You have mentioned Bittersweet several times in recent months. This is a plant that I have taken an interest in but don't recall seeing much information on them regarding bonsai culture. Care to elaborate? Perhaps in a new thread.
Both english ivy and bittersweet are probably classified as invasive, and may even be on the noxious weed hit list.
If they have a virtue as bonsai it is once staked up right the stem gets woody enough to support the plant tree-fashion. Virginia creeper leaves don't reduce, the other two do.
New growers seem to be perpetually in search of stock to collect (for free) these two that I have featured have enough good qualities to be starter plants, and need pruning on a regular basis, so they noticably grow.
And have the potential to regrow a bad trim. Beside being vital as a glutton (wolverine) in the woods.
Because they are invasive, nobody is "protecting" them, and most property holders will permit collection. Their north country cousin are larch, which will volunteer in any northern VT NH ME ditch.
Practice on the poorer country cousins. Buy better stock once you've kept a weed alive for a while.