When you’re spending hours every week nurturing, weeding, and hand pest-controlling your organic garden, the last thing that you need is for your raised garden bed frame to leach heavy metals into the soil. But that’s exactly what happens when you use pressure-treated wood. What are the alternatives?
The best option to extend the life of wooden raised beds is to select thicker hardwood like cedar, redwood, and cypress. 2 inches will last a lot longer than 1 inch, and adding a plastic lining will help too. If you’re in a wet climate, then treat with beeswax, tung oil, Eco Wood Treatment, or LifeTime Wood Treatment.
Is It Safe To Use Pressure-Treated Wood For Raised Garden Beds?
Studies have shown that the chemicals used to pressure-treat wood leach into the soil. The most toxic leak is from CCA, which leaches arsenic. ACQ leaches copper and CBA leaches copper boron azole, both of which are less toxic than CCA to humans but should not be used around ponds or aquatic life. Since the EPA restricted the use of CCA in 2004, you’re unlikely to buy CCA from a lumberyard, but CCA is commonly found in recycled materials. If you’re planning to upcycle an old fence or deck, double-check the origin to make sure it was not pressure-treated.
Pressure-treated wood should not be used for vegetable gardening or around aquatic life. If you need to use it for vegetable gardening, then line the wood with plastic sheeting, plant vegetables 12 inches away from the wood, and peel root crops before eating (roots absorb more copper than other plant parts, but most of it is stored near the peel).
You cannot use pressure-treated wood of any kind in certified organic gardens. For the home grower, certification doesn’t matter as much, but it’s still good to follow as many organic guidelines as you can.
6 Best Non-Toxic Wood Treatments for Raised Garden Beds
The good news is that you don’t need pressure-treated wood to extend the life of your raised garden beds. There’s many eco-friendly methods to choose from.
While not actually a wood treatment, it pays to invest a bit more money in FSC hardwoods like redwood, cypress, and cedar. These woods are naturally rot-resistant and will last much longer than cheaper softwoods like pine. As they rot, they’ll rot from the inside out, so your structure will remain in better condition for longer.
Also, the thicker the wood, the longer it will resist rot. If you can, go with a 2×4 rather than 1×6.
The biggest cause of rot in wood raised garden beds is from being pressed up against perpetually moist soil. By stapling either landscape fabric or plastic tarp against the wood on the inside, you’ll prevent some of the moisture from reaching the wood. If you’re in a relatively dry climate, this might be all you need.
Landscape fabric will allow more moisture to pass through than plastic will. If you need to cover the bottom, like with an elevated raised garden planter, then use landscape fabric over chicken wire to ensure adequate drainage.
Derived from the tung oil tree, tung oil creates a water-resistant coating for wood and is often used on wooden boats. While all parts of the tung-oil tree are toxic, the actual oil is not (it doesn’t contain the toxin albumin) and is approved by the FDA for use on surfaces that come into contact with food. It dries faster and darkens less than linseed oil, and creates a hard finish.
Yes, you read that right, beeswax. For millennia, people have used beeswax to waterproof everything from waterproof shoes to cutting boards to food wraps. Waterproofing the wood will keep it from absorbing water and thus from rotting.
To use beeswax, you’ll need to combine beeswax with walnut oil and wood charcoal. Keeping Backyard Bees has an excellent recipe.
Unlike the above, Eco Wood Treatment is a specific brand that’s made from a plant and minerals. It comes in a 2oz box of powder that you mix with water before applying. It’s safe for plants and pets, and it claims that you only need to apply it once to provide protection from sun and water.
Another specific brand product, LifeTime Wood Treatment is a non-toxic powder that will also age the wood gorgeously dark. The wood color will shift under the first treatment, then continue to darken or silver under the sun.
Wooden Garden Beds FAQs
How Long Will an Untreated Wood Garden Bed Last?
How long an untreated wood garden bed will last will depend on the type of wood, the thickness of the wood, and your climate. In relatively dry climates with cedar planks, untreated wood can last up to 10 years (sometimes even longer). In wet climates like Oregon and British Columbia, even with cedar planks, they may only last 2 to 5 years.
Does the Type of Wood Matter?
Yes, the type of wood will change how long it’ll last without treatment. Hardwood like cedar, redwood, and cypress is more expensive but are more rot-resistant than cheaper softwoods like pine. The thickness of wood will also affect how long it lasts, with a 2-inch thick board lasting years longer than a 1-inch board.
Read our 9 Best Wood Options for Raised Garden Beds in our detailed article.
Does My Zone Change How Long Wood Lasts in Soil?
Humidity is the biggest factor for how long untreated wood will last in moist soil. High humidity (especially paired with a lot of rain) will rot untreated wood faster (2 to 5 years) than in dry areas (10 years).
If you’re in a zone where the temperature frequently rises and falls below freezing temperatures, both untreated and treated wood won’t last as long. Wood swells and shrinks depending on the humidity, and humidity rises when it’s warmer and falls when it’s colder. Rapid fluctuations can cause structural problems that weaken the wood.
Can I Paint My Raised Garden Bed?
Yes, you can paint raised garden beds, although many people prefer their natural wood appearance. Select a non-toxic water-based paint and avoid painting inside the bed where the soil touches or use a plastic liner to keep the soil off the paint. Keep in mind that paint (even non-toxic paint) increases the risk of chemicals leaching into the soil, so you may wish to avoid the risk and leave your raised garden beds au natural, especially if you’re growing vegetables and/or an organic garden.
What Are Some Alternatives To Wood Treatments For Raised Garden Beds?
If you’re looking for long-lasting raised garden beds without the use of wood treatments, skip the wood and try these alternatives:
Stone, Brick, Or Concrete. Stone and brick are very attractive and durable options for raised beds, while concrete slabs can be cheaper but still durable. Although they’re more expensive than wood, you can save money by upcycling. Stone will also help heat garden beds in the spring and late fall, while also holding heat during the night.
Galvanized Steel Metal. Coming into vogue as a raised bed option, quality galvanized steel garden beds are an excellent and safe choice for wet and humid climates. The zinc coating will keep it from rusting, and there’s no leaching of toxic metals. You can even get them at different heights so you can actually garden while standing up. They’re also easy to assemble.
Mound soil. If the only reason you’re building a raised garden bed is because of weeds or crummy soil, then you can do the same by just mounding new soil and compost without a border. The design will become more flexible, and as a bonus for all of you gardeners suffering from slug and snail attacks, there’s no border there to become a habitat.
- Railroad ties. They’re treated with the carcinogenic creosote. If you don’t want treated wood in your garden, you definitely do not want railroad ties.
- Unknown origin recycled/reclaimed wood. Yes, it seems sustainable to upcycle old wood, but a lot of old wood is pressure treated with CCA. If you know the origin and you know it’s untreated, then use it. But if you don’t know, stay on the cautious side.
- Used car tires. Another trendy upcycling project, used car tires contain a host of heavy metals that they then leach into the soil. They’re also flammable and very hard to put out once they’re on fire (one reason they make a terrible mulch!).
Jamie is the founder of The Backyard Pros. When he was 15 years old he started working at a garden centre helping people buy plants, gardening products, and lawn care products. He has real estate experience and he is a home owner. Jamie loves backyard projects, refinishing furniture, and enjoys sharing his knowledge online.