I just finished the last Eat Your Dirt summit presentation I wanted to hear last night. Coincidentally, it was on hugelkultur with Paul Wheaton. During the discussion, Paul noted that cedar is an awful wood to use in hugels because it secretes something toxic (which is why it takes so long to rot) into the soil. Of course, during my life BP (before permaculture), I built two raised beds from cedar.
My questions are: what leaches out; how far does it penetrate into the soil when used as a raised bed; what harm does it do; is there a remediation for the harm?
I'm hypothesizing I could use micro-organisms as a way to salvage the raised bed soil. Would well-used worm bedding globs (coir and worm-treated kitchen scraps) buried and watered in about six areas in each 4x8 bed be enough to inoculate and get remediation going? I could water with a purchased mycorrhizal solution to add fungi, too. I use a worm tower type of vermicomposter with four bins ready now; the fifth will be ready once it's finally warm enough here in Zone 4a to set out my brassica seedlings. That's a month from now and it will be another month until our frost-free date.
I'm soooo happy I found this site. Love lurking and geeking out with you all.
MizEllie and her Service Dog, MaeMae
Zone 4, NW Wisconsin
My mother owns a vacant lot in a small city. By city order she was required to hire a company to come in an chip all the small brush for fire concerns. Since she eventually wants a house with a garden out there she was concerned that all that shredded cedar left on the ground would have a lasting impact on her garden plans. She took the time to research cedar toxicity to other plants. It's not toxic to every kind of plant (though definitely to tomatoes) but after a year on the ground the mulch no longer hurts tomatoes. I don't know if it leached away or broke down. I think boards and logs would take longer to break down or leach out all the toxic chemicals, but it happens even without any extra work on our parts.
As far as I understand, Cedar is not aleopathic (as other species like Black Walnut are), but rather pest & rot-resistant. This makes Cedar particularly good at raised beds & other outdoor structures (since they will resist rot for much longer than other woods) and particularly bad at Hugelkultur (since the pests and rot are what transform the buried wood into nutrients for plants). There should be no ill side-effects to growing in Cedar raised beds — it's possibly the best wood you could choose!
Cedars, especially Thuja species, have developed chemical weapons against a number of pests and pathogens. Researchers have found that Thuja plicata heartwood contains thujaplicin, a water-soluble tropolone not only inhibitory to various bacteria and fungi, but with anti-tumor activity as well. This antimicrobial activity is probably responsible for the rot-resistant nature of cedar wood. There is, however, no evidence that this substance harms plant tissues.
I always take a common sense approach to toxicity. Every plant is toxic to something, this is the nature of our world. Peppers are toxic to us, but we happily eat them. The variables involved are too numerous to say something is always bad or always good. If you don't see a problem, there probably isn't one. If there is a problem, it's probably more obvious (read: human in nature) than you'd expect. Humans are way better at creating aleopathic compounds than nature could ever be.
As an example: When using wood chips, remember that tree service companies are generally removing unwanted trees (ex: when they get in the way of power lines). Power companies don't want any trees near those lines. So they routinely spray herbicides in the same areas they're removing trees. Maybe those cedar wood chips are reducing microbial activity... or maybe they're covered in herbicides. Evidence is slim that Cedar wood chips have negative effects on plants, but evidence is pretty strong that herbicides kill plants.
The nice thing about Cedar is that it is rot resistant. So it should be used for raised bed boxes. The crappy thing is that it leaches toxic resins into the soil so it shouldn't be used in raised bed boxes. Maybe. Maybe not.
I built several beds of Western Red Cedar at a group home I was working at in Vancouver, B.C. As Cassie said, Tomatoes did not do well. The purchased mushroomcompost might also not have been the best soil to fill the beds with for tomatoes; I don't know... they failed that's all I know; I think it was the cedar. Not exactly scientific, but I'm running with the anecdotal... Carrots, beets, and greens all did well in other boxes. I figure that the tomatoes send out lateral feeder roots that engage in what they are hoping is good biology in or near the wood, and these toxic conditions eventually harm the tomatoes which can't stop the process once it's started. The other plants did not seem effected by it, and this might be simply because of a different feeder root pattern. Peas the next year did fine in the tomato bed. I figure that eventually the surface of the cedar will bleed off it's toxins and that small amount of soil that was toxic becomes re-inoculated with healthy bacterial communities, it will eventually thrive.
The way I look at it is like a cedar shake roof. If the shakes are brand new, they are bright and full of resins, and it's best not to drink the water from them. Once they have weathered to gray, most of the surface resins have been washed way, and the water is much safer to consume.
"Never doubt that a small group of thoughtful, committed citizens can change the world; indeed, it's the only thing that ever has."-Margaret Mead "The only thing worse than being blind, is having sight but no vision."-Helen Keller
I have 19 raised beds made from western red cedar, and have not had problems with tomato mortality or other plant problems that I can positively link to the cedar. I chose cedar because, like others have mentioned here, it is slow to rot, and bugs really don't want to eat it. I have had great results in my raised beds, and if I build more, will continue to use what has worked well for me. Hope this helps!
"Study books and observe nature; if they do not agree, throw away the books." ~ William A. Albrecht