The New Treated Woods: Safe for Garden Use?

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Q. I want to build some cold frames but am confused about what type of wood to use. I am concerned about using pressure treated wood around my veggies because of the arsenic. But I fear attracting termites with untreated wood. Any thoughts?

---Stephen in Huntingdon Valley, PA

Is the new pressure-treated "Nature Wood" safe to use as a framing material for a raised bed vegetable garden? We would also like to know about non-toxic wood preservatives we can use on an existing trellis. Thanks,

---Karen & Craig in Bingham, Maine

A. ORGANIC GARDENING campaigned heavily to make home owners aware of the dangers posed by old-school CCA (chromium, copper and arsenic) treated wood back when I was the magazine's Editor in the 1990s. (Even worse are old railroad ties and power poles; those massive reservoirs of cancer causing chemicals don't belong in any landscape.)

Thanks to the great work and applied 'pressure' of many dedicated groups and individuals (including OG's then Research Editor Cheryl Long—now the Editor of Mother Earth News), the industry cut a deal with the EPA in 2004 to begin voluntarily phasing out the use of arsenic in treated wood products available to home owners. (Although arsenic treated wood is still allowed for some applications, like marine construction. Sorry, fishies!) In CCA's place, a dizzying array of less-toxic treated wood products have come to market for home owner use, one of which is "Nature Wood".

The copper is still there, but the other C, chromium (a worrisome metal) and the big A, arsenic are gone; replaced by {quote} "Quaternary ammonium compounds," which an hour or so of research informs me is a form of ammonium chloride (as in ammonia and chlorine). I'm not wild about either of those elements, especially chlorine, which helped kill millions when used as a component of trench gas in World War I. And although this compound seems to be close to ubiquitous in household products, there appears to no data on its potential carcinogenicity, tera to genicity or mutagencity (translation: Potential to cause cancer, birth defects and mutations).

It's probably a big improvement over arsenic treated wood, whose health effects were clearly dire. But it's highly corrosive to certain metals, leading me to worry about its effects on soil life; and there is an excellent alternative that I like a lot: Wood treated with boric acid (a.k.a. borates). Boric acid is extremely safe for humans, yet protects wood against rot, decay and insects—acting first as a repellent, then a feeding deterrent, and finally being deadly to any insects what aren't repelled and/or deterred.

An Internet search found borate treated wood available under several brand names, including "Advance Guard" and "Hi-Bor" (both made by a company that used to sell a lot of CCA treated wood, so they should be just as easy to obtain).

Bill Quarles, director of the common-sense pesticide clearing house BIRC—the Bio Integral Resource Center in Berkeley, California —informs us that he has learned of a new treatment via the wonderful sustainable agriculture group ATTRA. Marketed as "Thunderbolt" (who thinks up these names?!) by a company in Riverbank, CA, the wood is treated with "a solution of 62.3% copper oxide and 35.8% citric acid dissolved in ammonia water". The company hopes to get this treated wood alternative accepted for use in organic agriculture, specifically for fence posts and grape stakes.

My research also found an intriguing new technology on the horizon: 'wood acetylation', a technique that uses acetic acid (as in vinegar) to make wood resistant to rot and insects. It dates back to the 1920s but only recently became cost-effective, when a company in the Netherlands reportedly began large scale production in 2007. We'll keep an eye out for it on our shores.

I remind every budding raised bed builder that bricks, field stone and pavers make excellent rot-and-insect-proof frames. And when used in cooler regions, the stone will store heat during the day and release it slowly in the evening—warming your soil up earlier in the season, keeping it warm later in the season, and longer into the evening.

If you like the look of wood, the composite 'lumbers' made from recycled plastic and recovered wood waste work extremely well. My raised bed frames made of Trex are over twenty years old and still look brand new. (I wish I could say the same of myself.) You can also buy 'timbers' made of 100% recycled plastic that look and work just like wood.

And for the question about the treatment of existing wood, we're back to boron! There are a wide variety of boric acid 'washes' that home owners can use to treat the wood of their choice, including "Bora-Care", "Jecta" and the wonderfully named "Tim-Bor" (get it?). (Timbor post treatment info.)

High concentrations of boric acid cause the rapid death of pest insects, but low levels cause no immediate symptoms, allowing the targeted ants or termites to take the material back to the nest, where it is shared by the entire colony—including the all-important Queen—wiping the colony out within a week or so. This same mechanism of action applies to the boric acid sweet-bait traps we recommend for household ant problems. The dose in the traps is deliberately low, to make sure the worker ants survive long enough to haul that Boric Acid Trojan Horse safely inside their walls.

Thanks to the great advocacy group Beyond Pesticides for their excellent fact sheet on termites, which supplied many leads and brand names for this article. Here's a link to that wonderful info. And here's a LINK to lots more info and detail about raised bed framing options—including naturally rot resistant woods—in a previous Question of the Week.

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