soil boxes--what type of wood?

joannaqcw(NY 4/5)July 26, 2012

My old plywood 1x2x3 soil boxes for greenhouse gardening through the winter are starting to decay badly. I want to build new ones and I'm looking for recommendations about materials. I'd like something rot-resistant, nontoxic (or anyway not likely to release toxins into the soil--know this cuts out pressure treated, don't know abut plywood) and as cleanable as possible . Any suggestions would be greatly appreciated.

To be clear, these aren't frames set on top of the soil, they're complete boxes with wood bottoms with drainage holes.

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Assuming you're limiting yourself to wood, here are some considerations:

1) There are forms of treated wood that should be totally fine with respect to vegetables.. but their availability is spotty for the general public. Sodium silicate treatment, or glass wood, is one example (TimberSil is one brand name). The new generation of heat treated wood is another (Thermowood is one brand name). One reason old pallets work better than expected for raised beds is because they are always heat treated. However, this heat treatment is minimal and mostly meant to eradicate bugs and pathogens. The new generation of heat treatment is mostly available in Europe, but if you can find it - it's perfect.

2) Naturally rot resistant wood has three main mechanisms:
a) The wood contains phenolic compounds or other secondary metabolites that inhibit decay and repel insects
b) The wood is dense and heavy with oil that physically excludes moisture
c) The wood contain tyloses in the xylem, small plugs that greatly decrease the penetration of water.

The first mechanism, and to a lesser extent the second, pose potential problems for plants. Polyphenols can be toxic to plant roots, as can oils from tropical hardwoods. A lot of times these problems prove to be so insignificant when they're actually tested that they aren't observed at all. Cedar and Redwood, for example, have long been used for planters with no apparent problems. The big exception is cypress - cypress mulch has been shown to be significantly allelopathic, and so I'd avoid making planters out of it.

The third mechanism is the one that makes white oak so great for wine barrels, and why wine barrels can be used at planters.

This leaves cedar (Western Red Cedar) and redwood (Coast Redwood), and white oak. Redwood is likely very expensive and hard to find in your state, leaving cedar as your probable best option for a natural, economical, long lasting wood. White oak is an option too, but mostly in the form of wine barrels. Buying white oak to make planters would be quite expensive.

The other option is to simply line the boxes with plastic, which will greatly increase the life of any wood you use. In that case, you can use regular plywood or even pine. This is most economical choice in your situation. You can also use a paraffin or wax coating on the wood underneath the plastic to make it last even longer. Don't use polyurethane, as this will only speed up decay in this kind of situation. Using this method will sacrifice some gas exchange through the walls of the wood, but I really doubt this is significant in a hothouse situation.

    Bookmark   July 26, 2012 at 11:35AM
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mea2214(z5 Chicago)

What kind of plastic do you suggest using to line the boxes and how would you apply this plastic to the inside?

    Bookmark   August 1, 2012 at 12:58PM
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