poplar, Any of at least 35 species and many natural hybrids of trees that make up the genus Populus (willow family). Poplars grow throughout northern temperate regions, some even beyond the Arctic Circle. They are rapid-growing but relatively short-lived. Their leaves flutter in the slightest breeze because of their laterally compressed petioles (leafstalks). The relatively soft wood is used to make cardboard boxes, crates, paper, and veneer. North America has three groups of native poplars: cottonwoods, aspens, and balsam poplars.