There is a supply of building materials which largely goes ignored and unclaimed.

old cedar fence boards

Those that were not stained must be safer to use. Culling may be necessary and if boxes of any size are being built, doubling might be a good idea. They are usually said to be 1" but are thinner. Anyway, they are often free for hauling away. I have used them for raised beds and they have lasted up to 10 years.

Of course, new cedar boards are available, as well. And, I used redwood for the bearing and south wall in my greenhouse.

Steve

Haha Steve beat me to it, I was going to suggest tje exact se thing. My beds are/were made of fence pickets. They may not last as long as rail road ties, but boy you sure can't beat the price!

We got a couple of large wood crates (appx 8' x 4' and 5' x 5', and about 18" deep) from The Container Store, and took off the bottom (wanted the plant roots to be able to go deeper). When we were shopping there, my husband noticed one of the crates by the employee entrance, and so asked if they wanted to get rid of it. They let us take it, and then we gave them our phone number to call us when they got another, which they did. They get these crates when they are shipped all the signs and banners for their indoor displays, and get rid of the crates.

I called the corporate office to ask if they wood had been treated, and they said they didn't have that information. We don't think it is, though - doesn't feel, look or smell like it. But to be safe, I won't put any plants too close to the edges (google searching found some sites that said any leaching would be most likely be only a couple of inches in from the edge).

So if you are interested in something like this, I'd suggest you contact your closest Container Store and ask. Great they are free!

Marj