The answer to using pressure treated wood for raised bed gardening really depends on the perspective of the gardener. Since 2003 the Environmental Protection Agency has banned harmful CCA pressure treated lumber for residential use due to the arsenic in the treating process and possibilities of leaching into soils. New pressure treated lumber, ACQ or AC-B, does not contain the harmful arsenic, but uses copper instead. The benefits of new, environmentally friendly pressure treated lumber have been scientifically tested to be sound for use in the garden. However, no pressure treated wood is allowed to be used in the gardens of those who grow certified organic foods. If you are a garden purist, perhaps all the science in the world will not convince you.
Alkaline Copper Quat (ACQ) lumber uses copper instead of Arsenic to treat the lumber. The copper acts as an insect repellent, and the quat is a fungicide and disinfectant which is commonly used in swimming pools. Tebuconazole is another fungicide which can be used and is often applied to food crops. All chemicals being used in ACQ or AC-B lumber are non-toxic and thus pose significantly minimized risk. The chemicals are all used in other recreational and industry setting with regular contact to people.
Testing done by the University of Washington has shown that copper can leach from both ACQ and AC-B wood into soils. However, copper is non-toxic and the levels of leaching are minimal. If a plant absorbed high amounts of copper it would die long before anyone could eat any food derived from the plant. Copper is a major concern when it comes to water systems however. Do not use ACQ or AC-B wood near streams or waterways since leaching will kill aquatic life.
The USDA states, "This provision prohibits the use of lumber treated with arsenate or other prohibited materials for new installations or replacement purposes in contact with an organic production site." ACQ and AC-B woods have not specifically been reviewed by the National Organic Program, but they have stated, "Some copper does leach from ACQ-treated lumber, making it unsafe for garden use."
- Natural Resistance - Some woods possess natural resistance to rot or insect pests though they can be expensive and are often limited by region. Cypress, Locust, Redwood and red cedar are all natural options for raised beds which do not pose any threat. The wood does not last as long as pressure treated lumber, but can always be burned or reused when replacements are necessary.
- Plastic Wood - As silly as it may sound, companies have found a way to create wood look-alike substances created from discarded consumer plastic. Not only do the "boards" have the same resistance of plastic, but also help to save pounds of waste from landfills. Often raised bed kits can be purchased as well as single pieces for individual projects.
- Stone or brick can be used to create walls around raised soils. Though more labor intensive, the structures are durable enough to outlast wood many times over.