You know the phenomena that, when you learn about something (in this case a plant), you begin to see it everywhere? I first encountered Japanese knotweed near the home of a family member. She said it reliably grew every year and was attractive. It took me awhile to identify the plant, I kept forgetting to do the research. Then, at a botanical garden, I learned that it was Japanese knotweed, an invasive import. While on trips to Pennsylvania I saw it. And again. And again. I saw hillsides of it, I saw it lining the highway. It is all over the northeast US too. It seemed to be everywhere. This plant can grow by the foot in a day and it crowds out native plants, in Great Britain, mortgages have been denied until the homeowner eradicates it from the property. Beware this plant!
http://www.hglivingbeautifully.com/2014/03/15/8184/
http://www.nps.gov/plants/alien/fact/faja1.htm
From Wikipedia: http://en.m.wikipedia.org/wiki/Fallopia_japonica :