Pressure-treated wood is a tempting building material for most people who want to build raised garden beds, as it lasts longer than most other materials and keeps your garden pest and fungus-free. However, there are many chemicals in pressure-treated lumber, and some are not safe for garden beds. Copper azole has risen to the forefront recently as a garden-safe pressure-treated wood, but is it actually safe?
Copper azole is safe for raised beds if it does not contain boric acid. Lumber treated with copper azole may leach some copper into your garden beds, but the levels are negligible and low enough that they will not interfere with the quality of your plants or your health.
Let’s dive into the facts and discuss whether using copper azole-treated woods in garden bed construction is safe. I’ll tell you about the types of copper azole, the benefits of using the safer varieties, and whether it’s appropriate for organic gardening.
The Toxicity of Copper Azole
Copper azole is toxic, as it is a fungicide and pesticide. There are two types of copper azole, and type B is much safer and less toxic than type A. Still, you should be cautious when cutting, sanding, or handling wood treated with either type of this substance.
The chemicals used to manufacture pressure-treated lumber were once more toxic than today. Micronized copper azole (MCA), or just copper azole, is an alternative to chromium copper arsenate (CCA), which used to be everyone’s go-to pressure-treated lumber for indoor and outdoor building projects. However, as the name suggests, CCA contains arsenic.
Since science has proved that arsenic is incredibly toxic, scientists recently went to work on developing other copper-based wood treatments, such as copper azole and some different chemical combinations.
Copper azole is a much safer alternative to CCA, and it’s one of the fastest pressure-treated woods to use in garden beds. Copper-treated wood has been popular for many years, and you’ll recognize it by its greenish-to-gray color.
However, you’ll still need to factor in that there are two types of copper azole, A and B, and one contains an extra ingredient that may make it unsafe for gardening.
Let’s look at the toxicity of these two types in detail:
Type A
Copper azole type A predates type B. It contains copper, tebuconazole, and one other ingredient – boric acid.
Boric acid is a common household chemical used in cleaning and soapmaking (among many other purposes). However, it’s also toxic, and you’ll find it in roach killers, rat poison, and other hazardous substances.
So, because copper azole type A contains boric acid, you should never use wood treated with it in your garden beds.
If you are unsure which copper azole type your wood contains, refrain from using it. It’s not worth poisoning yourself!
Type B
Copper azole type B is best for garden beds because it contains copper with a minimal amount of tebuconazole (usually around 4%).
The active ingredient is copper, which works as a pesticide and fungicide. However, tebuconazole is exceptionally effective as a fungicide. Still, according to research by the EPA, this chemical has low toxicity when people consume or touch it, making it safer for garden beds than most pressure-treated woods.
Experts at the University of Iowa have found that copper azole type B only raises the copper levels in some plants. For example, while the copper did not affect potatoes, carrots, and other highly absorbent root vegetables, the copper quantities in the leaves of these plants were higher.
While the levels were 10 to 100 times lower than the toxic level, they were still higher.
So, the concoction of chemicals in copper azole type B is safe for garden beds. Although it will still leach some copper and tebuconazole into your soil, it is not enough to poison you.
Pros and Cons of MCA-Treated Wood for Raised Beds
Micronized copper azole (MCA) does a lot for your garden bed, but you still have to consider the disadvantages of using chemically treated lumber in your garden. Depending on the plants you wish to grow, how you want to use those plants, and how wary you are of chemicals, copper azole might or might not be a good fit for your garden.
So, let’s weigh the pros and cons to determine the optimal application for MCA-treated lumber in horticulture.
The Pros
Fungicidal and Pesticidal
The most significant benefit of pressure-treated lumber is that it protects your garden bed from pests and fungal infections. Thus, you won’t have to deal with termites, caterpillars, moles, slugs, squash bugs, weevils, blights, white powdery mold, etc., when you use this wood for your garden beds.
