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.
Poa arida Poa arida is a species of grass known by the common names plains bluegrass and prairie speargrass. It is native to North America, where it occurs throughout western and central Canada and the central United States. It is most common east of the Continental Divide; specimens west are often misidentifications.