Dragon,
I have lived in the area all most of my life.What this will depend on is which part of the valley your in.If your in the Polson area to Somers west shore and your closer to the lake you can push your growing season from mid April to Nov, if it is a good year and with protection.Average years, late April to end of sept with a few things growing on into Oct...The east shore(Polson to Bigfork) is similar but tends to bake off a bit more from the west sunsets.It also gets hit quicker by freezes dropping down from the mts.If your further away from the lake it gets more in the lines of a standard zone 5 with micro climates.My garden in Kalispell runs a very different timeline then my Lakeside/Rollins gardens.When I lived in Polson I used the zone 6 guidelines and added protection.. When down in the charlo area stuck with 4 and 5 info..Of course as always we just have the usual Montana unpredictable weather that can dump snow or freeze everything even in July,,LOl thats a given here anyhow
If you would like just drop me a note on the side I would be happy to offer you some of my experience from the area.
Mary

I used to live in Bigfork. My ex and I owned the bowling alley over looking the lake. I think they either tore it down or are getting ready to.

Anyway, in Ferndale where my house was I didn't plant any annuals until Memorial Day weekend, as a rule. And the season was over by Laborday weekend. We had snow every month on the calendar in the 15 years i lived there, the lake even froze three times. That's cold !!!