Is treated wood safe for vegetable planters? What the label tells you

Lumber sold today for home use is treated with copper compounds, not arsenic, and the university tests that exist show no copper increase in the vegetables grown beside it. What is not safe is old or industrial wood: pre-2004 boards that may be CCA, railroad ties, utility poles, and anything treated with creosote or penta. So the decision is less “treated or untreated” than “which treatment, and how old is the board”.

For the wider question of which containers are fine to fill with soil, see food-safe containers for growing vegetables. Here the question is the lumber.

Read the board before you buy

Oregon State University Extension notes that all pressure-treated wood carries a label or end tag stating its intended end use, and that the preservative depends on that use. Lumber for home building gets a different treatment from railroad ties or utility poles. NPIC, the pesticide information center run by Oregon State with EPA, says some types of treated wood are not approved for use at home.

Two things to check on the tag:

• Residential use. Oregon State says to use only lumber treated with preservatives approved for residential use.
• Ground contact. Oregon State also says the lumber should be treated for “ground contact” applications. A planter holds wet soil against the wood all season, so that is the rating that matches the job.

Oregon State names copper azole (CA-C), micronized copper azole (MCA) and alkaline copper quaternary (ACQ) as the most common preservatives for lumber. It reports CA-C and ACQ are more common in western states for Douglas-fir, and MCA on southern pine in the east. EPA lists ACQ, borates, copper azole and others among the preservatives registered for residential lumber. Virginia Cooperative Extension recommends looking for ACQ or copper azole if you want treated wood for a plant container.

University of Maryland Extension describes MCA as not containing arsenic but containing copper, and considers it low in toxicity.

What the copper test found

Oregon State built four raised beds in a home garden, each about 4 by 10 feet from 2 by 12 lumber: two from untreated Douglas-fir and two from Douglas-fir pressure treated with copper azole to ground-contact specifications. The same vegetables and herbs went in each year, among them arugula, basil, beet, carrot, lettuce, radish, kale, pea, pepper, tomato and turnip. Soil was sampled within 1 inch of the edge, 3 to 4 inches from the wood, and in the center, and the crops were analyzed for copper. Four years of data had been collected at the time of writing.

Question	Oregon State result
Does copper reach the soil?	Yes, but only within 1 inch of the bed edge; about 20 to 68 ppm above the untreated beds, varying by year
How high is that?	Within the natural range of copper in the local soils
Further from the wood (3-4 inches, bed center)?	No excess copper found
In the vegetables and herbs?	No increase in copper in any season

Iowa State University Extension summarizes the research the same way: the copper increase is minimal, limited to about 1 inch from the wood, and does not increase uptake by vegetables, and it considers modern pressure-treated lumber safe for vegetable gardens.

Two limits are worth knowing. The Oregon State beds were 4 by 10 foot garden beds, not pots, and the study tested copper azole boards only; Oregon State says it will test other pressure-treated wood in future.

Old wood is the problem

CCA, chromated copper arsenate, was the arsenic-containing preservative. EPA says that in December 2003 manufacturers voluntarily discontinued CCA-treated wood products for homeowner uses. EPA does not require removal of existing structures made with it or the surrounding soil.

That makes age the practical test. Oregon State warns that reusing or salvaging lumber over 20 years old may bring CCA-treated lumber into your garden. On the risk to food, the sources are measured: Oregon State says there is no evidence that CCA-treated lumber increases arsenic accumulation in vegetables, but that manufacturers do not recommend it for this use. Iowa State says material produced before 2004 often contained arsenic-based preservatives and is not recommended for garden use. If an old board is already in your garden, Iowa State’s advice is a soil test, the best way to find out whether potentially toxic chemicals are present.

Creosote and penta are a separate case. Virginia Cooperative Extension says wood used around plants should not be treated with either; both may be toxic to plants as well as harmful to people. Railroad ties are the usual source: Iowa State says repurposed ties should not be used in gardens because they are treated with creosote, a known carcinogen. Shipping pallets raise their own questions, covered in pallet planter safety and the stamp codes.

Cut the exposure further

Even when the lumber is current, NPIC suggests a few cheap habits:

• Keep edibles away from the edge. NPIC says soil right next to treated wood is likely to hold more of the wood’s chemicals. In a narrow box that means planting toward the center.
• Wash soil off what you harvest. NPIC notes the soil can hold more preservative than the vegetables themselves.
• Seal or paint the wood. NPIC says a sealant or paint acts as a barrier against leaching; Maryland suggests exterior latex paint or a stain.
• Paint cut ends. Oregon State says to treat freshly cut ends with a wood preservative cut-end treatment.
• Wear gloves and a dust mask when sawing, and do not burn offcuts. NPIC and EPA both warn against burning treated wood because toxic chemicals can be released in the smoke.

A barrier between soil and wood is the other route. Maryland lists a heavy plastic liner, and Iowa State describes woven geotextile fabric that lets water drain while separating soil and roots from the lumber. Which one, and how to fit it without creating a bathtub, is worked through in lining a wooden planter. If you go the fabric route, you can browse woven landscape fabric on Amazon.

When another material is the better call

Oregon State’s own advice for gardeners who are not comfortable with the small amount of leached copper is to choose cedar or redwood, or use concrete blocks or stone. It adds that redwood and cedar are more expensive but resist decay. Maryland’s list of non-treated options also includes oak, black locust and bricks. University of Illinois Extension advises caution with treated lumber when growing food or when toddlers are around, and points to naturally decay-resistant wood.

Cost is the trade. University of Maryland Extension gives the following lifespans for a salad-table build: Western red cedar 12 to 15 years and MCA-treated lumber 15 to 20, with untreated spruce, pine or fir at 4 to 8 years. In Oregon State’s trial, after 6 years its untreated Douglas-fir boxes had severe decay on the inside lower half, with over half the cross section decayed, while the copper azole boxes were mostly intact.

A short way to decide:

1. Buying new lumber: residential, ground-contact rated, and the precautions above. Cedar or redwood if the budget allows or the copper bothers you.
2. Reusing old lumber or an unlabelled board: treat it as possibly CCA. Skip it for a food planter unless you line it and test the soil.
3. Railroad ties, poles, creosote or penta: no.
4. A small pot for herbs: the material question is easier to sidestep with a pot; see best containers for herbs.

Sources

• Oregon State University Extension: Pressure-treated wood for raised bed construction in Oregon (EM 9879)
• US EPA: Chromated arsenicals (CCA)
• US EPA: Overview of wood preservative chemicals
• NPIC: Home and garden use of treated wood
• Iowa State University Extension: Creating raised bed planters
• University of Maryland Extension: Safety of materials used for building raised beds
• University of Maryland Extension: Growing vegetables in containers and salad tables
• Virginia Cooperative Extension: Vegetable gardening in containers
• University of Illinois Extension: Container material choices