Food-safe Wood Choice for Raised Planter Beds?
Issue: Issue 321
Posted Date: 3/5/2013
After many years as a subscriber and half a century of woodworking, I am stumped. This problem is part finishing and part organic gardening. My wife is handicapped and has a hard time trying to attend to our garden. So I made her raised bed gardens. I have my own band saw mill and 37 acres of timber. I used the cutoff slabs from the mill to line the new raised beds, out of hemlock, as this is our most prominent species. They only last a couple of seasons due to rot, being in constant contact with the moist soil. My wife’s garden cart helps but she still can’t reach the produce very well.
Now I have plans to redo it all with stackable 6X6 timbers. These will be rough cut hemlock, stacked 2 high, for her to sit on as she gardens. I have no wish to replace these every year or two (approximately 1,500 linear feet), so I would like to put a preservative of some sort on them to slow down the rotting process.
Here’s the catch: we do organic gardening, and I cannot come up with a way to preserve them that will not leach into the food chain. I would normally soak the timbers in fuel oil/used motor oil, as this works well, except like any chemical, it will get into the soil. Is there a coating of some kind that is food grade and will stop or slow down the rotting process? I cannot line the garden with plastic because it gets rototilled regularly. Will a vegetable oil of some sort work? Any advice along this line will be greatly appreciated. - George White
Tim Inman: There is no finish which will satisfy your needs. They will be either ineffective, or toxic. So, my best suggestion is that you shift over from hemlock and use a rot-resistant lumber for this project. We have burr oak on our farm, along with osage orange and some other woods that would suffice very nicely. Maybe you have some other wood, too. You would be better served to trade off or sell some of your hemlock for a better, more rot-resistant wood for this job. I'd definitely recommend you stay away from plant-toxic woods like walnut.
Chris Marshall: George, how about using another material choice for your raised beds? Interlocking retaining wall blocks or stone can also create attractive raised beds that will never rot or require chemical treatments that could leach into your vegetables. I'm a wood aficionado as you are, of course, but sometimes certain applications guide me away from wood because of its long-term limitations. For me, it's satisfying to put my energies into projects that won't have to be redone in a few years — if ever. Especially when those projects require as much hard labor as laying timbers or rock does. And that leaves me more time to keep making sawdust on new and different projects.