The hydrangeas that throw seedlings in my garden are H. serrata and H. paniculata (tardiva) - do you have those? I've never had an H. macrophylla (mophead) or an H. arborescens produce offspring.

My seedlings are normally shorter than your plant, which seems more like a vine, maybe a honeysuckle or bittersweet, but ... it's hard to tell.

I am not sure what it is, but it doesn't look like a hydrangea to me. Does the bruised foliage have an odor?

There are some pretty amazing IDers on the Name That Plant forum if you want to ask there.