Wooden garden beds break down – that’s just nature. Many people wonder if they can use pressure-treated woods to ensure their garden bed stays intact for many years. However, pressure-treated woods often contain chemicals, and some of these compounds are highly toxic to humans.
Some treated wood can contaminate vegetable gardens. The most common toxic pressure-treated wood is chromium copper arsenate, which contains arsenic. However, there are a few other poisonous wood treatment chemicals such as creosote, pentachlorophenol, methyl bromide, and copper HDO.
So let’s get down to the facts and talk about what types of pressure-treated wood you should avoid when building a garden bed. I’ll tell you about all the most popular wood treatment chemicals and how safe they are, list some salvaged wood types to avoid, and help you find a treated wood option safe for garden beds.
What Types of Treated Woods Contaminate Vegetable Gardens?
The types of treated woods that contaminate vegetable gardens are those treated with chromium copper arsenate, methyl bromide, creosote, pentachlorophenol, and copper HDO. These chemicals are common in railroad ties, utility poles, decks, gazebos, and other outdoor applications.
So, there are quite a few different chemicals that you can use to pressure-treat wood, and only some of them are toxic. Let’s take a closer look at these harmful chemicals and see what they contain:
- Chromium copper arsenate. This bright green chemical pesticide includes chromium, copper, and arsenic, making it bug and fungus-proof. It is common in construction for beams, shingles, posts, etc. This chemical has been standard for many decades, although the EPA has never approved it. The arsenic in this chemical makes it dangerous for homeowners and wood treatment workers.
- Methyl Bromide. Most pallet boards include methyl bromide, which sanitizes the wood and makes it pest and fungus-resistant. However, methyl bromide has high toxicity to humans, and it can cause liver, kidney, adrenal, and cardiovascular damage when ingested through vegetables or any other garden plant.
- Creosote. Creosote consists of wood or coal tar and is most common in railroad ties and utility poles. It gives the wood a darker finish. This chemical is not as toxic or volatile as the others, but it can still leach into the soil in a garden, making its way into your vegetables. You may experience a burning in the throat, digestive ailments, dizziness, and confusion if you consume it.
- Pentachlorophenol. Pentachlorophenol used to be the most popular wood treatment for killing and preventing pests. However, you should never use wood with pentachlorophenol for garden beds. Even the smallest amount can cause side effects such as cardiovascular irregularities, convulsions, confusion, kidney or liver failure, and other potentially fatal symptoms.
- Copper HDO (Bis-(Ncyclohexyldiazeniumdioxy- copper)). Copper HDO is another copper-based pesticide and fungicide, and it’s safer to use than the other chemicals on this list because it is less likely to leach from your wood into your garden. However, it is highly toxic, and when it does leach out, it may be carcinogenic. So, it’s best to avoid Copper HDO in garden bed construction.
What Types of Salvaged Wood Should You Avoid for Garden Beds?
The types of salvaged wood you should avoid for garden beds are railroad ties, utility poles, decking materials, construction or industrial-grade treated wood, pallet boards, shingles, and roofing materials.
The wood used for industrial and commercial purposes often contains toxic chemicals such as chromium, copper arsenate, or creosote. These chemicals ensure that structures such as utility poles and railroad ties don’t succumb to decay or termites.
However, since these wood structures rarely come in contact with gardens and food, companies usually use the longest-lasting, cheapest wood treatments, paying little attention to toxicity.
So, if you are going with salvaged wood, don’t choose woods that someone once used in commercial or industrial applications. Never repurpose old telephone poles, and don’t try to make use of an old railroad tie.
Likewise, you should never use green-looking wood if you do not know what chemicals it contains. Copper-based wood treatments like chromium copper arsenate, copper azole, and alkaline copper quaternary have a green finish.
While the latter two chemicals are safe for garden beds, chromium copper arsenate has one of the highest toxicities of all lumber treatment chemicals. So, never use green wood if you don’t know what’s in it.
Likewise, older woods likely contain chemicals that we now know to be toxic. So, reusing antique woods like barn wood is risky when building a garden bed.
For the best results, purchase new wood from a hardware store or lumber shop and ask a sales associate for details about the wood’s chemicals. That way, you can be sure that you won’t poison your plants or yourself.
What Kinds of Treated Wood Are Safe for Garden Beds?
