Safety of Pressure Treated Wood - Knowledgebase Question

Eagle River, AK
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Question by amerson
May 15, 1999
I want to build raised beds for my garden. Should I use pressure-treated wood?

Answer from NGA
May 15, 1999
There are several perspectives to consider. Regarding safety of nearby food plants, children, or yourself, research provides no clear answer. Apparently, the toxic chemicals can migrate from the wood to nearby soil, but at very low rates. Once in the soil, the chemicals are bound tightly and not very likely to pollute ground water. On the other hand, prudence suggests not planting edible crops right next to pressure-treated timbers; likewise if you have children or animals that would be playing near the wood.

Working with pressure-treated wood requires safety precautions. You should wear gloves and dust mask, and dispose of the sawdust and scrap. Never burn pressure-treated wood.

Also consider the type of chemical used. Chromated copper arsenate, or CCA, is most common and the one most people mean when they refer to pressure-treated wood. Both arsenic and chromium are classified by the EPA as hazardous chemicals. Wood treated with CCA is a light green color. Two other chemicals are now available that have many of the same benefits without the hazardous chemicals. Ammoniacal copper quaternary compound (ACQ) also produces a light green color, and copper dimethyl-dithiocarbamate (CDDC) produces a brown color.

From an environmental-impact perspective, the primary problem with CCA-treated wood comes at the disposal end of the wood's life cycle, especially if it is incinerated. Some of the CCA (particularly the arsenic) may become airborne, but most will end up in the incinerator ash, where it is highly leachable so likely to contaminate ground water.

Finally consider the alternatives. Pressure-treated wood does conserve forest resources, but at some expense. Naturally rot-resistant woods such as redwood and cedar are available but in short supply. Wood-like timbers made of recycled plastic are available.

Consider non-wood materials such as stone, brick, or cement blocks. But if you want wood, look for a local, renewable wood. In the northeast, hemlock is an example; in the northwest, it would be Douglas fir. Without any treatment the beds should last five to seven years.

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