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.
Looking through endless pretty garden magazines and books, we found a lot of suggestions for raised bed materials. Here are the options we considered:
The more we explored, the more commercial lumber started looking like the right option for us. 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?
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.
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. 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.