The short answer
Extension services treat both as usable for food, with precautions. Zinc from galvanized steel may leach only in highly acidic soil, according to the University of Minnesota Extension. Today’s copper-based treated lumber showed no copper increase in vegetables in an Oregon State University trial. The arsenic-based treatment that caused the worry was withdrawn from almost all home uses in 2003.
Galvanized and coated steel
The University of Minnesota Extension says zinc from the coating inside a galvanized bed may leach into the soil in highly acidic soils, below pH 5. If zinc is a concern, it suggests adding a liner or choosing stainless steel. It also warns that some prefabricated metal beds sold online have been found to contain lead, and advises caution when buying metal raised-bed materials.
On durability, Virginia Cooperative Extension notes that most metal beds are 20 to 24 gauge galvanized steel, and cites research that 14-gauge galvanized steel could last 35 to 50 years or longer before it first rusts through.
Treated lumber
The EPA says CCA, the treatment containing arsenic, was cancelled for virtually all residential uses from December 31, 2003. It describes ACQ, a copper-based preservative, as relatively low risk. The University of Maryland Extension says micronized copper azole, now the most widely available residential treated lumber, is considered low in toxicity.
In Oregon State University Extension’s trial, beds of copper-treated lumber raised soil copper only within 1 inch of the boards, with no increase in copper in the vegetables and herbs grown over several seasons. Washington State University Extension calls the risk from ACQ-treated beds negligible and suggests lining the inside walls with plastic if you are concerned.
Precautions extension services give
- Use treated lumber labelled for ground contact (University of Minnesota Extension; Clemson Extension recommends AWPA UC4A or higher).
- Plant vegetables 12 inches in from the inside edge of a treated bed (Clemson Extension).
- Line the edges or inside walls if you want to limit contact (University of Minnesota Extension; Washington State University Extension).
- For a galvanized bed, zinc is a concern only in highly acidic soil, below pH 5; add a liner or choose stainless steel if it worries you (University of Minnesota Extension).
Choosing a bed
The best wood guide compares untreated woods by decay resistance, and the materials guide compares metal, cedar, composite and fabric beds in our catalogue. Nothing here is a test of any product.
References
- Raised bed gardens, University of Minnesota Extension
- Comparison of Raised Bed Methods, Materials, and Costs (SPES-425NP), Virginia Cooperative Extension
- Overview of Wood Preservative Chemicals, US Environmental Protection Agency
- Growing Vegetables in Raised Beds, University of Maryland Extension
- Pressure-treated wood for raised bed construction in Oregon (EM 9879), Oregon State University Extension
- Raised Beds: Will They Benefit Your Vegetable Garden? (FS075E), Washington State University Extension
- Treated Wood in the Landscape (HGIC 1732), Clemson Cooperative Extension