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ARE VEGETABLES GROWN IN TREATED-WOOD GARDEN BEDS SAFE?
Gardening in raised beds can be a smart alternative to trying to grow vegetables and other plants in compacted soil or soil interlaced with tree roots.
Building materials that work well to contain soil above ground include treated wood, cedar and redwood, interlocking landscape blocks, and recycled plastic boards.
The most common raised-bed material is CCA-treated wood. CCA stands for Chromated Copper Arsenate, a chemical mixture consisting of three pesticidal compounds (chromium, copper and arsenic) registered for wood preservative use.
These three compounds protect wood against bacterial, fungal and insect attack. In the U.S., CCA is the treatment found in nearly all wood sold as containing wood preservers.
Creosote-treated timbers (including railroad ties) and penta-treated timbers should not be used for raised gardens.
Copper in preserved wood is used to prevent deterioration from fungi and arsenic acts as a repellent to insects. Copper, chromium and arsenic are all naturally occurring elements in nature. All garden soil contains traces of arsenic. Copper and chromium are metals that are needed by the human body and are often found listed as ingredients in supplemental vitamins.
The question gardeners often ask is whether CCA-treated wood is safe. Can these chemicals leach from the wood into garden soil and get absorbed by vegetables?
The University of Minnesota conducted a study of soils collected from six Twin Cities raised gardens constructed of CCA-treated wood. All of the beds were at least 10 years old to ensure that, if leaching does occur, it would be detectable in these soil samples.
I won't go into all the details but the controlled study was very thorough and included various soil types and a mix of vegetables (carrots, spinach, bush beans and buckwheat). Soil samples were collected from the six gardens at very precise distances from the wood. Plants were grown in the collected soil samples in pots in a greenhouse.
Of the three elements, arsenic is considered the most dangerous to human health. Arsenic concentrations in soil generally range from 3.6 to 8.8 ppm (parts per million) and previous studies have demonstrated that concentrations up to 40 ppm are safe.
Results of this study showed that vegetable crops grown in raised garden beds constructed of CCA-treated wood can accumulate arsenic from the wood, but based on U.S. Public Health Service standards, these vegetables would be safe for human consumption. Since the study found that plants grown in soil samples collected 1 inch from the wood had significantly higher concentrations of arsenic than those grown in control soil samples collected 45-50 inches away from the wood, gardeners may want to consider spacing vegetable plants at least 15 inches away from treated wood to reduce accumulation of arsenic in food crops. For plants with extensive root systems, it may be helpful to place a plastic barrier between the wood and the soil.
Arsenic accumulation is not a human health concern when growing non-vegetable plants in a raised bed.
As stated above, an alternative to treated timbers is redwood or cedar. These two woods are more costly than CCA-treated pine, and they don't last as long, but they are longer-lasting than untreated wood.
A newer treated wood product that doesn't contain arsenic is called ACQ, or "Kodiak Wood." ACQ stands for Ammoniacial Copper Quaternary and contains a blend of ammonia, copper and quaternary ammonia. It is more expensive than CCA but it is said to be effective against rot and insects. ACQ wood is the wood-preservation formula used in Japan and Europe where CCA-treated wood is banned. ACQ wood is available at some lumberyards.
You may also want to consider recycled plastic boards or interlocking landscape blocks that are made specifically for edging and retaining walls.
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PUTTING DOWN ROOTS:
A Delightful Blend of
Gardening Wisdom, Wit
and Whimsy
$10 + $2 for shipping
by Cliff Johnson |
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