Aster is a genus of about 180 species. Asters are popular perennial flowers, commonly seen in bouquets and gardens. The flower is actually a combination of tiny tubular flowers, grouped together in a central yellow disk and surrounded by petals, the ring of ray flowers. The flowers can be white, blue, purple, and red, mostly with yellow centers. Asters are 60-180 cm (2-6 ft) tall. Asters grow best in moist and well drained soil, with a pH ranging from 5.5 to 7.5.