Suggestion for type of woood to use for grow boxes

I bought some regular 2x10x12 lumber, not pressure treated, and I haven't
done anything to the wood. A few days ago I looked at the wood on the
interior that is damp most of the time. After four years I can't see any rot
at all. It is my theory that having one side of the lumber totally exposed
helps keep the lumber healthy. If it was damp and in the shade I suspect it
wouldn't be in such good condition.