i planted an american bittersweet several years ago ( and, yes, I'm sure it's american) but, now, i'm noticing MANY new bittersweet vines in the scrubby woods surrounding my property. I've been on invasive plant patrol here - zone 6 or so- for 8 years (keeping such plants as grandiflora rose, russian olive, and wild grape to a minimum), so, i am sure this is a new development. is it likley that these new starts are the american or is it just a coincidental proliferation? the leaves are not as substantial or as dark as the one i planted. it seems to grow faster too.