What is the name of the broad expanse of flat land in the western United States and western Canada  where the species of plant called the Polygonum sawatchense of the buckwheat family be found?
Polygonum sawatchense Polygonum sawatchense is a North American species of plants in the buckwheat family. It in the western United States and western Canada, from the Pacific Coast to the western Great Plains, so from British Columbia south to California and east as far as New Mexico, Colorado, Saskatchewan, and the Dakotas.
North American Prairies Province The North American Prairies Province is a large grassland floristic province within the North American Atlantic Region, a floristic region within the Holarctic Kingdom. It lies between the Appalachian Province and the Rocky Mountains and includes the prairies of the Great Plains. It is bounded by the Canadian coniferous forests on the north and the arid semideserts to the southwest. The province itself is occupied by temperate grasslands, savannas, and shrublands (including such ecoregions as the Flint Hills tall grasslands, Sand Hills, High Plains). Endemism is rather limited in this province, and its boundaries are vague. During the Pleistocene much of the province was glaciated.