The answer is simple- Bittersweet Factories.
A "bittersweet factory" is a land area, usually 1/2 acre or more, that has not been managed to control oriental bittersweet infestation. Typically there is no residence or business on the property so it is a case of "out of sight, out of mind" allowing an absentee property owner to ignore the problem. The land may be slated for eventual development so there is no motivation for the owner to invest in managing the natural area.
It only takes 5 years or so for vines to climb trees, mature, and turn into a wooded area into an out-of-control bittersweet factory. Some of the bittersweet factories in town have been producing hundreds of thousands if not millions of berries for 30 years and more. The berries are carried all over town by birds. They pass through the bird's digestive system and are dropped, undamaged and ready to come up in the spring as seedlings.
It's easy to spot a bittersweet factory in the winter after the leaves have fallen, by the hundreds of vines hanging from the trees. The older the bittersweet factory the more pronounced the tree damage is.
How dense is berry development in a bittersweet factory? The following picture, taken in late June, shows an area about 3' by 5' in a large bittersweet factory several acres in size. Imagine how many berries this "factory" produces. No wonder seedlings appear everywhere in the spring!
The proof that nobody escapes the Bittersweet Reaper? Even the shrubs in the median of the McDonalds parking lot are sprouting oriental bittersweet! (see below):
How can large bittersweet factories be managed to limit damage to the rest of the town? The simplest step is to cut the mature vines. This won't stop additional vines from coming up but it will stop the development of additional berries on the cut vines.
The purpose of this post is to encourage "bittersweet factory" owners to take steps to stop berry development by cutting as many mature vines as possible. This one step will dramatically reduce the spread of bittersweet to your neighbors and across the town.