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?

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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.

Polygonum cascadense: Polygonum cascadense is a species of flowering plant in the buckwheat family known by the common name Cascade knotweed.  It has been found only in the State of Oregon in the northwestern United States, in the Cascades and in the Blue Mountains.

Polygonum californicum: Polygonum californicum is a species of flowering plant in the buckwheat family known by the common name California knotweed.  It is native to the west coast of the United States from Washington, Oregon, and northern and central California in the Coast Ranges and Sierra Nevada foothills as far south as Napa and Tulare Counties.  It can be found in many types of open habitats.

Polygonum austiniae: Polygonum austiniae, common name Mrs. Austin's knotweed, is a plant species in the buckwheat family.  It is native to western Canada and the western United States, from Alberta and British Columbia south as far as California, Nevada, and Wyoming.

Polygonum majus: Polygonum majus is a North American species of flowering plant in the buckwheat family known by the common name wiry knotweed.  It grows in the western United States and western Canada, from British Columbia south as far as the Sierra Nevada of northwestern Inyo County in California, east as far as Montana.

Polygonum erectum: Polygonum erectum is a North American species of annual plant species in the buckwheat family, with upright or ascending stems, called erect knotweed.  It was once cultivated for food by Native Americans as part of the group of crops known as the Eastern Agricultural Complex.  It is found primarily in the northeastern and north-central parts of the United States, but with scattered populations in other parts of the US and also in Canada.

Polygonum engelmannii: Polygonum engelmannii, common name Engelmann's knotweed, is a North American species of plants in the buckwheat family.  It is native to western Canada and the western United States, usually at high elevations in the mountains.  It has been found in Alberta, British Columbia, Idaho, Montana, Wyoming, Nevada, Utah, and Colorado.

Polygonum pensylvanicum: Polygonum pensylvanicum (syn.  "Persicaria pensylvanica") is a species of flowering plant in the buckwheat family, Polygonaceae.  It is native to parts of North America, where it is widespread in Canada and the United States.  It has also been noted as an introduced species in parts of Europe and South America.  Common names include Pennsylvania smartweed and pinkweed.

Polygonum spergulariiforme: Polygonum spergulariiforme is a North American species of flowering plants in the buckwheat family known by the common name spurry knotweed or fall knotweed.  It grows in western Canada (British Columbia and Saskatchewan) and the western United States (primarily Washington, Oregon, and northern and central California but with a few isolated populations in Idaho, Wyoming and Montana).

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.  It embraces

Supporting paragraphs: Polygonum sawatchense Great Plains
Answer: The Great Plains