The research does not give you a clean yes or no here. Three extension services look at the same boards and land in three different spots, so pressure treated lumber safety comes down to how much doubt you are willing to live with. That is a frustrating answer if you are standing in the lumber aisle with a loaded cart. None of them says the wood poisons your vegetables. None of them waves it through without a caveat either.
University of Maryland Extension points to Oregon State University research that put numbers on the worry. Soil copper rose by a minor amount, and only within 1 inch (2.5 cm) of the bed edge. The plants growing in those beds showed no rise in copper at all. That single inch is the entire footprint of the leaching people picture when they imagine chemicals creeping through their soil.
The wood on the shelf changed a while back. Chromated copper arsenate held arsenic along with copper and chromium. It left the residential market in 2004 and has not been sold for home use since. What sells for home projects now is micronized copper azole. It carries copper and no arsenic, and it rates low in toxicity. The board you buy today is not the board that started the arsenic scare.
The three positions split from there. Maryland rates the current preservative low in toxicity. University of Delaware Cooperative Extension calls new treated pine safe. It also rules the wood out of organic gardening, which matters if you hold to that standard. Jonathan Foster at the University of Maine moves the concern off the harvest and onto you. Skin contact, sawdust you breathe while cutting, and dust you swallow by accident are the bigger worry. He is plain that the risk in both cases is certainly not zero.
- Oregon State research measured a copper rise only within 1 inch of the bed edge.
- Plants grown in those beds showed no increase in copper.
- Micronized copper azole holds no arsenic and rates low in toxicity.
- Delaware rules new treated pine out of organic gardening.
- Jonathan Foster of Maine says the risk is certainly not zero.
- The exposure that matters most is the gardener, not the food.
You do not have to settle that argument to build a bed. Raised bed wood safety is one problem you can build your way out of in an afternoon. Put a barrier between the boards and the soil and you cut the contact the sources worry about.
- Paint: Coat the inside faces with exterior latex paint before you fill the bed.
- Stain: A semi-transparent oil-based stain does the same job and keeps the wood grain visible.
- Liner: Hang a heavy plastic liner that still lets water drain out of the bed.
- Skip treated wood and build with a decay-resistant species instead.
- Build the walls out of a non-wood material.
Two materials stay out of a food garden no matter which side of the evidence you find convincing. Creosote lumber and pentachlorophenol lumber, the kind sold as old railroad ties, must not be used for a bed you eat from. If a neighbor offers you free ties for the cost of hauling them away, pass. That line is not a matter of comfort level, and Delaware states it without qualification.
So pick your comfort level and build to it. If treated pine is what you can afford and haul home, use it, paint the inside faces, and wear a dust mask while you cut. If the unresolved risk sits badly with you, spend more on decay-resistant wood or put up a non-wood wall, and stop reading arguments about copper. Both beds grow the same tomatoes.
Read the full article: Raised Garden Bed Specs Backed by Research
