Question: 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?
Sentence (from "Great Plains"): The Great Plains (sometimes simply "the Plains") is the broad expanse of flat land (a plain), much of it covered in prairie, steppe and grassland, that lies west of the Mississippi River tallgrass prairie states and east of the Rocky Mountains in the United States and Canada.
Relevant evidence: Yes