The kinds of treated wood that are safe for garden beds include Wood treated with Alkaline Copper Quaternary (ACQ), Copper Azole, and heat. Most other wood treatments pose some risk to your health.
So, let’s look at the safety of each of these wood treatments.
Alkaline Copper Quaternary (ACQ)
Alkaline copper quaternary is a compound that comprises copper and ammonium. Because these components are safe for human consumption in small quantities, ACQ-treated wood is a safe alternative to most other treated woods.
ACQ kills bacteria, insects, and fungus, giving your garden bed more longevity and pest resistance.
For more information and evidence about the safety of using ACQ-treated woods in garden beds, check out this video from Alberta Urban Garden Simple Organic and Sustainable:
Copper Azole
Copper Azole is a relatively new wood treatment, and it first rose to popularity in the 1980s. This pesticidal, fungicidal chemical contains 96% copper, 4% tebuconazole, and water. Since it is predominantly copper, it is safe for gardening applications. Likewise, this chemical doesn’t leech from wood very readily, so the copper and tebuconazole won’t significantly impact your soil.
As this chemical ages, it takes on a gray or silver color. So, if you’ve ever seen gray wood around landscape boundaries, it’s probably copper azole-treated.
Note: Older woods treated with copper azole may contain boric acid, which is not safe for garden beds. So, avoid salvaging or repurposing timber treated with this chemical.
Heat-Treated Lumber
Heat-treated wood is the safest treated wood to use for gardening applications. Heat-treated lumber isn’t as resilient or long-lasting as other boards that contain pesticides and fungicides, but it rarely includes any chemicals. So, this option is the only treated wood that is entirely organic.
How To Make Pressure Treated Wood Safe for Garden Beds
Although I will never recommend using woods that contain toxic chemicals such as arsenic, creosote, pentachlorophenol, or methyl bromide, there are ways to make a treated wood safe for gardening.
These methods will work for a little while, but they aren’t long-term solutions and may interfere with your bed’s longevity. However, they can keep chemicals from leaching into your bed’s soil for a few years. It is best to use these methods on untreated, heat-treated, and ACQ-treated woods.
Place Tarps on the Interior Sides of Your Garden Bed
Although this option may interfere with your soil’s drainage, it is a safe, inexpensive, and simple way to ensure that no chemicals from pressure-treated wood leach into your vegetable garden. You can use any simple non-permeable tarp, such as a clear or colored plastic sheet.
To use this method, slide the tarp around the sides of the garden bed, ensuring that you leave the bottom bare to allow for as much drainage as possible. Then, add soil to your garden and plant your veggies.
Paint or Seal the Wood To Form a Protective Barrier
Paint or a wood sealer can create a barrier between the wood and your soil, ensuring that the toxic compounds in your lumber can’t escape.
However, there are some disadvantages to using this method. Here are the most significant cons:
- Reduces the longevity of your wood. Since sealer and paint won’t allow your wood to “breathe,” using this method may make your wood less durable.
- Paint and sealants will wear away, crack, and chip. Latex paint and wood sealers will eventually chip and wash away, so they are not a permanent fix.
- You’ll still need to line the garden bed. Because latex paints and wood sealers may release volatile organic compounds and toxic chemicals, you’ll still need to line your garden bed with a liner or tarp when painting your pressure-treated wood.
When sealing the wood, use latex paint or a high-quality wood sealer. Here’s how to get the job done:
- Move your wood, paint, sealer, and tools outdoors to a well-ventilated location.
- Use a paint roller or brush to coat the side of the wood that will face the exterior of your garden bed.
- Allow the paint or sealer to dry for the entire curing time specified on the label.
- Place a plastic liner along the sides of your garden bed, ensuring that the bottom of the bed is bare to allow for drainage.
- Place your wood along the sides of your garden bed, tucking the plastic liner around the wood.
- Add soil back to your garden bed and plant your vegetables.
Final Thoughts
Treated woods can leech harmful, toxic chemicals into your vegetable garden, where plants absorb these toxins and poison your produce. However, some treated woods, such as alkaline copper quaternary, copper azole, and heat-treated lumber, are safe since they are not toxic.
If you have been using toxic pressure-treated wood for your garden beds, you can create a barrier with a tarp and paint your garden bed’s wood to keep the chemicals from seeping into your soil. However, it’s best to avoid using pressure-treated woods due to potential health hazards and inorganic compounds in the wood.
