Reynoutria sachalinensis (giant knotweed or Sakhalin knotweed Japanese オオイタドリ ooitadori, Russian Горец сахалинский, Гречиха сахалинская; syns. Polygonum sachalinense, Fallopia sachalinensis) is a species of Fallopia native to northeastern Asia in northern Japan (Hokkaidō, Honshū) and the far east of Russia (Sakhalin and the southern Kurile Islands).