Grass of genus Poa (also called meadowgrass), of the family Graminae (Poaceae). About 50 species are natural to the United States, and 4 are of economic importance: Kentucky bluegrass (P. pratensis), used in lawns and pasture; Canada bluegrass (P. compressa), used for erosion control; roughstalk bluegrass (P. trivialis), adapted for turf in cool wet shade; and annual bluegrass (P. annua), a weed of cool moist sites. Kentucky bluegrass was introduced from Europe to Illinois by French missionaries, and spread rapidly throughout the Midwest, thriving where mean July temperatures are below 68°F (20°C).