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.
Eriogonum gossypinum Eriogonum gossypinum is an uncommon species of wild buckwheat known by the common name cottony buckwheat. It is endemic to California, where it is known from the southern Central Valley and some of the adjacent foothills. It is often found on clay soils. It is an annual herb producing a gray or reddish branched stem no more than about 20 centimeters tall. The leaves are oblong in shape and woolly in texture. The scattered inflorescences are small clusters of white to pink glandular flowers buried in a layer of cottony fibers.