A number of years ago, lumber treated with chromate copper arsenate – pressure treated wood (known as CCA) came as an answer to a gardener’s prayer. It bragged of longer life than rot-resistant species such as redwood. You could purchase it anywhere. Additionally, manufacturers said the treatment chemicals, though toxic, safely stayed in the wood.
But word started spreading out that those CCA chemicals were not so well bound up after all. Word had it that some of them moved from the wood into the surrounding soil. And that’s when pressure-treated wood began receiving close scrutiny.
So, is pressure treated wood safe for your garden? Well, let’s find out!
How Are Pressure Treated Wood Made?
In the processing of pressure-treated wood, the lumber is sealed in a tank, and the air is extracted, creating a vacuum. Thereafter, a solution containing copper, chromium, and arsenic is added. The vacuum created forces the chemicals to get deep into the wood.
Are Pressure Treated Wood Safe for Your Garden?
All three chemicals used in pressure-treated wood are poisonous. Chromium is a bactericide, arsenic an insecticide, and copper a fungicide, and all arrest decay of some kind.
Although the three chemicals are poisonous, copper and chromium do not raise any concerns. Chromium may not particularly harm you if you do not inhale it. On the other hand, copper is not very toxic to mammals, although it is to fungi and aquatic life.
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The main concern with the safety of pressure treated wood is arsenic. If arsenic – a gray, metal-like element – reacts with chlorine, oxygen, and sulfur, it is considered inorganic arsenic. If carbon is part of the combination, then the arsenic is organic.
Inorganic arsenic is a source of concern for the safety of pressure treated wood. Compared to organic arsenic, inorganic arsenic is more likely to pile up in living tissues where it interacts with cell enzymes, impairing metabolism. Organic arsenic does not seem to do this and is largely excreted before it can do any harm.
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Daily, you are exposed to organic forms of arsenic that are present in small amounts in water, soil, and food. Typically, you eat 25 to 50 micrograms of mostly organic arsenic in a day.
Inorganic arsenic may also be present in foods because of the residues in the soil since the days when arsenic was approved as a pesticide. Root crops tend to accumulate arsenic because of the minute particles of soil that stick to the skin of their roots, even with brisk scrubbing.
The chemicals used in treating pressure treated wood are pesticides. Therefore, you should handle the wood with the same precautions as befit any potentially hazardous material.
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Consider protecting yourself while handling CCA wood. Always put on gloves, eye protection, and most importantly, a dust mask. Long sleeves are an excellent idea, as well. You can also wash your clothes and yourself afterward.
Additionally, clean up every speck of sawdust you can. Sawing and drilling over paved surfaces make dust retrieval easier. Bag up wood scraps and sawdust and send them to the landfill. Do not consider these steps optional.
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By all means, avoid burning CCA-treated wood. Burning sends some of the arsenic up in smoke, which contaminates and poisons the air. The ash, as well, contains high concentrations of arsenic.
You can do the following on CCA-treated wood to minimize migration or leaching of toxic chemicals:
- Scrub the wood with detergent or power wash it to remove surface residue
- Allow the board to weather for several months after they have been cut and drilled before assembly
- Predrill holes for screws. This will prevent cracks in the wood. Cracks are places where preservatives can leach
- Line the inside of the bed with heavy-duty plastic before filling it. This will create a physical barrier to any CCA compounds moving into the soil
- You can paint the exposed wood surfaces with a water-repellent finish, paint, or stain that will protect your skin if you lean or kneel on the sides
- Avoid growing spinach and root crops, particularly carrots and radishes, close to CCA-treated wood. You can plant a band of compact flowers along the edge of the bed.
