Gardeners commonly rely on pressure-treated wood to line raised vegetable beds. An older form of lumber called Chromated Copper Arsenate (CCA) lumber has chemicals which could potentially be toxic to produce.
CCA-treated lumber releases arsenic, copper, and chromium into the soil where plants absorb it without becoming toxic to humans over time.
Arsenic-treated wood
Up until early 2000 CCA or Chromated Copper Arsenate was a standard used in pressure-treated wood products to help protect against rot and insect damage. The bad news is that CCA was found to contain arsenic, which was eventually determined a human carcinogen as leaching from timber products that were treated into soil and water sources ultimately led to arsenic creeping into vegetables and plants. Absorption of this could result in stomach problems, kidney and liver failure, other toxic symptoms resulting in stomach inflammation and failure.
Since CCA was banned for residential use in 2003, new preservatives have come onto the market to replace it. Two key compounds include copper and fungicide – no arsenic! These products are commonly used by lumber manufacturers to slow the rate at which lumber decomposes. When lumber treated with these products is bought for vegetable gardening it is still safe.
However, cases of arsenic poisoning from working with old CCA pressure-treated wood have occurred rarely for gardeners using raised beds made from modern pressure-treated lumber. Children tend to become exposed to this chemical through breathing in sawdust or eating unwashed vegetables grown near it and eventually fall ill from its effects.
Arsenic-treated wood may be used in the construction of a vegetable garden, but this should be done sparingly and only using newer non-arsenic wood (which won’t leach into soil or water). If necessary, old lumber should be lined with plastic before placing it in your growing areas.
Rosen suggests planting compact flowers or vegetables along the edge of your raised bed about one foot from any treated wood, to prevent their roots from meeting its arsenic-rich surface. He further recommends mixing soil from several inches away with that closer in to reduce arsenic leaching into food crops.
Some gardeners may worry that copper and chromium present in their soil will have an adverse impact on their vegetable gardens, but this is unlikely. These metals are more readily absorbed by plants than arsenic; their concentration levels tend to be much lower; additionally, these metals tend to move into vegetables more gradually than arsenic would and any amounts absorbed would fall far below U.S. Department of Agriculture consumption standards for arsenic consumption.
Borate-treated wood
Up until the 1990s, toxic chemicals such as creosote and copper azole were the go-to treatments for wood rot and termite outbreaks. Since then, more nontoxic treatments such as borate have been implemented, and are being widely used by pressure treated lumber companies when producing landscape timbers and other building materials having logs or dimension lumber for landscape applications or as construction materials themselves. Borate’s presence in wood cells kills decay fungi while protecting it against future infestation by wood eating insects such as termites or carpenter termite colonies consuming it all the way.
However, borate preservatives still pose more of a toxicity risk than older arsenic products; organic gardeners should therefore use other means to mitigate it. One such method would be using heavy plastic under wood to isolate it from soil; on top of which an organic-rich soil such as potting mix could further lessen chances of leaching into their garden.
Even with liners in place, gardeners who rely on treated wood may want to consider using it sparingly in vegetable beds; in fact, it may be best to do without altogether if you intend on producing certified organic produce.
To minimize preservative exposure, always wear rubber gloves when handling treated wood and work in a ventilated environment whenever possible. Also use a dust mask when cutting or machining lumber to minimize sawdust inhalation. In case preservatives or sawdust contaminate clothing before reuse; launder it before reuse before cleaning your hands regularly.
When buying treated wood, always check for its “Use Category” label at each end of each board to ascertain its intended use: indoor, outdoor, ground contact or above ground contact and what preservative has been applied to it (May, 2022). In addition, when connecting components to treated lumber using corrosion-resistant connectors ensure these are used – otherwise metal could potentially corrode through the wood into soil or water below.
Copper-treated wood
After the EPA banned CCA use in residential use, many gardeners became wary about using pressure-treated wood or lumber for raised beds treated with this chemical. It is now considered safe to use lumber treated with copper in vegetable gardens provided certain precautions are taken – these include lining your raised bed’s interior with heavy plastic and not letting wood touch soil directly; in addition to wearing protective gear like dust mask and gloves when working with it.
Now, however, residential applications of copper chloroadioxide preservative (CCA) have much safer alternatives compared to their predecessor. Copper azole preservative is becoming an increasingly popular choice as a safer replacement; it has lower toxicity than CCA and serves to protect wood against fungal decay and termites while simultaneously slowing degrade rate of treated wood. Though these preservatives leach some copper into soil through leaching action they do not absorb efficiently by plants so any small amount that comes into contact will unlikely have adverse effect on vegetables or people.
Alkaline copper quat is another alternative to copper azole used for ground contact applications, replacing Penta product that had been commonly used. While similar in chemistry to CCA, ACQ does not have arsenic or chromium and has not been linked with any health concerns in humans; it leaches small quantities of copper into soil that plants may take up readily.
Wood is an excellent material to build raised beds from, however it should not be used to construct benches or seats as it can become rotted over time. Instead, use rot-resistant lumber such as western red cedar or redwood when creating these types of structures in your garden.
Raised beds not only help prevent disease in your plants, but they can make vegetable gardening much simpler as well as easier. They make managing soil conditions simpler as you have complete control of how much nitrogen and phosphorus is added; additionally they allow you to add compost or manure which binds up excess metals that accumulates in the soil.
Creosote-treated wood
Wood is an ideal landscape material, often used to define flower or vegetable beds and construct compost bins. Wood products that go through treatments with preservatives can produce negative impacts on both people and the natural environment. This includes use of railroad ties, telephone poles, and of course, garden beds that use creosote or pentachlorophenol elements which have been decided toxic to plants and wildlife. It’s suggested that the use of these types of products be avoided when making plans for your yard or vegetable garden.
Today’s CCA-treated lumber products are much safer than earlier versions of this product. Creosote was once widely used as an alternative to CCA; however, due to its toxic effects on plants and humans alike. Creosote can release fumes for up to 24 hours after application and seep into soil and groundwater sources that then enter our food chain – ultimately harming both humans and plants alike.
Gardeners today can buy lumber treated with alkaline copper quaternary (ACQ) or copper azole preservatives that are free from arsenic, and water based. While copper leaching from ACQ-treated lumber may harm aquatic life, its leaching poses minimal threat to warm-blooded vertebrates like humans (Can Treated Lumber Be Used for Your Raised…, n.d.).
Though non-treated lumber can be constructed out of various types of wood, most gardeners prefer using treated wood in their raised bed gardens. While non-treated wooden raised beds typically only last one or two years before disintegrating completely, those constructed using treated wood typically last three to five years more effectively.
References
Can treated lumber be used for your raised…. (n.d.). Treated Wood. Retrieved December 23, 2023, from https://www.treatedwood.com/news/can-you-use-treated-lumber-for-your-raised-vegetable-garden-beds
May, N. (2022, January 11). How to Tell if Lumber is Treated – 4 Easy Ways | Conner Industries. https://www.connerindustries.com/how-to-tell-if-lumber-is-treated/
