Annuals vs. Perennials
The distinction between an annual plant and a perennial plant is in how long it takes the plant to complete its normal life cycle.
An annual completes its entire life cycle in just one growing season. It grows from a seed at the beginning of the season, produces flowers and seeds later in the same season, and dies completely at the end of the season. New plants grow the next year from the seeds produced by the previous year's plant.
A perennial stretches its life cycle over multiple growing seasons. Often, the leaves and stems of the plant die back at the end of each growing season and the roots remain dormant through winter, but new foliage grows from the still-living roots at the beginning of the next growing season.
Sometimes perennials are grown as annuals in climates too cold to allow the plants to survive the winter. In these cases, an entire plant, including its roots, dies when the weather turns cold, and new growth does not emerge from the root system in spring.
Chrysanthemums (Chrysanthemum spp.), commonly called mums, have been grown in their native Asia for thousands of years, but they were first imported to the West in the 1600s. Early mum cultivars resembled daisies, with single-petaled ray flowers surrounding central discs. Modern cultivars have been developed with more complex flower structures and varied growth habits. The development of some cultivars has emphasized cold-hardiness while the development of others has focused on flower production.
Garden mums (Chrysanthemum x morifolium), also commonly called hardy mums, are perennials and are well-adapted to make it through the winter in cool climates. They are generally hardy in U.S. Department of Agriculture plant hardiness zones 5 through 9, and in those zones, the plants regrow from their existing root systems year after year. Some varieties, such as the "Color Echo" cultivar, have been developed to be especially hardy and can withstand winters in USDA zones 4 through 9.
The hardiness of these plants is attributable to the fact that they develop robust root systems, with spreading underground shoots that tolerate cold soil temperatures.
Some varieties of mums, commonly called florist mums, are significantly less cold-tolerant than hardy mums. These varieties are often sold as cut flowers or potted plants in fall. Like hardy mums, they are varieties of Chrysanthemum x morifolium and technically perennials, but unlike hardy mums, they do not develop substantial root systems and are easily killed by cold soil temperatures.
Some species within the Chrysanthemum genus, such as the tricolor chrysanthemum (Chrysanthemum carinatum) and the garland chrysanthemum (Chrysanthemum coronarium), are true annuals. They complete their life cycles in a single growing season, regardless of winter temperatures, and must be grown from seed again the next season.