Clematis pitcheri Torr. & Gray
Growing to an ultimate height of 13 feet (4 meters) Clematis pitcheri, a viny species with urn-shaped bluish to reddish purple sepals, covers a native range in the southwestern part of Texas for the variety Clematis pitcheri var. dictyota. For the second variety, pitcheri var. pitcheri, territory includes sweeps from Indiana, Iowa, and Illinois through far western Kentucky, Missouri, Nebraska, Kansas, Oklahoma, Arkansas to Texas and Arizona. Foliage is highly variable in size and thickness with simple, compound, and lobed leaves all included. Flower color of both outer sepal and recurved tips is also variable. In the wild, it is often found in limestone outcrops in wooded areas of dry to moist soil.
A close relative is Clematis versicolor. However C. pitcheri has more coarsely reticulate leaves than versicolor.
From the Flora of North America description, pitcheri flowers from spring to autumn. However, my experience is that it begins to flower very late (August) compared to the species viorna, glaucophylla, versicolor, crispa in my southeastern PA garden.