Daisy one of the most well-known kinds of flower, but it's not necessarily a simple matter to nail down what exactly a daisy is. It is not a species of plant or even a genus; a daisy is one of dozens of species in many genera with only a few common characteristics among them. The most obvious common feature of all daisies is their flowers, the familiar sunny disc with a prominent center surrounded by brilliant petals.
Daisies' common characteristic, their flower head, superficially appears to be a single flower but actually is a cluster of tiny flowers surrounded by larger, showy petals. Each of the tiny flowers in the center of the cluster produces a single seed. A typical daisy can have as many as 250 flowers in a single flower head. So one plant can produce a huge number of seeds. Each seed is dispersed by means of a hairy structure called a pappus, which is attached to the seed and acts like a parachute, allowing the seed to float on wind.
Some daisy species are annual, lasting only one year, and some are biennial. A biennial daisy grows from seed its first year, doesn't flower that year but flowers during its second year before dying. That kind of daisy species perpetuates itself by producing and dispersing seeds. Still other daisy species are perennial, coming back year after year; the ox-eye daisy (Chrysanthemum leucanthemum), for example, is hardy in U.S. Department of Agriculture plant hardiness zones 3 to 10 and spreads by means of underground rhizomes as well as seeds.
Although 38 plant species spread across 22 genera have common names that include the word "daisy," all daisy species are members of the family Asteraceae, which also is called Compositae. The plants in this family all produce daisylike, composite flowers. The true flowers in the center of each flower head are surrounded by a ring of specialized flowers, each of which produces a single, raylike petal that protrudes from the flower head; the ring formed by these rays gives the flower its distinctive sun-shaped form.
It's not difficult to propagate daisies in a garden, and sometimes it may be difficult to control them. Annual and biennial species spread easily on their own with their parachute-clad seeds, and they tend to come back reliably season after season. Perennial daisies often benefit from some help from gardeners, however; dividing clumps of daisy plants every two or three years keeps the daisies from becoming crowded and encourages them to grow stronger.