The shrub you are enquiring about is a Japanese knotweed (Polygonum cuspidatum Sieb. & Zucc.). It is an invasive species from the Buckwheat family.  Native to Japan but it found a new home in Europe and the U.S. in 1800’s. There are two species: Japanese knotweed (Polygonum cuspidatum Sieb. & Zucc.) and Giant Japanese knotweed (Polygonum sachalinense) that has more prominent heart-shaped leaves. Knotweed is commonly found along the river banks, meadows, railroad tracks, at the edges of wooded areas along the creeks, any wet disturbed areas, etc. Originally, it was sold through the seed and plant catalogs but soon become noxious weed. It sends roots, rhizomes to be exact, 10 feet deep, up to 60 feet sideways and shoots above ground grow about 3 feet a month. With such growth habit it does not take very long before it takes over. It is very difficult to get rid of. On the positive side, it is edible and loaded with vitamin C and resveratrol. Young growing tips and shoots could be eaten fresh or cooked. (http://www.eattheweeds.com/japanese-knotweed-dreadable-edible/ ).