Pressure-treated wood is the most common material used for raised bed frames because it is cheap, widely available, and lasts for decades. Whether it is safe for vegetable gardening depends almost entirely on which treatment was used — and the answer has changed significantly over the past twenty years.
The Old Formulation: CCA
Until 2004, the most common wood preservative was CCA — chromated copper arsenate. This treatment was banned for residential use in the United States and most of Europe because it contained arsenic, a known carcinogen that leached from the wood into surrounding soil. If you have old pressure-treated timber from before 2004, do not use it for food beds and handle it with gloves.
Modern Treatments: ACQ and CA
The treatments used today — ACQ (alkaline copper quaternary) and CA (copper azole) — contain no arsenic or chromium. The active component is copper. Research from the University of Rhode Island and several European institutions has found that leaching from modern treated timber into adjacent soil is minimal, and resulting soil copper levels fall within safe limits for food production. The EPA and most European regulatory bodies consider ACQ and CA treated timber safe for use in raised vegetable beds. Organic certification bodies including the USDA NOP take a more cautious position — certified organic produce cannot be grown in beds with pressure-treated timber regardless of treatment used.
How to Identify Your Timber
Modern ACQ and CA treated lumber is typically marked with a stamp indicating the treatment type and retention level. Look for "ACQ," "CA-B," or "MCA" on the stamp. If there is no stamp, ask at the yard for the product data sheet.
Practical Risk Reduction
To minimise exposure: line the interior of the bed with heavy-duty HDPE pond liner before filling, maintain soil pH above 6.0 to reduce copper mobility, and avoid using treated timber for beds growing root vegetables like carrots or potatoes, which have the most direct soil contact.
FAQ
Is ACQ-treated wood safe for growing salad leaves?
Current research suggests yes. For added peace of mind, line the bed interior with HDPE liner. Most food safety agencies consider the risk negligible compared with other environmental exposures.
Can I use old railway sleepers for raised beds?
Not for food growing. Traditional railway sleepers were treated with creosote, a probable human carcinogen that leaches into soil for decades.
Does painting or sealing treated wood reduce leaching?
An exterior paint or sealer slows moisture penetration and reduces the wet-dry cycling that drives leaching. It does not eliminate copper movement, but it is a practical additional precaution.
How far from the timber does leaching affect soil?
Studies show elevated copper levels within 50-75mm of the timber face. The centre of a standard 1.2m wide bed is well beyond this zone.
What about composite lumber?
Composite decking made from plastic and wood fibre does not leach copper because it contains no copper. It is a viable option for raised beds and lasts longer than natural timber.
See also: White Oak vs Cedar for Garden Projects and Safest Materials for Edible Gardens.