Treated Lumber:
Is it Bad for SIP Wicking Beds?

The very suggestion of using pressure-treated lumber is enough to trigger a few gardeners into an utterly vehement rage. My use of treated lumber has elicited some of the most passionate, and at times derogatory, responses from random viewers on my YouTube channel.

What's the deal? Is pressure-treated lumber an absolute no-no in the garden? Will it slowly poison your vegetables and anyone who consumes them? Where did all of the concerns come from? And are they still relevant today? What about when it comes to SIP raised beds?

Does Treated Lumber Contain Toxic Chemicals?

CHROMATED COPPER ARSENATE: When people get all bent out of shape over treated lumber, it's really chromated copper arsenate (CCA) that started it all. CCA wood treatment contains chromium as well as arsenic. This is a super-old wood preservative which has been around since 1933. Quite understandably, people don't like the idea of toxic heavy metals leaching into their soils. For that very reason, the US lumber industry hasn't been selling CCA to consumers since 2003. Yet, more than two decades later, consumers may still act as though the treated lumber they get at a big box store is going fill their soils with lethal toxins.

NEW TREATED FORMULATIONS: These days, the only metal commonly used in pressure-treated lumber is copper. Perhaps you've read about alkaline copper quaternary (ACQ). This is a much safer alternative since it will not expose you to chromium or arsenic. Of course ACQ is not the only formulation out there. In addition to copper quaternary, you may also come across copper azole (CA). On treated wood labels you might see copper azole listed CA-C. Be careful not to confuse this with the old CCA. Those are not the same thing. Unlike arsenic, copper is an essential trace element. It is generally safe and not regarded as a toxic metal. In fact, copper is even permitted for use in organic gardening!

Pressure Treated Lumber Tag - MCA Ground Contact Use
This micronized copper azole wood treatment does not contain heavy metals and is approved for ground contact.

MICRONIZED COPPER: As a further refinement in wood treatments, the lumber industry now offers MCA (micronized copper azole) and MCQ (micronized copper quaternary). With micronized copper treatments, the particles are so small that they are able to seep more deeply into the wood pores. They become physically embedded in the wood, reducing the amount of copper required. That means the copper is less likely to escape through leaching. This makes micronized copper a great option for raised bed gardening.

Will Treated Lumber Chemicals Leach Into Vegetables?

As we've already seen, modern formulations of pressure-treated lumber don't pose a threat like older arsenic / chromium formulations. Even still, out of an abundance of caution (or perhaps paranoia), some gardeners still worry that these treated "chemicals" will get sucked up into their plants. No doubt they fear cancer or some other debilitating disease and eventual death... Currently, there is no credible research to justify such fears. However, even if there had been some legitimacy to those concerns, this doesn't apply to SIP raised beds.

NO DIRECT CONTACT: The inner walls of SIP raised beds are completely lined with an impermeable barrier. The bottom reservoir section has a thick, water-tight liner. The upper sidewalls can be lined with whatever lining suits your needs. So the garden soil in an SIP never touches the pressure-treated sidewalls. If there is no direct contact, then how would you expect the copper or other treatment chemicals to migrate into the soil? In my most recent design revision, I even wrap the lining over the upper edge of the side walls. Then a thin strip of wood is applied to secure it. I use ripped pieces of fence plank. If you are afraid of pressure-treated chemicals, then you could use cedar fence planks.

SIP Raised Bed -Wrapping Treated Lumber with Plastic Barrier
The inside of SIP beds are wrapped with a lining which prevents soil from ever touching treated lumber.

Is Treated Lumber Safe for Organic Gardens?

SAFETY vs OMRI CERTIFIED: Are you a home gardener who likes to do things organically? Or do you run a USDA certified organic agricultural operation? Depending on who you are, you might have very different needs and standards that you must meet. Organic certified operations adhere to very stringent criteria in order to maintain their organic status. On the other hand, home gardeners may simply be focused on growing pesticide-free vegetables that are safe for their family to eat. Secondly, they might like the idea of maintaining healthy soils.

ORGANIC CERTIFIED? Can you expect to maintain USDA organic status if you are growing food in contact with pressure-treated lumber? No. According to the National Organic Program Regulations, "a producer must not use lumber treated with arsenate or other prohibited materials for new installations or replacement purposes in contact with soil or livestock" -Section ยง205.206(f). The focus seems to be on arsenate, which is no longer a concern with modern pressure-treatments.

However, "other prohibited materials" apparently includes formulations like alkaline copper quaternary (ACQ) wood. This does not mean that food grown in beds constructed from ACQ lumber would be unsafe or toxic to humans. But still, it would not meet the organic standard in its current form, since such formulations do not appear on the USDA National List of synthetic substances allowed for use.

SAFE FOR HOME GARDENS? A home gardener is not required to jump through all sorts of fiery hoops in order to grow their own food. There's no red tape and no inspectors dictating what you must do. It's perfectly fine to take an organic approach to your gardening, even while using pressure-treated lumber for your garden beds. You can use organic fertilizers and organic pest control methods. You can promote healthy soil. Treated lumber will not sterilize your beds, killing all worms and beneficial microbes. And as long as you're not using old CCA boards, there's no need for worrying about heavy metal contamination.

Raised Bed Garden Built with Pressure Treated Lumber
The lumber in this raised bed garden received modern wood treatments which do not cause arsenic exposure.

Why Not Use Cedar Instead of Treated Lumber?

You can! Cedar is certainly a popular alternative, preferred by some gardeners. It is completely compatible with OMRI organic standards. If you want cedar that will hold up as long as possible, then it's important to select boards that are cut from dark colored heartwood. The lighter sapwood is not nearly as resistant to insects and decay.

The darker heartwood is from the inner portion of the trunk and it is full of... chemicals. Well, naturally occurring compounds which protect the wood. Things like resins, phenols and terpenes. Cedar wood even contains a unique chemical called plicatic acid which can damage lung tissues. That's why you should wear a respirator if you're cutting the wood and kicking up a lot of sawdust.

PRICING? Still though, probably the biggest reason to opt for treated lumber is because cedar can be so much more expensive. At a local box store, four 2x4x8 cedar boards were going for $68.72. Meanwhile, you could get one 2x8x8 and one 2x10x8 (pressure-treated, ground contact boards) for $24.38. So the cedar would cost more than DOUBLE the price! And it's not even approved for ground contact.

LONGEVITY: Despite the claims of durability, cedar is still capable of degradation. Compared to pressure-treated lumber (rated for ground contact) most modern cedar won't last as long. Such cedar is not intended for ground contact. Sure, it should hold up well when suspended vertically on a fence. In that application, it gets good air flow and never touches the ground. But even cedar fences are typically attached to treated fence posts. The pressure-treated wood simply lasts longer than most of the cedar boards you'll find at the store.


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