Longer Lifespan
Pressure-treated lumber lasts longer than heat-treated or untreated wood. Since copper azole resists and kills fungus, it won’t rot. Thus, beds made from copper azole-treated lumber can last for many years, removing the necessity of replacing and rebuilding your garden beds.
Low Toxicity Levels
While copper azole still leaches some copper into your garden, it’s not enough to affect your health. Compared to other pressure-treated woods, it’s the best option.
The Cons
It’s Still Pressure-Treated Wood
Pressure-treated wood invariably contains some chemicals. So, if you want to reap the benefits of using it in your raised beds, you’ll have to be comfortable with some substances leaching into your garden.
Unclear What’s Inside
It can be challenging to determine what’s inside pressure-treated wood. Knowing what chemicals are inside your wood is critical whether you are salvaging or purchasing your wood at a hardware shop.
However, some sellers don’t know whether their copper azole wood contains boric acid or other chemical preservatives. So, you’ll have to find a reputable source, which may take some patience.
Chemicals Can Leach Out
The chemicals in pressure-treated wood leach out more quickly when the soil pH is too high or too low. When your soil is between a pH of 6 and 7 (the ideal conditions for growing plants), the chemicals in copper azole will not leach into your soil very readily.
However, if the pH is too low or high, the copper will mobilize into your soil, raising the potential for toxicity. This concept is true for all other chemicals in pressure-treated lumber.
Disposal Difficulty
Because of the substances in copper azole and other pressure-treated woods, you cannot compost or burn them safely. So, you’ll have to throw them away, and they will ultimately end up in a landfill (which isn’t the most sustainable way to dispose of wood).
Copper Azole Isn’t Suitable for Organic Gardening
Copper azole is not good for organic gardening. It is not organic-approved since copper azole contains tebuconazole and may contain other preservatives or chemicals, such as boric acid.
Since the USDA National Organic Program lists that pressure-treated wood is unsuitable for organic gardening, it’s not suitable for commercial organic gardens. In addition, if you want to feel confident that your crops are chemical and pesticide-free, you should not use pressure-treated lumber in raised garden beds.
If you want to learn more about growing vegetables in raised beds made with pressure-treated wood, you can read my article: Can Treated Wood Contaminate Vegetable Gardens?
Tips for Using Pressure-Treated Lumber for Raised Beds
Now that we’ve been through the nitty-gritty of how copper azole-treated wood affects your garden let’s discuss some precautions and tips for creating your garden beds.
I’ll give you some recommendations for keeping your garden bed from becoming toxic and tell you some of the peculiarities of using pressure-treated wood.
Use This Wood for Ornamental Gardens
Copper azole is safe enough for vegetable gardens but best kept for ornamental gardens. Since you will not consume your ornamental plants, it won’t matter how much copper is in the soil in the bed.
Line Vegetable Garden Beds With Plastic
If you want to eliminate the chances of any copper leaching into your soil, line the sides of your garden beds with a tarp or other plastic lining. The plastic will serve as a barrier to keep the chemicals in your wood from entering your growing media.
Wear Masks & Eye Protection When Sanding or Cutting
The chemicals in pressure-treated wood are in the wood’s core, protecting people from inhaling and handling them. However, when you sand or cut the wood, you release potentially toxic dust particles that may affect your eyes and respiratory system. So, be cautious and always wear your safety gear when constructing a raised bed.
Dispose of Pressure-Treated Woods Responsibly
Although copper azole is not particularly bad for the environment, burning it will produce toxic fumes, and the copper can negatively affect compost. So, check with your local government on how to properly dispose of pressure-treated wood.
Final Thoughts
Copper azole is one of the safest pressure-treated woods for raised garden beds. However, it still contains chemicals that may slowly leach into your garden’s soil, which is inappropriate for organic gardening. In addition, only copper azole type B, which does not contain boric acid, is safe enough to use in vegetable gardens.
Copper azole is ideal for ornamental gardens since it may raise the copper levels in your soil. However, using it in a vegetable garden will not prove toxic, especially if you keep the soil’s pH balanced and line the bed with plastic.
