Chrysanthemums (Chrysanthemum spp.) and dahlias (Dahlia spp.) belong to the same family of flowering plants, but they have distinct characteristics that make them suited to very different roles in the garden.
One of the most commonly grown Chrysanthemum species is the garden mum (Chrysanthemum morifolium), also often called a hardy mum; it is widely cultivated as fall-blooming ornamental for both garden and container growing.
The dahlias most commonly grown in gardens today are descendants of pinnate dahlia (Dahlia pinnata), a native of central Mexico.
Hardy mums, as their common name suggests, are able to survive winters in relatively cold climates. They are generally winter hardy in U.S. Department of Agriculture plant hardiness zones 5 to 9, although in colder climates they may not return reliably and are often grown as annuals. Dahlias, in comparison, are much more tender and are winter hardy only in USDA zones 7 to 10.
Dahlias may be grown in climates colder than USDA zone 7, but in those climates, the tubers must be lifted in the fall and stored over the winter. Sometimes gardeners in warmer climates lift the tubers as well because the dormant period the tubers experience while stored sometimes increases flower production the following season.
Garden mums tend to be compact plants, growing in mounding clumps ranging between 1 and 3 feet in height and spreading to a similar width. Dahlia species and cultivars vary widely in height, but larger varieties can reach heights of up to 6 feet. The plants have an upright growth habit, and tall varieties usually need support from stakes or cages to keep them from falling over.
Flower Forms and Bloom Time
Both dahlia and mum cultivars produce flowers that differ considerably from cultivar to cultivar in terms of size, color and form. Dahlia flowers are perhaps more diverse, and cultivars are classified into 10 distinct groups based on their flower forms. The flowers of both dahlias or mums may be either single or double.
Garden mum flowers are typically about 2 3/8 inches in diameter, and the plants flower prolifically from early fall until the first frost. Dahlia flowers may be as small as 1/2 inch in diameter or, in the case of "dinner plate" varieties, up to a foot wide. In general, dahlias bloom from mid-summer to fall.
Both dahlias and mums prefer moist, well-drained soil that's rich in nutrients. Mums flower best when grown in full sun, but they can tolerate light shade. They do well when shielded from the afternoon sun in warm weather. Dahlias may also benefit from relief from afternoon heat when temperatures are above 90 degrees Fahrenheit, but they perform best when planted in a location that gets at least eight to 10 hours of full sun each day.