Signup to receive email updates




or follow our RSS feed

follow our RSS feed

Blog Banner

Hort in the Home Landscape

A blog devoted to sharing timely horticulture topics and answering the questions of gardeners and homeowners.
2012-09-08 16 40 14

Using Treated Lumber in the Garden


A frequent question that I get from gardeners is about the safety of using treated lumber in the garden, especially for raised beds. The good news is that there is plenty of research that has already been done on this exact subject and the new treated lumbers have shown to be safe for garden use. Check out a blog post about the safety of the wood in pallets too.

The advantage of treated lumber of course is the fact that it has excellent decay resistance, so it is often used in situations when wood needs to be in contact with soil. However, many gardeners are still concerned that the chemicals used to preserve the lumber could harm garden plants and the people who eat them.

A publication from Penn State University Extension explains exactly what treated lumber is, but essentially it consists of taking preservative chemicals either purchased by the consumer and applied themselves, or by using chemicals intended only for use in commercially pressure-treated lumber.
 
Chemical preservatives can be divided into two major groups: organic (or oilborne) and inorganic (or waterborne). Organic preservatives include pentachlorophenol, creosote (commonly used on railroad ties), and coal tars, while inorganic preservatives include chromated copper arsenate (CCA), ammoniacal copper arsenate (ACA), and acid copper chromate (ACC). The color of the wood is usually the best indicator of the preservative used. CCA-treated lumber is usually green in color, unless dyed to look brown.
 
CCA-Treated Lumber
The toxicity concerns regarding pressure treated wood have been primarily focused on the toxic effects of arsenic in CCA-treated lumber. Studies have shown that, although most of the elements used in CCA are fixed in the wood, some amount of arsenic, chromium, and copper can be dislodged from the lumber as a result of exposure to rain, deck washes, containing brighteners, and irrigation water (Source).
 
Good news is that On February 12, 2002, the Environmental Protection Agency (EPA) announced a voluntary decision by the wood preserving industry to phase out the use of wood preservatives that contain arsenic for any wood products destined for consumer use. That means that CCA treated-treated wood is no longer available to the public and no longer an issue for gardeners unless using older wood purchased before 2003.
 
If you have an exisitng CCA-treated structure in your garden or landscape, the EPA does not require or recommend replacing existing CCA-treated structures at this time. Although the EPA does say that any reduction in arsenic exposure is desirable, it has not concluded that there is unreasonable risk associated with CCA-treated products. Those concerned about existing structures in their yards or gardens can either seal the treated wood every two years with an oil-based stain, or insert plastic liners into the containers to eliminate contact with soil. (Source)
 
Other Wood Products
So what other wood products are available to use in the garden? The EPA has approved alkaline copper quaternary (ACQ) for use in garden structures. This product is higher in copper than CCA, but is free of arsenic. How do you know what the wood has been treated with? If you are purchasing lumber there should be a tag stapled to it that tells you what type of chemical has been used to pressure treat it:
 
Another lumber choice for gardening applications today is naturally rot-resistant wood such as redwood or cedar. These are always a good choice for gardens where food plants (vegetable, fruits) are grown.
 
Reducing Your Risk
Still worried about exposure to CCA-treated or other chemically treated wood? Here are a few tips to reduce your risk (Source):
  1. Use alternative materials. Any possible risks from exposure of plants or humans to CCA metals can be eliminated by not using CCA-treated wood in vegetable garden applications. Alternative materials include:If you choose to use CCA-treated wood for gardening purposes, do not allow sawdust or wood scraps to fall onto garden beds and do not put CCA sawdust in your compost pile.
    • Naturally decay-resistant wood such as eastern or western red cedar, northern white cedar, Osage orange, white oak, locust, or redwood (none of these will last as long as CCA-treated wood, however).
    • Plastic lumber, concrete blocks, brick, or stone; wire mesh for compost bins.
    • Wood treated with ACQ (alkaline copper quaternary ammonium). This is an alternative wood-treatment chemical that contains no arsenic, chromium, or any other chemical considered toxic by the EPA. However, ACQ contains more copper than CCA, and some copper will leach from ACQ-treated lumber as it does from CCA-treated lumber.
  2. If you choose to use CCA-treated wood for gardening purposes, do not allow sawdust or wood scraps to fall onto garden beds and do not put CCA sawdust in your compost pile.
  3. Cover CCA-treated wood used for raised garden beds or borders with heavy plastic to prevent contact with garden soil.
  4. Manage your garden soil to reduce plant availability As, Cr, and Cu.Plant vegetables, especially root crops, at least 12 inches from CCA-treated wood. Concentrations of CCA metals will be highest in soil immediately adjacent to the wood. If plants are some distance from the CCA-treated wood, most of the root systems will be in soil with normal As, Cr, or Cu levels.
    • Maintain soil pH in the near-neutral range (pH 6-7). Solubility of Cr and Cu is greatly reduced in neutral soils.
    • Maintain adequate phosphorus fertility levels. Plant uptake of As is reduced by the presence of phosphorus.
    • Maintain high soil organic matter levels by adding compost or manure. Organic matter strongly binds As, Cr, and Cu and thus reduces their availability to plants.
  5. Plant vegetables, especially root crops, at least 12 inches from CCA-treated wood. Concentrations of CCA metals will be highest in soil immediately adjacent to the wood. If plants are some distance from the CCA-treated wood, most of the root systems will be in soil with normal As, Cr, or Cu levels.
  6. Thoroughly wash all soil from vegetables grown in close proximity to CCA-treated wood. In general, soil will have much larger concentrations of CCA metals than will plant tissues. Thus, human intake of CCA metals can be reduced by removing all soil from vegetables immediately after harvest.
  7. Peel root crops grown in close proximity to CCA-treated wood. Plant tissue concentrations of CCA metals will be highest in roots, especially at the root surface. Thus, peeling root crops such as carrots, potatoes, and turnips will remove much of any As, Cr, or Cu that the plant may have taken up.


Please share this article with your friends!
Share on Facebook Tweet on Twitter Pin on Pinterest

COMMENTS



Email will not display publicly, it is used only for validating comment