Short answer: modern residential pressure-treated lumber (copper-based ACQ, copper azole or micronized copper) is generally considered acceptable for raised vegetable bed frames and for the skids, sills, posts and framing of animal shelters. Buy ground-contact (UC4A) rated stock for anything touching soil or sitting less than about 6 in (15 cm) above it. Keep treated wood out of feed troughs, mangers and anything animals chew. Never reuse lumber that may be old arsenic-treated (CCA) stock, never burn treated offcuts, and skip treated wood entirely if you're seeking organic certification.
What's in today's treated lumber
Until the end of 2003, most treated lumber sold to homeowners was chromated copper arsenate (CCA). Under an agreement with the EPA, manufacturers stopped treating wood for residential uses with CCA after December 31, 2003. What's on the rack now is copper-based: alkaline copper quaternary (ACQ), copper azole (CA) or micronized copper versions of either. None contains arsenic. That's why the old warnings about treated wood in gardens mostly come from the CCA era.
The practical consequence is about salvage, not new lumber. If a board is old enough to be CCA (more than 20 years old), don't build a food bed or an animal pen from it.
Read the end tag: above ground vs ground contact
Treated lumber carries a tag or stamp with an American Wood Protection Association (AWPA) Use Category. The category tells you how much preservative went in, and where the board will last.
| Tag | Meaning | Farm and garden use |
|---|---|---|
| UC3B | Exterior, above ground | Rails, siding, rafters, gate boards |
| UC4A | Ground contact, general use | Raised bed walls, skids, sills, bottom plates, fence boards near soil |
| UC4B | Ground contact, heavy duty | Structural posts set in the ground, members that are hard to replace or wet all the time |
Since 2016, the AWPA standard has called for ground-contact treatment even on parts that never touch the soil if they sit less than 6 in (15 cm) above grade, stay wet, have poor air circulation, or are hard to replace. For a coop floor frame on blocks, the sill of a hog house or the bottom board of a raised bed, that means UC4A. The AWPA also warns that some lumber 2 in thick or thinner is only treated for above-ground use, so read the tag rather than assuming.
Raised vegetable beds
Oregon State University Extension tested raised beds built from copper azole-treated Douglas-fir over four growing seasons. Soil copper rose measurably only within about 1 in (2.5 cm) of the boards, stayed within the natural range for local soils, and did not rise in the vegetables grown in the beds. Other extension services reach similar conclusions for ACQ, the other common copper treatment.
- Use UC4A ground-contact lumber for bed walls. Above-ground stock rots out quickly in damp soil.
- Brush a cut-end preservative onto every saw cut, as Oregon State recommends. The cut exposes untreated wood at the core.
- If you'd rather keep soil and wood apart anyway, staple heavy polyethylene to the inside faces of the walls and leave the bottom open for drainage.
- Avoid creosote railroad ties and any salvaged treated lumber of unknown age.
- Certified organic growers can't use treated lumber where it contacts soil or crops. Untreated cedar, redwood heartwood or black locust are the usual alternatives.
Animal housing
Treated lumber's consumer information sheets say not to use it where the preservative could become part of food or animal feed. In practice, that splits a shelter into two zones.
Treated wood is right for: skids and runners, sills and bottom plates, posts set in the ground, and framing within 6 in of soil or manure. The Cattle Feeder rides on treated 4×6 skids, and the Fenceline Feeder for Sheep hangs its trough on treated 4×4 posts, because those parts sit in the wet.
Use untreated wood for: trough bottoms and sides that hold feed, hay mangers, salt and mineral troughs, and any edge that horses, goats, rabbits or pigs will chew. Where a chewer can reach structural treated wood, cover it with a strip of sheet metal or hardware cloth. Poultry rarely chew framing, but don't use treated wood for feeders or for a roost they'll stand on all night if you can easily avoid it.
Working with it safely
- Cut outdoors, wear a dust mask, gloves and eye protection, and sweep up sawdust so it doesn't end up in bedding, feed or garden soil.
- Wash your hands before eating.
- Never burn treated wood, in a stove or in a brush pile. Dispose of offcuts through ordinary trash or as local rules direct.
- Use fasteners rated for treated lumber. Copper preservatives corrode plain and electro-galvanized steel.
Frequently asked
Is pressure-treated wood safe for a chicken coop?
Modern copper-treated lumber is fine for the coop's skids, sills and framing. Build feeders and nest boxes from untreated wood, and keep sawdust out of the run.
Do I need to line a treated raised bed?
The Oregon State research found no copper increase in the vegetables, so a liner isn't required. Some gardeners add one for peace of mind; if you do, leave the bottom open.
How can I tell if old lumber is CCA?
Often you can't by looking. Faded green color doesn't settle it either way. If it's more than about 20 years old and you don't know its history, keep it out of food beds and animal pens.
Is cedar better than treated pine for a garden bed?
Cedar is untreated and acceptable for organic growing, and its heartwood resists decay naturally. It also costs more, and sapwood cedar rots almost as fast as plain pine.
Guidance summarized from AWPA, EPA and university extension sources. Follow the tag on the lumber you buy and any local rules.