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.
Spartina gracilis Spartina gracilis is a species of grass known by the common name alkali cordgrass. It is native to western North America from northwestern Canada through the western United States and eastern California, and into central Mexico. It grows in moist alkaline habitat, such as evaporating streams and shorelines, alkali flats, and inland marshes.