Clematis montana's in Zone 6
Has anyone in Zone 6 grown any montana clematis? Some catalogs and sources say it is hardy in zone 6 - 9, others zone 7 - 9. I bought one anyway, the 'Mayleen' variety. It is said to grow 20, 30, even 40 feet tall, making it excellent for growing into a tree. I've got it below a white birch tree, hoping it will make it at least to 20 feet. I'm wondering if it could get to 40 feet in my zone, 6. Any thoughts?