I am building two large box gardens - 16 by 4 ft. and about 3 ft deep.
I was wondering about how concerned I should be about slowing the wood
rotting over time. I am using redwood. I was thinking of lining the
interior with plastic, but I don't want it to leach anything toxic
into the soil. Do I need to be consider a lining, and if so, is there
a kind of plastic that is recommended?
Lining won't really help you much. Some, but not much. Plastics also
deteriorate with UV exposure fairly quickly.
Do you have termites? If so, you might consider concrete block or similar
instead. Heartwood is +/- more resistant than sapwood, but certainly not
termite-proof. If you're in a termite zone, sooner or later, they'll
find your wood in contact with soil. Likewise, sapwood is not nearly
as resistant to decay as heartwood, and old growth is more resistant then
second or third growth.
Here's the Forest Products Lab book on wood properties... I think
you want Chapters 3 and 13, especially table 3-10 and the map in ch. 13.
Depending on what you're buying, you may be paying premium prices for
a product that won't do as well as something less expensive.
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