Using Wood in Raised Garden Beds

What kind of wood (if any) is suitable for garden beds? November 28, 2006

Hopefully someone can help me with a serious problem. I landscape and put in kitchen gardens. I constructed 3' wide x 9' long raised gardening beds made from 1 x 12 red oak boards. The beds were lined with PVC pondliner and filled with soil. Unfortunately, the top edge of the raised beds started to cup outwards by approx. 1" from the original flat dimension. I guess that red oak is not the best material and the difference in moisture would cause this to happen. Garden sprinklers are hitting the outside of the raised beds while vinyl pondliners keep the inside dry.

My question is how do I bring the warped upper section back? Can I clamp the boards and then glue/screw 11" long 2 x 4 red oak pieces into place which is what I am planning to do? I am afraid that the oak boards might crack, so do I have to wet them before clamping them? Or is it best to wet the inside with towels to bring back the warped upper section before I clamp and install the red oak pieces. Any help would be most appreciated.

Forum Responses
(Sawing and Drying Forum)
From contributor J:
Your right, red oak was a poor choice - not because of the cupping, but because it is not suited for outdoor use. White oak, black locust, hedge, cypress, or redwood would have been a better choice. You may be able to cure the problem in the short term but it will continue to occur because of the moisture differential. If you used a good outdoor urethane finish on all surfaces after it has dried out you may be able to prolong the life of the wood and minimize the cupping.

From Gene Wengert, forum technical advisor:
Here is something to think about and perhaps discuss further. I see the use of a vapor and water resistant poly finish as being a negative. If you coat the entire piece, but then in a short while a crack or two opens up (which is likely with oak), you will get water into the lumber. Then, because of the poly, the water inside the wood will have a hard time escaping and will keep the wood at a very high MC, suitable for decay. This is called the envelop effect. So, it is my guess that coating it as suggested will actually result in more rapid decay. Does this make sense?

From contributor J:
This is kind of like six of one - half a dozen of the other. The same cracks would also offer an escape route for the moisture but decay would still likely occur. If the boards were dry when finished and sealed completely wood movement would be decreased but not eliminated. Either way the red oak will decay in a few years or less in its present location, given the fact it is constantly wetted with a sprinkler. The finish may help stabilize the cupping of the wood do to the moisture differential.

From Gene Wengert, forum technical advisor:
Here is a possible short term solution. Cut off the top 2 or 3" all the way around and then joint a new edge on the top piece that has been sawn off so that when it is put back, it will be straight (vertical). Also, consider turning the piece 180 degrees so any cup is the other direction. Then use epoxy to glue it back.

I doubt that the cracks will help much in drying. It is similar to a balloon that gets moisture inside it. The opening does not really remove much moisture, so the moisture inside the balloon stays forever. Similarly, this is also why we use a vapor barrier only on the inside of a home wall. Use one inside and outside both, and you will have moisture problems within a year. The envelop effect is very serious.

From contributor D:
Trying to fix your mistake of using the wrong wood to begin with, will only prolong the agony. My suggestion is to bite the bullet and replace it with pressure treated or some wood that is suited for the specific outdoor use. Anything else is a waste of time and money.

From contributor T:
Perhaps if you have enough space forget using boards entirely and use bales of hay or straw. When the hay decays in a few years put another set of bales on the outside and push them towards the center. Great insulation in the cold and will feed your plants as the hay/straw bales decay.

I wouldn't recommend cypress unless the cypress is old growth cypress. And I wouldn't recommend using old cypress because it is valuable. Outdoor use of new cypress is almost an urban legend. I find slash pine to be more rot resistant than new cypress.

From contributor G:
For a kitchen garden I would definitely not use pressure treated wood. The arsenic adds a bad taste as well as causes death (cancer at the very least). I used aromatic cedar, cut to 6/4 for much the same purpose. The wood was almost as new and there is very little warp and no rot.

From Gene Wengert, forum technical advisor:
Arsenic in the amount found in CCA treated wood does not move easily into plants (almost all stays in the wood) and even if it does, the amount is far less than is considered safe. However, you can no longer buy treated wood with arsenic in it anyway, so the info provided about not using treated wood by Contributor G is not true. You can use pressure treated wood and not have to worry about arsenic.

A lot of drinking water has arsenic. Further, high doses of arsenic do not cause cancer. I have never found any documentation that arsenic in the soil, which there is a lot of all over the USA and it is natural, causes flavor problems in food.

From Gene Wengert, forum technical advisor:
Has anyone ever wonder how a wood like cedar or cypress can resist insects and decay? The reason is because there are naturally occurring insecticides and fungicides in the wood. Are these naturally occurring chemicals safe for humans? Are they any safer than pressure treated wood? Will these chemicals leach out with water (yes they will) and get into plants and affect the plant safety and flavor?

From contributor A:
About the treated can warp too. I had to repair a south facing fence once in an emergency. Every new board I put up cupped, warped and twisted within a week. Here in Texas the sun worked the south-facing side really well and those boards checked and fell off. I now keep several in reserve in the barn and already dry.

For my wife's raised beds I used 6 x 6 WRC posts. They aren't warping but they are decaying slowly. I never thought about the bales of straw/hay idea. I might have to try that next if I can't find some cheap rock or brick.

From contributor K:
I don't see how chemicals or substances in any wood could get into the gardening beds in this case since the soil is in contact with vinyl pond liners instead of wood. One way to minimize cupping is to build with thinner strips of wood, perhaps ripped from the original boards. 4" or 6" wide boards will not cup nearly as much as 12", and probably not at all if quarter sawn.

From the original questioner:
I would like to thank all of you for your kind responses. I guess I will wet the inner edge with some pieces of cotton cloth and hope it will bend back. Also, I will wait for prolonged rain and try and clamp the boards before putting in support brackets. Thank you again, this forum is very helpful.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor A:

I use cedar or cypress. No need to line beds with plastic. I hope the bottom of the bed isn't lined as rot will occur in roots. I've been doing it organically over 20 years and rarely use cedar anymore as it isn’t environmentally sustainable and is pretty pricey.