A.Paul Fisette responds: In general, pressure treated wood will not rot. The chemical treatment is poison to the decay organisms that attack wood. Most treated wood is Southern pine, which is an easy-to-treat species. Although the chemical penetrates deeply into the wood during treatment, the heartwood is somewhat resistant to penetration. So if a 4x4 is cut from the central portion of the tree, the innermost core of the 4x4 may not be completely impregnated with chemical, so the core could rot.
I think that it is best not to set posts in concrete. Pouring concrete around the post may save time backfilling and tamping, but it will trap water around the post. This could lead to core rot, and may encourage carpenter ant infestation. Instead, backfill the post hole with free-draining soil or stones to encourage drainage. The best way to...
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