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.
I'd suggest taking some pictures of the new plants and posting them in the Plant Identification forum--someone there will likely be able to tell you whether they're seedlings from your American bittersweet, or if they're the un-American Oriental bittersweet (which does occur in the wild in PA so is also a distinct possibility).
I had bittersweet coming up in the lawn from the main plant. Glysophate slowed it up but it came back until i dug a trench about 6" deep and cut the roots. I haven't seen any coming from seeds in any other part of the yardHerman