If you are interested in a smaller variety I can attest to the continuing floriferousness of "Blue River". For me, for the past 2 seasons, it has bloomed from June 1 until Oct 1 while remaining less than 6 feet in height.
Rooguchi blooms from June to October for me - I can imagine what he would do in your zone ^_^. I don't have to cut him back after each flush - he just keeps going. Rooguchi is a real blooming machine for me and one I would definitely replace if I were to move.
I can concur with the long bloom time of Rooguchi. Another small flowered clematis that blooms for me from late June to early September is Betty Corning. However, beware the size estimates for that plant are usually understated. Mine climbs up the 8 foot deck post and the spreads about 10 feet in two directions. It is almost the same size as sweet autumn clematis.
I also agree that Rooguchi blooms a long time another that bloom for a long time here is small flowered Duchess of Albany starts later here than some of them but continues for a good season. Duchess of Albany is planted on each side of an arbor and is well established after years in this location, although for some reason one side was way behind last year while the other side was it's usual robust self.
I have Dutchess of Albany, whiich is a member of a small group, she blooms a long time for me and I think I see a signicant response to fertilization increasing bloo.m ing timeI have two plants of what I believe to be...then again I could be nuts!!I have two plants of what I believe to be Betty Corning and for me they had no labels they are white with a vertical bell shape and a purple band around bottom,they start in May and bloom through August. Lee McDonald
Betty Corning does bloom almost as long as Roguuchi for me as well. I do have BC on a northern exposure and last year put one in where it will recieve more light. It should be interesting to see the differences with the plant in different exposures.