservatives used in railroad ties are not intended for garden or residential use.
OSU researchers study leaching
Many gardeners are hesitant to use pressure-treated wood for raised beds. The information available through web resources is often conflicting, and much of the information available on web pages and blogs is not supported by research.
Oregon State University researchers began conducting a study in 2021 to answer two common questions:
- Do raised beds constructed from pressure treated wood leach copper into the soil?
- If leaching does happen, do the vegetables and herbs grown in the raised bed absorb the copper?
What is the role of copper in gardening?
- Copper is an essential plant micronutrient and is important in plant growth and development.
- Copper is a naturally occurring element that is present in Oregon soils. Different soil types interact with copper differently. Clay soil (like those in the Willamette Valley) and those high in organic matter can bind copper.
- Copper is also an active ingredient in some pesticides used to manage fungal and bacterial diseases — including those used for organic gardening and agriculture.
- High levels of copper can have a negative effect on plant growth and the environment.
How was the study set up?
Researchers at Oregon State University set up a study in a home garden. They constructed two raised beds out of untreated Douglas-fir lumber and two beds from Douglas-fir lumber that was pressure treated with copper azole (CA-C) to ground contact specifics (Figure 2). Each raised bed was about 4 feet wide by 10 feet long and was constructed from 2-inch x 12-inch lumber. The research site is located in the Willamette Valley.
The beds were filled with native garden soil amended with compost upon installation and annually thereafter. The beds were irrigated as needed using drip irrigation. Trellising was added to support larger plants. (The trellises were either galvanized steel or untreated wood and didn’t contain copper.)
The treated and untreated raised beds were planted with the same vegetables and herbs each year (Figures 3 and 4). Included in the study so far were arugula, basil, beet, carrot, lettuce, radish, kale, parsnip, pea, pepper, tomato and turnip. Each vegetable type was collected as it matured and was analyzed for copper content. So far, four years of data have been collected and analyzed.
How was copper leaching and plant uptake tested?
Water-soluble copper may leach from the treated wood when it gets wet from rain and irrigation. Some will move vertically down into the soil that the treated wood directly rests on. In this study, however, the researchers wanted to determine if there was any horizontal spread into the raised bed soil where the herb and vegetable roots were present.
To measure the spread, the researchers collected and tested soil samples from inside each bed in the spring before planting and in the fall after all vegetables had been harvested. The soil was tested within 1 inch of the edge of the raised bed, 3-4 inches away from the wood, and from the center of the bed (Figure 5). All four beds in the study were sampled and tested using this technique.
The researchers also directly tested the vegetables and herbs to see if the plants absorbed copper. Different plant parts were tested depending on the plant. For example, root crops like carrots, beets and parsnips had the leaves and roots tested separately. Vegetables were harvested throughout each year as they matured. The soil and the plants were tested in a lab using standard soil testing methods to determine the copper levels.
What did the researchers find?
Results from this study are now available through four growing seasons. This study showed that raised beds made from pressure-treated lumber increase soil copper concentrations statistically significantly but only within 1 inch of the bed edge, and that observation was the same for each of the four years when comparing the soil taken before spring planting and soils taken after harvest. The increase in copper was small but varied across the four growing seasons, ranging from about 20-68 ppm above the level measured in the raised beds made from untreated lumber. This increase in soil copper concentration while statistically significant did not result in abnormally high copper levels in the garden raised beds, and even the highest levels detected within 1 inch of the bed material were well within the natural range of copper found in Willamette Valley soils.
There was no excess copper found in the other locations tested (3-4 inches from the edge and the center of the bed) in either the treated or untreated beds across all four growing seasons. This means that the increase in copper was limited to the soils in direct contact with the wood.
The study also showed no increase in copper concentration in each of the vegetables and herbs grown in raised beds made with pressure-treated lumber in any of the growing seasons. So, the small amount of copper that did leach from the pressure-treated wood did not result in an increase in copper in the vegetables grown in the beds.
How are the beds holding up?
The main advantage of utilizing pressure treated wood for garden box material is to extend the life span of the beds. So how do the treated beds hold up compared to untreated beds? The OSU study is going into its 6th year at the time of writing and treated garden boxes have started showing their value. After 6 years, untreated garden boxes are showing signs of decay (brown rot) in the lower portions of the 2 x 12 inch lumber (Figure 6). On the inside lower half of the boxes this decay is severe and over half of the wood cross section is fully decayed. By contrast, the treated wood garden boxes are mostly intact and show only minor signs of fungal decay and some signs of abrasion. The untreated boxes will likely need to be replaced after the 6th growing season as their structural integrity has been noticeably compromised and the treated boxes appear ready for many more years of service.
What does this mean for gardeners?
Pressure-treated lumber is an economical, long-lasting choice for constructing raised beds. This OSU study suggests that Willamette Valley gardeners should not be concerned with copper accumulation in vegetables and herbs grown in raised beds constructed with copper azole treated wood intended for “ground contact” use.
Only use lumber treated with preservatives approved for residential use, and make sure the lumber is treated for “ground contact” applications to ensure good performance. If you’re not comfortable with the small amount of copper leached from pressure-treated wood, choose a rot-resistant wood like cedar or redwood or use concrete blocks or stone.
Oregon State University is continuing this study and will seek to test other types of pressure-treated wood in the future.
Resources
- Basics of Pressure Treatment of Wood from Oklahoma State University
- American Wood Protection Association’s residential use category infographic
- Overview of Wood Preservative Chemicals from the Environmental Protection Agency
- Home and Garden Use of Pressure Treated Wood from the National Pesticide Information Center
- Copper migration from treated wood garden boxes into soil and vegetable biomass Part I: The first two growing seasons after installation
- Copper migration from treated wood garden boxes into soil and vegetable biomass Part II: The third and fourth growing seasons after installation
Related Extension publications
This article is based on ongoing research by Oregon State University’s Department of Wood Science and Engineering. This article will be updated as additional results are available.
