Generally, any wood that we use for outdoor structures of any kind should be properly processed using pressure treatments in the factory. There are many different treatments available. These range from traditional treatments that are known to be toxic, to modern treatments that don’t create unreasonable risks to the environment or human health.
As long as you use wood that has been pressure treated with highly toxic chemicals, it is safe to use for vegetable gardens and raised garden beds. These new chemicals are dramatically different from the old treated wood products that leached poisons into the soil.
What is treated wood?
We use treated wood outdoors to protect it from insect infestations, fungi, and rot. And unless we use a rot-resistant wood species, if we don’t use treated wood for structures like decks, pergolas, fences, screens, and so on, it’s not going to last.
When we talk about treated wood, we usually mean pressure-treated lumber that is processed commercially. There are also wood preservatives available for DIY use, but they don’t penetrate the wood thoroughly.
Historically, the only options for pressure-treated wood were highly toxic chemicals. From the 1980s, alternative wood preservatives were developed, but they too are highly toxic.
Today there are several chemical wood preservatives that the United States Environmental Protection Agency (EPA) considers safe to use.
Toxic wood preservatives
The three heavy-duty wood preservatives used throughout the 20th century were creosote, chrome arsenicals, and pentachlorophenol (also known as PCP or Penta). All three were used for pressure-treated wood, although creosote, made from coal tar, was also available for DIY use.
Chromated arsenicals include a bunch of preservatives that contain chromium, copper, and deadly arsenic. Chromated copper arsenate (CCA) poles were commonly used for children’s play equipment, like swings and jungle gyms, and for garden frames.
CCA-treated lumber was also commonly used to make decks and other outdoor structures. A water-based product, it gave timber a distinctly green tinge.
CCA was used from the 1940s until December 2003 when manufacturers voluntarily canceled almost all residential uses. Today, its use is severely restricted to only certified pesticide applicators.
Creosote, also used in the 1940s, is oily, has a strong smell, and is highly toxic to plants. It turns the timber black and is known to cause some materials, including shade cloth to rot.
It is now a restricted-use pesticide that may only be used for industrial applications like utility poles and railway ties.
Penta was registered as a pesticide in December 1950 and was, according to the EPA, one of the most widely used biocides in the U.S. until 1987. Now it has restricted use for commercial purposes only, mostly for utility poles. Only pressure and thermal PCP treatments are allowed.
Safe wood preservatives
The EPA has been the major role player in terms of minimizing the risks of wood treatments and their safe use. As a result, there are several new active ingredients for treated pine and other woods.
Those approved for use in the residential timber and lumber market are:
- Alkaline copper quaternary (ACQ)
- Copper azole
- Copper naphthenate
- Copper – HDO
- Polymeric betaine
Toxicity is low but variable, some treatments make the wood safe, or safer than others. The use of different treatments for wood to be used in the vegetable garden is not specified.
ACQ is a water-based wood preservative that works as a fungicide and insecticide. It has “relatively low risks” and is registered for use in landscaping applications, for garden structures, fences, and aquatic areas.
Today, ACQ is the most widely used product for wood used for residential applications.
Copper Naphthenate is also suitable for fences and landscape timbers and may be used where the wood comes into contact with the ground or with water. It’s regarded as a safe garden product for raised beds.
Copper – HDO is a very new wood preservative that was only registered in 2005. It is registered for garden structures used, for decking, gazebos, fencing, and so on, but not for use in aquatic areas. It cannot be used to build beehives.
Can you build a vegetable garden with pressure-treated wood?
You can build raised vegetable gardens or enclose vegetable gardens with pressure-treated wood, provided the chemicals used definitely are low in toxicity. But researchers do maintain that even the treatments that supposedly keep wood safe have chemicals that may leach into the soil.
The Oregon State University Extension experts maintain that there isn’t any research that suggests the level of chemicals leaching to be a significant threat.
The EPA doesn’t claim the newest treatments aren’t toxic, it states they have significantly lower toxicity than the older types. If you are uneasy about the risks of leaching, you can line the inside walls of raised beds with heavy plastic.
Alternatively, use rot-resistant wood or untreated wood that will likely break down relatively rapidly.
Issues with using treated wood for vegetable garden
The National Pesticide Information Center (NPIC) warns that some preservatives can leach into water or soil. If this happens, there’s a risk of it being taken up by plants.
They have a list of useful links that will give you more information on the topic. One, from the Iowa State University’s Extension and Outreach, which talks about small farm sustainability, also mentions research.
For instance, a study undertaken in 2014 showed that the copper in wood treated with copper azole did leach into the soil right next to the sides of the wooden planter. But the copper levels weren’t higher in the tubers and roots of vegetables like carrots, potatoes, and radishes than they were in these plants growing in beds built with untreated wood.
They were, though, elevated in the carrot foliage. But we don’t normally eat it!
Nevertheless, they recommend using liners with any sort of pressure-treated wood.
Other alternative wood for building A vegetable garden
There are rot-resistant wood species, including redwood that will probably last at least 50 years when exposed to the elements. It is, though, relatively expensive.
Cedar is another option, but while less expensive than redwood, it will only last about 15-20 years. Pine doesn’t weather very well and is prone to attack by insects
Until relatively recently, it wasn’t safe to use pressure treated for vegetable gardens or any kind of raised garden beds. Now there are treatments that utilize chemicals with very low toxicity levels.
The EPA considers some safe, but it’s up to you to decide.