Our Friend Carol Lim has published a great web site about the North Amercan Species of Clematis in Section Viornae. Often called "Leather Flowers" or "Americal Bells" many species are of very restricted range so few people see these beautiful flowers in the wild although hybrids using the red genes from C.texensis are common in cultivation.
We have several plants of, what I am pretty sure is, C. addisonii in our garden. I took these pictures in our graden the last week of May and the first week of June 2007.
I think everyone should try to grow these plants when seed is available or nurseries sell plants. Carol has links to some nurseries on her web site.
Update: Link to Larger version of my Pictures of Clematis addisonii.
2016-2017 Winter Bird Feeder Count Results
2 weeks ago