Gardeners' Dirt: Pressure treated lumber in the garden
By William J. Yanta - Victoria County Master Gardener Edited by Charla Borchers Leon
Feb. 9, 2012 at midnight
Updated Feb. 8, 2012 at 8:09 p.m.
Lumber is often used in gardens for a variety of reasons. It might be used to provide a raised bed for better drainage, to minimize bending or to raise the entire bed to a height accessible even for wheelchairs. Raised beds may be used in an aesthetic fashion to delineate various parts of the garden. If untreated lumber is used, though, it will soon decay or be eaten by insects. Only redwood, cypress and red cedar woods are rot-resistant and relatively resistant to insects.
Pressure treated lumber
A good way to increase the longevity of lumber is to treat it with chemicals, which prevent destruction by fungi, insects or exposure to moisture. This treatment is commonly called pressure treated lumber or PTL. But gardeners often ask these questions: Do the chemicals leach into the soil, and is it safe to use PTL in a garden?
Pressure treated lumber involves a process that forces chemical preservatives into the wood. Untreated lumber is sealed inside large tanks with air being pumped out that creates a vacuum. Since wood is porous, air is also sucked out of the wood fibers. A chemical preservative solution is added, and because of the vacuum, the chemicals are absorbed deep into the wood. These preservatives help protect the wood from fungal decay, termites and other insects.
Three types of treatments
In general, there are three types of treatments used on the wood: waterborne, creosote and oil-borne (pentachloro-phenol). Wood treated with waterborne preservatives is typically used in residential, commercial and industrial building structures. Creosote is primarily used for treating railroad ties, guardrail posts and timbers used in marine structures. Oil-borne (penta) is most often used for treating utility poles and cross arms. Before 2004, waterborne preservatives typically consisted of chromated copper arsenate.
CCA lumber became very popular with gardeners and had been used for decades. The chemicals did not harm plants, unlike creosote and penta. Chromium is a bactericide; copper a fungicide, and arsenic an insecticide. After decades of use, questions arose about the safety of these chemicals. All three are toxic; however, chromium and copper do not pose any major concerns. If we do not inhale it, chromium is not particularly toxic to humans. Copper is not very toxic to mammals.
It is arsenic which raises concerns, although it is found all around us in very small amounts. It occurs naturally in the soil in two forms - organic and inorganic - and occurs at lower levels in water, food and air. Because of the concerns about arsenic in the food grown in a garden bed surrounded by boards with arsenic, numerous tests were carried out by various universities, state extension services and various labs in the United States, Canada, Britain and New Zealand to determine the safety. Studies done by Texas AgriLife Extension Service showed insignificant movement of these compounds into surrounding soil.
General findings - CCA banned
The general conclusions were that arsenic levels in vegetables were well within safety limits and only slightly above the naturally occurring levels, but, since children's playground equipment was made out of CCA-treated wood and arsenic would leach out and a lot of "hand-to-mouth activity" was present, there was a higher chance of youth ingesting arsenic. Because of this, on Dec. 31, 2003, the U.S. Environmental Protection Agency banned the use of CCA to preserve wood for residential use.
Present day wood preservatives
Two compounds have now replaced CCA wood in the residential market: alkaline copper quaternary and copper azole - both contain copper and a fungicide, but no arsenic. Small amounts of copper and quaternary do leach, but nothing in ACQ is considered hazardous by the EPA. At this point there has been very little published research on the effects on chemical leaching of these compounds into foods grown in gardens with the new PTL.
If a gardener still has concerns about leaching, then a similar practice can be used as was done with the CCA wood - insert a layer of heavy plastic sheeting between the soil and the wood to minimize leaching of chemicals into the soil. Another option is use a waterproof stain on the wood to minimize leaching. Avoid paint since paint doesn't bond to pressure treated wood like untreated wood.
PTL or expensive alternative
One can use redwood, cypress or cedar; however, this lumber typically is three to five times more expensive than PTL. The use of concrete blocks or bricks is also expensive. Plastic lumber made from recycled plastic is another alternative although very expensive, and there are concerns about the volatility of the plastics and the binders. In general, the alternatives to PTL are more expensive and may have side effects that are not desirable or well-documented.
Research results indicate that there is minimal transference of PTL chemicals to garden vegetables. EPA still indicates the chemicals used in post-2003 PTL are not hazardous. Since there is never a situation where there is zero risk, it comes down to the question of whether the gardener is willing to take some risk (probably negligible) in using PTL or an alternative in the garden.
Either way, various examples of materials used for raised beds can be seen at Victoria Educational Gardens located by the airport control tower. Come take a look for yourself.
The Gardeners' Dirt is written by members of the Victoria County Master Gardener Association, an educational outreach of Texas AgriLife Extension - Victoria County. Mail your questions in care of the Advocate, P.O. Box 1518, Victoria, TX 77901; or firstname.lastname@example.org, or comment on this column at VictoriaAdvocate.com.