Should I Use Treated Lumber For Raised Garden

With all their benefits, it is easy to see why raised bed gardens are so popular. They can create fertile garden spaces for people with limited mobility, poor soil, small yards, balconies, or rooftops. Furthermore, they can be custom-built to meet the gardeners' needs.

If you have the skills and equipment, you may be interested in building your own raised garden beds, which raises the question, "Where do I begin?"

The 7 best wood choices for raised beds:

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Pressure-treated (Yes, really)
Cedar
Cypress
Black Locust
Pine*
Oak
Redwood

Now there are certain considerations with some of these woods and we will explain that but cost, availability, and durability are important factors when building raised beds. There are other options as well including wood alternatives which we will look at as well as the three key types of wood to avoid. Let's dig in with what is probably the most controversial wood choice on our list:

Pressure Treated Wood

Is pressure-treated wood safe for raised beds? Yes. It is possible to find pressure-treated lumber that is safe for use even in a vegetable garden! Depending on your needs, budget, and growing zone, this may be a good option for you to consider.

The purpose of pressure-treating lumber is to make it more resistant to rot and decay. The chemical treatments applied to the wood repel fungi, insects, and bacteria that would find a moist environment like a garden very hospitable. Untreated wood, depending on type and quality, begins to decay within the first year of use.

Since the discontinuation of CCA, preservatives like micronized copper azole (CA) and alkaline copper quaternary ammonium (ACQ) are the most common treatments for residential lumber. Both of these compounds are safe to use, even in vegetable gardens.

There is a warning with this, however. If the chemical treatments were cosmetically applied to the wood's surface, it should not be used for "ground contact." In other words, it is not safe to make direct contact with the soil.

Wood that was truly pressure-treated–the preserving chemicals are equally distributed throughout the wood–will have a label indicating that it is safe for "ground contact use." Reputable building supply retailers can provide you with Safety Data Sheets for their treated lumber which will give you more information about the chemicals used.

Pressure-treated wood is widely available, long-lasting, and more economical than some of the hardier alternatives. If you decide to build your garden with pressure-treated lumber, but still have safety concerns, take the following steps:

Have your soil tested regularly for heavy metals. If there are heavy metals in the soil, take out the vegetables and replant with flowers instead. You can also replace your soil, then replant with veggies.

Use plastic sheeting to line the interior of your garden beds to prevent direct contact between the wood and the soil. As long as you allow for drainage holes, this won't affect the overall quality of your garden.

Note that pressure-treated wood is not acceptable for organic growers under the USDA Organic guidelines (source).

Alternatives to Pressure Treated Wood

If you want to avoid pressure-treated wood, you can still build a high-quality raised garden from a variety of untreated woods. Keep the following considerations in mind as you decide which is best for you:

Cost
Availability
Durability
Resistance to rot and decay

Cedar

Cedar is one of the most popular untreated woods used in raised gardening. Whether you build your own, or purchase a kit, cedar is definitely worth your consideration.

Cedar is a "gold standard" for raised beds because of its resistance to insects and rot. Even untreated, it has a long lifespan.

West coast cedar (Thuja plicata) is the variety you are most likely to find at a lumber retailer. Eastern red cedar (Juniperus virginiana) is even more rot-resistant, but it is much more expensive and difficult to find.

The main problem with using cedar is the expense. It may be worth the price, but if you choose cedar, expect some high costs upfront (source).

Cypress

Cypress (Taxodium distichum) is an alternative to cedar that you may be able to find if you live in an area where cypress trees are grown.

Cypress does not last quite as long as cedar, but it is more resistant to rot and decay than cheaper woods like pine. It is sturdy, attractive, and easy to find if you want to purchase a raised garden kit.

If you want to build your own raised beds, however, you may have a harder time finding it. Even though cypress, along with cedar, pine, and redwood, is a frequent recommendation for raised beds, its lumber is not widely available. Gardeners who live near cypress mills, in the southeastern United States, for example, will have much better luck finding it than gardeners who live in other regions.

If you want to use cypress lumber in your raised beds, you may be able to order it through a local retailer, although this will likely be expensive.

Black Locust

Black locust wood is another excellent choice because of its resistance to rot and decay. In fact, black locust is becoming an increasingly popular lumber for many outdoor projects like fences and decks because it holds up well in all kinds of weather.

The major drawback with black locust is availability. Some states classify it as an invasive species and restrict the sale and propagation of black locust trees. This means that depending on where you live, black locust lumber could be hard to find and the high demand means higher costs.

Check with a local Extension agent or other experts to learn more about the availability of black locust lumber in your area. If you can find it, its hardiness makes it a good candidate for your raised beds (source).

Pine

Pine is by far the easiest wood to find and one of the least expensive. Pine is strong and easy to use, which makes it a popular choice in a variety of building projects, including raised beds.

However, pine is not very resistant to rot, decay, and insects. Many gardeners replace pine beds within five or six years.

If you have a small budget, are a novice builder, or simply do not mind having a more temporary raised bed, pine may be a good option for you (source).

Other Hardwoods

Depending on your location and the availability of materials, you might consider using another wood like oak or redwood.

Be aware, however, that while it seems like a great idea to build a raised bed out of such durable wood, the costs can be quite high.

