Is this 'Elsa Spath' ?
I believe that I looked this up last year. It's a lovely plant and this year will get more sunlight as we (sad) cut down a large oak that was too close to the house (but gave wonderfully cool summer shade).
Does anyone recall the features of 'Elsa Spath' that differentiate it from, say, other lavender, maroon-anther clematis? Thanks.