Treated woods for garden use: What is safe?
The mild, moist, maritime climate that we enjoy in western Washington shortens the life of wood products used in garden construction. Decay organisms, primarily fungi but also insects, usually begin the degradation process very quickly on wood used outdoors, especially if it is placed in contact with the soil.
Some woods such as redwood, western red cedar, black locust and bald cypress have been used, and still are used, under the assumption that they possess inherent qualities that protect them from decay and insect invasion. Black locust and cypress are not easily obtained in our region, and the other two may not last very much longer than other woods. Only the heartwood of these species is very resistant to breakdown. Old-growth trees, which contain the most heartwood, are a diminishing item. This makes decay-resistant wood difficult to get and very expensive when it is available.
Besides the obvious structural and aesthetic problems that rotting wood causes, carpenter ants and termites can use outdoor wood as springboards or highways to invade your home or other structure. A raised bed constructed of unprotected lumber near your house may very well serve as a pathway inside.
Old railroad ties are frequently used to build raised beds or to terrace slopes. This wood has been treated with creosote, a product derived from coal. Creosote has certainly proven itself as wood preservative over a long period, but because it is toxic and because it has become a restricted-use pesticide, questions have arisen about its safe use around plants.
Creosote can volatilize into the air, especially during hot weather, and plant foliage in the vicinity of the ties may be damaged by the vapors. It can also leach into the soil near the ties, but it will not be absorbed by the roots and will therefore not get into the plants' tissues.
When using railroad ties for raised beds or similar areas, avoid putting plants too close to the wood. Keeping plants three of four inches away should prevent damage. Also, don't ever use creosote-treated wood in interior locations such as greenhouses, since the vapors will damage or kill plants in them.
You may find another big problem is that creosote will seep out of ties in warm weather. When you sit on or lean against the timber as you garden, your clothes will pick up black tar stains.
Pentachlorophenol, another restricted use chemical, has also been used to treat wood to prevent decay and insect infestation. Do not use such lumber to construct planters or raised beds, since it is likely to severely damage plants. As with creosote-treated wood, absolutely never use it inside greenhouses. For treating wood yourself to retard decay, both copper and zinc compounds are available in various proprietary formulations made to be brushed or painted on the surface. Although they do work to a certain extent, they do not really possess much long term effectiveness. It is questionable whether either one will protect wood in contact with the soil. They are safe to use around plants and can even be used to treat wood in greenhouses.
Within recent years, the availability and use of pressure-treated lumber has become common. Chromated copper arsenate (CCA) is forced into the cells of the wood under high pressure. Wood treated with this material has been shown to last a very long time indeed, even when it has been in contact with soil.
University research on CCA-treated wood indicates it is safe for use around plants. The copper and arsenic protects the wood from both insects and decay organisms. The chromium in the compound is the agent that fixes it in the wood. Since it is chemically bound to the wood, it cannot leach into the soil or vaporize.
During construction with CCA-treated wood, you should use appropriate safety precautions. Eye protection, a dust mask and gloves should be used. Wood scraps and sawdust should be disposed of properly, never burned in fireplaces or wood stoves.
CCA-treated wood should never be used as a food preparation surface, where chopping and cutting food might cause particles of wood to mix with food. Acceptable uses include raised beds, terraced gardens, trellises, compost bins, and planter boxes.
Some organic gardening publications have claimed that these garden uses are unsafe. The U.S. Environmental Protection Agency has approved the use of CCA-treated wood for gardens. (Tests have confirmed that arsenic does not leach from timbers used in gardens. Soil was collected from near timbers that had been place from 6 months to 9 years.)
Some gardeners who want to be very cautious and are unconvinced of the safety of treated wood can consider the following options:
- Use untreated wood and plan to rebuild every few years
- Use treated wood, but grow only ornamentals near the timbers and grow edibles to the center of the beds.
- Line treated wood with plastic before filling beds with soil.
- Use masonry, concrete, "lumber" made from recycled plastic or some other building material.
*Information courtesy of Washington State University, Cooperative Extension