I grew this in my garden for at least five years before I knew what it was and, to be honest, I'm still not positive.  It was mislabeled when I purchased it, and as hard as I tried, I had no success in identifying it, soI just referred to it as the "mystery plant."  This mystery may be solved.  This hardy, showy perennial may be Kitaibelia vitifolia, also known as Cedar Cup or Chalice Flower, as well as Yugoslavian Hibiscus, although it is not a Hibiscus. This shrub-like perennial grows to at least five feet tall in my garden (see photo below), with large palmate leaves, and it is smothered in beautiful white, chalice-shaped flowers (with absolutely no trace of pink or any other color) all summer long.  It likes full sun, although mine receives quite a bit of shade each day and doesn't seem to be bothered by that in the least.  Though not a requirement, I often cut mine back in late fall just to keep the garden tidy.  This plant is a rarity in gardens - I've never seen it in anyone's garden but mine.  It's a spectacular perennial and one that needs to be grown more often.  Full sun/partial shade in Zones 5-8.