Tall and short varieties of grass and some flowering perennials dominate grasslands, including the prairies of North America. Big bluestem grass, blue grama grass, buffalo grass and Indian grass are abundant. Flowering perennials like milkweeds, goldenrods, asters, blazing star, sweet coneflower and purple coneflower bloom where rainfall is sufficient. These plants survive cold winters and fires in grasslands by developing underground storage organs and thick stem bases.
Average annual rainfall can support an occasional tree, but drought and grassland fires that sweep through every one to five years prevent the growth of large forests. The tallest prairie grass, big bluestem, or turkey feet, can reach 11 feet tall with a root system 7 feet long. It grows where the climate is wet, while shorter varieties of grass dominate drier areas with hotter summers and colder winters. The deep root systems fortify the plants against strong winds and help keep the soil in place. Grassland plants die back in winter but are kept alive by the robust underground support system made up of roots that can be longer than the plant is tall. Two-thirds of a prairie plant may be hidden underground in a dense tangle of roots, bulbs and stems. Some roots die each year and decompose, adding organic matter to the fertile soil.