We get a little more frost here than you but this is how the good guys do it.
7' hole with 12" of crete in the bottom. Let it set then place the post on top and fill with sand or gravel. There are other methods but that is one of the only ways the frost won't jack the polls out of the ground.
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Ditto - except I'm a bit further south, and when they built the original big pole shed (40X96) the holes were drilled to 6'... I've never had an issue to date - and that was done 40 years ago..
