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.
I do a bunch of groundwork for a local carpenter and he has had zero issues with that method. Some skimp on the depth and get paid but the frost giant rears it's ugly head eventually.
