This species has been rather cryptic in our flora, closely resembling P. lapathifolia and P. maculosa. It has just recently been detected and is expected to expand in range.
For a detailed study see Atha, D. E., M. H. Nee & R. F. C. Naczi. 2010. Persicaria extremiorientalis (Polygonaceae) is established in the flora of the eastern United States of America. J. Torrey Bot. Soc. 137(4):333-338.