polygonum
noun
po·lyg·o·num pə-ˈli-gə-nəm 
: any of a genus (Polygonum) of herbs of the buckwheat family with a prominent tubular sheath around the base of each petiole, thickened nodes, and flowers that are solitary and axillary or in spiked racemes
called also knotweed
Word History
Etymology
borrowed from New Latin Polygonum, genus name, going back to Latin polygonus, polygona "knotweed," borrowed from Greek polýgonon, from poly- poly- + -gonum, derivative of góny "knee, joint or knot (in the stem of a plant)" — more at knee entry 1
Note: The genus name was introduced by linnaeus in Species plantarum (Stockholm, 1753), tomus 1, p. 359.
First Known Use
1562, in the meaning defined above