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 concerns about the potential for the chemicals used to preserve the wood to contaminate the soil and plants.
What is pressure-treated wood?
Pressure-treated wood has gone through a process to force a liquid chemical preservative into the lumber and between the wood cells using vacuum and pressure. Pressure-treated wood has a different color and appearance from untreated wood due to the addition of the chemical, most often copper. In some cases, a finish is applied to the outside of the wood that distinguishes these products from untreated lumber.
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 wood. 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.
It is important that gardeners only use pressure-treated wood labeled for “ground contact.” The treatments used will ensure the wood will perform well as a raised bed. Pressure-treated wood in the western United States sold for ground contact applications is easily identifiable at the lumber yard by the regular cut marks on the surface (Figure 1). The cuts, or incisions, help the liquid preservative enter the lumber. Confirm by checking the end tag for the words “ground contact.” As with all wood products, users should wear a dust mask when cutting pressure-treated wood to avoid inhaling the dust. Fine particulate sawdust from cutting any wood product can be carcinogenic with repeated, excessive exposure.
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 was used for wood available to the general public.
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 leaching and accumulation in vegetables, manufacturers do not recommend it for this application. It is also not recommended that gardeners use old railroad ties for garden boxes. 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 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 soils (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 another 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 study is located in the Willamette Valley.
The beds were filled with native garden soil amended with compost. 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 raised beds were planted with the same vegetables and herbs each year (Figures 3 and 4). Included in the study were arugula, basil, beet, carrot, lettuce, radish, kale, parsnip, pea, pepper, tomato and turnip. So far, two seasons 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. The soil was tested within 1 inch of the edge of the raised bed, 3-4 inches away from the wood, and at 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. 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?
This study showed that raised beds made from pressure-treated lumber do increase soil copper concentrations but only within 1 inch of the bed edge. The increase in copper was small — about 20 ppm above the level measured in the raised beds made from untreated lumber. This increase in soil copper concentration was minor, and even the highest copper concentrations seen within 1 inch of the bed material were well within natural range of copper levels for Willamette Valley soils.
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 the soils in direct contact with the wood.
The study also showed no increase in copper concentration in the vegetables and herbs grown in raised beds made with pressure-treated lumber. 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.
What does this mean for gardeners?
Pressure-treated lumber is an economical, long-lasting choice for constructing raised beds. This OSU research study suggests that Willamette Valley gardeners should not be concerned about copper accumulation in vegetables and herbs growing in raised beds constructed with wood treated with copper azole for “ground contact” use.
Only use lumber treated with preservatives approved for residential use, and make sure your bed material 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
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.
