The American twin is virgin's bower (Clematis virginiana)
is a native clematis in the buttercup family. It ranges from Manitoba, Quebec
and Nova Scotia south to at least Florida. (There's a western variety
Clematis ligusticifolia, with the Spanish name "yerba de chivat", so
technically we've got triplets).
While most sources say that sweet autumn clematis is "Asian", New Zealand claims
it as a native, with the Maori name "puawhananga".
Both vines grow rapidly to about 20' and look wonderful draped over a fence or
climbing up a tree. The bees, butterflies and other pollen gathering insects
love the fragrant flowers; birds like to nest in the thick, tangled vines. The
flowers are followed by attractive feathery seeds. Neither plant appears to
have much forage or medical value. Since many clematis are high in poisonous
alkaloids, this may also be true of virgin's bower and sweet autumn clematis.
Sweet autumn clematis is readily available for sale in local nurseries and via
the Internet. Many garden experts recommend it.
The flowers look alike so the best way to tell the vines apart is by their
leaves: the native plant has toothed leaves and the foreign one has round,
smooth edged leaves.
Since it's much harder to find a seller of the native plant, I suspect that the
Asian plant is easier to propagate, like the Asian bittersweet. So is sweet
autumn clematis a kudzu in the making? It has gotten loose and is said to
found in the wild from Southern Connecticut to Florida and west to the
Mississippi. I see it climbing over hedges and fences in Stamford urban areas.
While it's not high on the Canadian and USA invasive menace lists, it is
starting to make a few, particularly in the Southeast.
BTW: there are many, many lovely highbred clematis vines that make great garden
plants. They come in a wide range of colors and if you shop around you can find
ones that bloom most of the summer and a great mixer with roses (that tend to
quit in the heat). Most these "garden" clematis like their roots kept cool and
moist in shade but must have their tops in good sun to produce flowers. They
take a couple of years to get started but are worth the wait. To my knowledge,
in the northern USA and Canadian, none are invasive; but in the South and West,
a few varieties are listed as potential trouble makers.