Can You Use Pressure-Treated Wood For Garden Beds?

When it comes time to build raised beds, wood is one of the most popular materials. It’s readily available, inexpensive, and easy to work with. However, microorganisms can cause untreated pine to crumble in a single growing season, leading to disappointment and frustration.

If you want a long-lasting wood garden bed, you may wonder if you can use pressure-treated wood. You know this type of wood will hold up longer in your garden, but is it safe to use? I’ll dive into the science of the composition and risks so you can determine whether or not you feel comfortable using it to build raised beds.

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The Short Answer

If you want to err on the safe side, you should avoid using pressure-treated wood for garden beds. While the risks to human and environmental health depend on what the wood was treated with, it can be difficult to determine what chemicals are present. Choosing naturally rot-resistant wood like cedar and cypress allows you to enjoy long-lasting wooden garden beds without exposure to potentially toxic chemicals.

The Long Answer

People use the term pressure-treated wood to refer to wood containing various chemicals. However, the chemicals used to treat wood vary depending on the treatment date and intended use. Some of these chemicals have been proven harmful to humans at low doses, while others are more benign.

Learning more about the chemicals used can help you determine whether or not you feel comfortable using this material to build garden beds.

What Is Pressure-Treated Wood?

Treated wood can contain all sorts of chemicals, but most people use this term to refer to pressure-treated lumber regularly found in hardware stores. It’s important to recognize that the lumber industry has changed over time, so the chemicals found in pressure-treated wood from 2000 are different from those found in today’s lumber. Regardless, people use a similar process to produce pressure-treated wood.

While you probably think of a piece of 2×4 lumber or a log as a solid object, wood is filled with hundreds of tiny holes. Pressure-treating involves applying preservatives to wood and then exposing it to high pressure. The pressure forces the chemicals into the wood pores until the wood, so the entire piece of wood is protected.

There are two main types: ground-contact and non-ground-contact. As you might expect, ground-contact lumber is designed to withstand prolonged contact with the soil, so people use it for items like fence posts and dock piers. Non-ground-contact lumber can withstand moisture and insects but is not intended for soil contact. People often use it for decking boards, porches, and outdoor furniture.

What Does Pressure-Treated Wood Contain?

The chemicals used to pressure treat wood have changed over time. Just like humans have recognized the harm of products like asbestos and lead-based paint, we’ve also learned that some of the chemicals we previously used to treat wood were harmful to human and/or environmental health. Therefore, producers have phased out these chemicals in favor of less harmful materials.

With that said, you can still find old wood that contains these phased-out chemicals. Therefore, it’s helpful to know about the products found on old and new pressure-treated wood. Here are some of the most common substances used to treat wood.

Chromated Copper Arsenate (CCA)

Until 2003, chromated copper arsenate was one of the most commonly used substances. However, due to health concerns, manufacturers voluntarily stopped using CCA to treat wood used in residential settings at the end of 2003.

CCA contains the elements chromium, copper, and arsenic. Humans are exposed to these elements when they touch treated wood, and the elements also leach into the environment over time. If you use CCA-treated wood to create garden beds, the elements can leach into the soil and become absorbed by plants. If you’re growing edible plants in your garden bed, then you may consume chromium, arsenic, and copper when you eat the plants.

Chromium and arsenic pose numerous risks to human health. Exposure to both elements increases the risks of developing certain types of cancer, and arsenic can damage the central nervous system, skin, and cardiovascular systems. Therefore, you should avoid using CCA-treated wood to construct garden beds.

Copper Azole (CA)

After manufacturers stopped using CCA to treat wood available to homeowners, they turned to other chemicals. One of these is copper azole (CA). There are two types of copper azole: type A and type B. Type A consists of copper, boric acid, and tebuconazole (a fungicide), while type B contains only copper and tebuconazole. 

Farmers use tebuconazole to treat fungal diseases on crops, including cucumbers, turnips, and sunflowers. Research surrounding this use suggests that the fungicide doesn’t present acute risk, but chronic use does pose a risk to birds, aquatic invertebrates like snails and stoneflies, and fish. It’s important to note this risk of tebuconazole assumes the fungicide was sprayed on crops rather than used to treat wood.

The real risk of CA-treated wood is related to copper. Although copper is a naturally occurring trace mineral required for human and plant health, too much copper poses health risks. However, CA-treated wood is unlikely to release toxic levels of copper into the soil.

Alkaline Copper Quaternary (ACQ)

Alkaline copper quaternary (ACQ) is another chemical currently used to pressure-treat residential lumber. ACQ contains multiple compounds, mainly copper, DDA Carbonate, and MI/MCI. None of these elements are carcinogenic, but they can be toxic if humans ingest or contact them in large quantities.

With that said, studies have shown that ACQ-treated lumber poses a minor risk to human health. Even if children wipe the treated boards with their hands and then lick their hands, they will be exposed to non-toxic levels.

Is Pressure-Treated Wood Harmful to Human Health?

As I mentioned above, there are many different types available! Knowing which type of pressure-treated wood you’re working with is the first step in determining whether or not it’s harmful to human health. Some of this wood poses both acute and chronic risks to human health, and some of it poses little threat.

Pressure-treated wood used to create garden beds can pose multiple risks to human health. Children may touch the wood and then put their hands in their mouths, leading to exposure. Chemicals may also leach out of the wood and into the soil, where they are taken up by plants. If you consume these plants, you can be exposed to elements like arsenic and chromium.

If you’re concerned about toxins in your wood, you can use untreated wood or an alternative material.

Is Pressure-Treated Wood Harmful to the Environment?

Similarly, different types of pressure-treated wood pose different levels of harm to the environment. Modern pressure-treated wood available for residential use poses little risk to the environment, but older wood treated with CCA and creosote poses a higher risk. Therefore, you should avoid using these older types of wood in and around your garden.

Alternatives

If you’re set on using wood for raised beds but want to avoid the chemicals, don’t grab some untreated pine boards and hope for the best. The wood will quickly deteriorate, leaving you with a pile of rubble in a year or two. A better option is to choose a naturally rot-resistant wood like cedar, cypress, or black locust.

You can also use a non-wood material like galvanized metal. Raised beds made from this type of metal last for years without decaying or leaching toxic chemicals into the surrounding soil.