These woods do offer a bit more rot resistance than pine, and they are quite attractive. However, even if you are willing to pay for it, you may have a hard time finding it in sufficient quantities (source).

Natural Wood Preservatives

If you want to extend the life of your untreated wood, there are a couple of natural preservative options. Their results will vary, but in general, they will not provide your wood with the same protection that ACQ provides.

Raw Linseed Oil

Raw linseed oil, a flaxseed extract, is relatively inexpensive and protects wood from some sources of decay. It works as a sealant that keeps moisture out of porous surfaces. It also acts as a natural pesticide, repelling fungi and other microorganisms (source).

Tung Oil

Extracted from tung tree seeds, tung oil is also a moisture-repellent preservative. However, it is more expensive than raw linseed oil and often manufactured in combination with chemicals that are not safe for gardens. Use caution when purchasing tung oil to avoid some of these concerns.

Other Alternative Materials

If you are not comfortable with pressure-treated wood, but you want a material that will last longer than untreated wood, plastic, metal, or masonry could be the answer.

Plastic

While plastic does not last forever, sturdy plastic planter boxes, plastic barrels, and plastic "boards" hold up well over time. Even child-sized swimming pools, if they are robust enough, make decent raised beds as long as drainage holes are added.

Metal

Barrels cut in half, aluminum and steel planters, discarded cattle troughs, sheet metal, and other metal containers are also common alternatives to wood.

There is some concern about chemicals from metal, particularly galvanized metal, leaching into the soil. This is another instance when it is important to know the origin of your materials before planting anything edible. If you purchase sheet metal or metal containers, make sure you buy from a reputable source who can answer any safety questions you may have (source).

Masonry

Native stone, bricks, and cement blocks are by far the most durable and most expensive building materials for raised gardens. If you want something that will last longer than any wood, and if you are willing to pay for it, masonry could be the way to go.

Cement blocks may affect your soil's pH level, making it more alkaline. This will not last forever and be offset with acidifying fertilizers, if necessary (source).

Woods to Avoid When Building Raised Beds

Despite what you may have seen on social media, the following woods are not safe to use in a raised garden:

Railroad ties and utility poles
Recycled woods of unknown origin
Recycled woods that were pressure treated prior to 2004

Railroad Ties and Utility Poles

Railroad ties and utility poles have been popular options for gardeners because of their sturdiness and availability. However, whatever chemicals have been used to treat the wood will leach into your soil. Because railroad ties and utility poles have been treated with creosote, you should avoid using them in your garden.

Creosote is a tar-like preservative that is widely used on industrial wooden structures because it protects the wood against a wide number of insects and fungi. Creosote contains somewhere between 200 and 400 chemical compounds, some of which may be linked to cancer. Furthermore, creosote is toxic to many plants, which means using creosote-treated wood in a garden is self-defeating.

Creosote-treated wood also can't be stripped and refinished.

If you intend to grow herbs or vegetables, railroad ties and utility poles are not the woods for you! Flowers may fare a little better, but it is best to avoid creosote-treated lumber altogether.

Recycled Woods of Unknown Origin

Using recycled or reclaimed wood is another popular option because it is usually economical and seems eco-friendly, as well. But if you do not know where it came from, it is better to avoid using recycled wood.

Again, whatever chemicals have been used to stain, finish, or treat the wood will leach into the soil. If you do not know where the recycled wood came from, it will be difficult or impossible to know which chemicals are lingering on it.

One common preservative used on both industrial and residential lumber was pentachlorophenol (penta). Penta is a powerful pesticide and herbicide that is now only used as a restricted pesticide. It used to be quite common on fence posts, laminated timbers, and foundation pilings.

Penta is toxic to humans and plants. If you do not know where your recycled lumber came from originally, or what it was treated with, do not use it in a raised garden. Other chemicals may be safe, but if your wood was treated with Penta, it is not safe to use (source).

Recycled Woods That Were Pressure Treated Prior to 2004

If you want to use recycled wood, and you know its origin, there is one last question to answer: was the wood pressure-treated before 2004?

Starting in the 1970s, the most popular wood treatment for residential projects like decks, play structures, and raised gardens, was chromated copper arsenate (CCA). However, growing concerns prompted the EPA to investigate claims that wood treated with CCA leached arsenic into the soil. Furthermore, plants like basil, tomatoes, onions, and lettuce were accumulating arsenic when exposed to CCA in raised beds.

The EPA's investigation showed enough evidence of danger that woods treated with CCA have not been available for residential uses since December 31, 2003.

If the wood you want to use for your raised bed comes from a fence, deck, or other pre-2004 residential structure, it is not safe to use in a future home project.

Conclusion

Before you build a raised garden bed, learn as much as you can about the origins of the wood you choose. Recycled woods, especially industrial materials like railroad ties, are often unsafe for your garden.

Wood that was pressure-treated after 2004 is not only safe for garden use, but is also widely available and inexpensive. If you use untreated lumber, choose cedar, cypress, black locust, or pine, knowing that untreated lumber will not hold up forever against insects and decay.

And there's one other option you may not want to overlook. Have a look at our step-by-step guide to building a raised garden bed with pallets.

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