Using Pressure Treated Wood for Urban Garden Raised Beds
Readers have noticed that our raised beds are made from commercial lumber, which is chemically treated. Before we designed our urban garden I did quite a bit of research on the various options for garden construction materials. Here's why we decided on pressure treated wood for raised beds.
Raised bed options
Raised beds are a smart option for urban gardens, especially if they have poor natural soil. Looking through endless pretty garden magazines and books, we found a lot of suggestions for raised bed materials. Here are the options we considered:
Brick - looks pretty, but it holds a lot of heat for our already hot climate. It's expensive and difficult to build, and once it's up it's not moveable.
Logs - also pretty, but they would take up more room width-wise in our tiny space. Plus, living in the city we didn't know where to find perfect straight logs the right size.
Cedar - pretty, and ages well, but after calling around I only found one place in our town that carried it. and it was twice the price of commercial lumber.
Woven willow - so very pretty. So impermanent (rots quickly). So difficult to make. So hard to find the materials.
Stone - expensive, better for a terraced garden or retaining wall than our freestanding raised bed plan.
Composite wood - looks great, but it's expensive and more bendy than wood. We would have to reinforce our 9-foot long beds in the middle with cross bracing.
The more we explored, the more pressure treated lumber started looking like the right option for our raised beds. It's cheap, readily available to city dwellers, rot resistant, easy to work with and doesn't take up much space.
But is it safe?
The risks of pressure treated wood for raised beds
I started reading up on gardeners' objections to pressure treated wood for raised beds. Primarily, warnings centered around the chemicals injected into the wood. The main problems seem to be arsenic and copper.
When I dug a little deeper, I found that chromated copper arsenate (CCA) is no longer used in pressure-treated lumber. Instead, manufacturers have shifted over to arsenic-free salt and copper-based preservatives called alkaline copper quat and copper azole. So I focused on researching copper's effect on edible plants.
First, from stories I've written about organic farming, I know that organic farmers often use copper-based insecticides in farming. In fact, this is a common objection to organic farming - these copper-based chemicals may be organic, but they do leech into the soil and they don't go away. So there is already copper in our commercially-farmed organic produce.
Second, the studies I found (including this great article in Fine Gardening) conclude that copper seepage from pressure treated lumber is negligible. The levels are low, they are limited to the soil area immediately next to the wood, and plants in the study died before they accumulated toxic levels of copper.
Pressure-treated lumber is not certified organic, but since we aren't running a commercial farming operation we aren't interested in certification. In all other aspects we run an organic operation, so we feel pretty "clean" about our veggies overall.
The choice is personal
Once we decided on pressure-treated lumber, we built our beds and tried to give them a little extra time to weather before we added the soil. Our latest bed will end up sitting in the rain and sun for three months before we add soil. Perhaps that will allow some of the copper to seep out from the surface before the soil and plants hit it.
I'm not advocating that everyone use pressure-treated lumber to build raised beds. That's just what works for us, after careful consideration. You may have more space, or more money, or better resources than we did. Let us know what works for you! I always love looking at more pretty pictures of raised beds.