Most pressure-treated lumber no longer contains arsenic
Q: I want to build a raised bed for vegetable gardening this year. However, I have heard a lot of bad publicity about pressure-treated lumber. How safe is it? Is there anything else that will last as long for the price?
M.A. in Springfield
For many years, the only real choice of pressure-treated lumber was wood treated with Chromated Copper Arsenate (CCA). As the name would seem to indicate, it did contain arsenic, which leached into the soil and could contaminate the plants in your garden.
However, CCA has become a thing of the past (except for certain specific commercial and industrial markets), and you will no longer find CCA-treated wood at local hardware stores.
The most common replacement wood preservative treatment you will find today in treated lumber is ACQ, which stands for alkaline copper quat (a mix of copper and a quaternary ammonium compound) that protects against rot, decay and termite attack.
Quat is a synthetic product commonly used in commercial disinfectants and cleaners and is not on the list of allowed substances for organic growers.
Small amounts of copper and quat do leach into the soil, but nothing in ACQ is considered hazardous by the EPA, and no ingredient is a known or suspected carcinogen. Because copper is highly toxic to marine life, ACQ is not recommended for aquatic environments.
The wood is expected to last as long as CCA-treated lumber.
In late 1997, ACQ-D came out on the market. The "D" formula has a built-in water repellent. ACQ-treated wood was only about 10 percent more expensive than CCA because it contains more copper. The product caught on pretty quickly and that made it possible for consumers and stores to replace the more dangerous CCA-treated wood.
ACQ corrodes aluminum but that is because copper doesn't get along with aluminum chemically.
There is an even newer wood treatment known under various names, such as MCQ, Smart Sense, MicroPro, MicroShades, or Micronized Copper Quaternary that first appeared in production in 2006.
While ACQ is a liquid, MCQ is made of tiny (micro) particles of copper which are forced into the wood cells or pores during the pressure cycle. The leaching of chemicals out of MCQ is practically non-existent, so much so that aluminum fasteners are approved for use on this type of treated wood because they won't corrode.
According to Sally Brown at the University of Washington, even arsenic from the old CCA treated wood won't be taken up by plants unless the soils are deficient in phosphorus, which is usually not a problem for gardeners who use lots of compost or other organic matter in their garden.
As for the new copper-based wood treatments, Brown believes the actual risk is minimal. First of all, if plants take up too much copper, they will die before a gardener can eat them. In addition, if homegrown vegetables make up a small percentage of the diet, exposure to any metal taken up is insignificant.
If you want to stick with the pressure treated lumber, but are still nervous about the chemical contents, you can line the sides of a raised bed with heavy plastic to prevent leaching of chemicals from the wood into the soil. The bottom of the bed needs to be left with no liner, or you will have a pond instead of a garden.
You could also consider using cheaper non-resistant woods and sealing them with linseed oil to add some resistance and just replacing them more often.
Another option would be to use stacked concrete blocks to construct your beds.
Just to give you a little price comparison, CCA was expected to last more than 40 years while the ACQ-treated wood (which costs about 10 percent more) is projected to have the same longevity.
Redwood costs almost twice as much as ACQ. Cedar costs more than four times as much. Each of these options is expected to last for 15 to 25 years. Composite products (plastic boards) cost 2 1/2 times as much as ACQ and would last about 30 years
www.news-leader.com/story/life/home-garden/2009/04/26/most-pressure-treated-lumber-no-longer-contains-arsenic/77200928/
