What blue clematis will grow best in Denver area zone 5 alkaline soil and bloom the longest? Anyone have blue clematis from 'Great Garden Plants'?

Welcome to ATP, BlueGardener. Did you mean Crystal Fountain or Franziska Marie?

Bloom time is about three weeks with any of the class 2 clematises, longer by far with Jackmanii (class 3). I've never gardened in your zone so I'm not much help to you.

Maybe if you checked out the ATP database for clematises you might find others you like.
http://garden.org/plants/brows...