Pressure-treated (PT) wood is lumber infused with chemical preservatives through a high-pressure process to resist rot, decay, and insect damage. This chemical infusion allows the wood to withstand harsh outdoor environments for decades, making it a common choice for landscaping projects. However, using PT wood for structures like raised garden beds, where edible plants are grown, has raised significant public concern regarding chemical leaching into the soil. The safety of using PT wood depends heavily on the specific chemicals used and the year the wood was treated. Modern treatments have sought to address the public health concerns created by original formulations.
Historical Chemical Concerns
The historic concern about pressure-treated wood centers on the use of Chromated Copper Arsenate (CCA). For decades, CCA was the standard preservative, but it contained arsenic, a heavy metal and known human carcinogen. Studies showed that arsenic could leach out of the wood and into the surrounding soil, raising fears about contaminating garden soil and food crops. In response to these public health concerns, the wood treatment industry and the U.S. Environmental Protection Agency (EPA) agreed to a voluntary phase-out.
This regulatory shift, announced in 2002 and finalized in 2003, effectively banned CCA for most residential applications, including decks and landscaping timbers. Since 2004, CCA-treated wood has not been available to the general public for residential projects. This means wood purchased today does not contain arsenic, but older structures built before 2004 likely used the CCA formulation.
Safety of Current Treatment Methods
The pressure-treated wood available for residential use today employs newer, copper-based preservatives. Common modern residential treatments include Alkaline Copper Quaternary (ACQ), Copper Azole (CA), and Micronized Copper Treated Wood (MCA or MCQ). These formulations rely on copper compounds to function as a fungicide and insecticide, preventing decay. These modern treatments do not contain arsenic or chromium. The copper compounds are bound differently within the wood matrix, which reduces the rate they leach into the soil.
For non-edible landscaping, such as retaining walls and fences, modern PT wood is accepted as safe and long-lasting. For raised beds growing food, the risk to plants is very low; studies show no detectable increase in copper concentration in vegetables grown in these beds. The main debate centers on the long-term accumulation of copper in the soil immediately adjacent to the wood. While copper is a necessary micronutrient, excessive amounts can become toxic to soil biology and plant health over time.
Mitigation Techniques for Use in Gardens
Gardeners using modern pressure-treated wood for raised beds can implement several techniques to minimize chemical transfer. The most effective method is creating a physical barrier between the treated wood and the garden soil. This involves lining the interior walls of the raised bed with heavy-duty plastic sheeting or a food-grade pond liner before adding soil.
When lining the bed, secure the plastic sheeting at the top, ensuring it covers the entire inner surface of the wood. The plastic should not cover the bottom of the bed, as this impedes drainage. Another mitigation technique involves sealing the exposed wood surface with an oil-based sealant or water repellent. This sealing process should be completed on the interior and exterior and repeated every few years to maintain effectiveness against leaching.
Gardeners should also avoid planting food crops directly against the interior wall of the treated wood. Maintaining a small buffer zone of a few inches between the wood and the closest plant roots reduces the chance of chemical uptake.
Non-Treated Wood Alternatives
For gardeners who wish to avoid chemical preservatives, several naturally rot-resistant wood species serve as excellent alternatives for garden construction. These woods offer a completely chemical-free option for food production, though they often have a higher upfront cost than pressure-treated lumber.
Naturally Resistant Wood Options
- Cedar, particularly Western Red Cedar, contains natural oils and tannins that resist decay and insects.
- Redwood is a highly durable option with similar rot-resistant properties.
- Black Locust is known for its exceptional longevity in ground-contact applications.
- Cypress is another durable species suitable for garden use.
Gardeners can also consider non-wood materials, such as stone, concrete blocks, or composite decking materials, all of which are inert and pose no chemical leaching risk to the soil or plants.