The genus Chrysanthemum comprises over 200 species, including the fall-blooming plants we commonly refer to as chrysanthemums or mums. In the late 1990s, after much work, taxonomists split the genus up into many groups, putting those common mums into the genus Dendranthema. Other popular garden plants were also placed in different genera: marguerite daisies became Argyranthemum; silver and gold chrysanthemums became Ajania; the wild ox-eye daisy is now Leucanthemum; Montauk daisy is Nipponanthemum; and tansy and feverfew are now Tanacetum. However, after much discussion it was decided to reinstate the name Chrysanthemum for all those fall-blooming "mums," since the name was so widely used. As a result, they are now properly Chrysanthemum once again.
Adding to the confusion, Korean mums were hybridized in Connecticut in the 1930s. A nurseryman, Alex Cummings, was working on hybridizing cold-hardy varieties that would flourish in New England temperatures. A tall plant--a wild species he mistakenly identified as Chrysanthemum coreanum--fell into his hands and the results were the lavish Korean mums you see today. The plant turned out to be a Chrysanthemum sibiricum, a wild mum with white-pink, daisy-like flowers, vigorous growth, and good branching. This species is also native to Korea, so the popular name Korean mums is correct. Korean hybrids tend to be 4 feet tall with spectacular daisy-like flowers that come in a range of colors from pale yellow and dusty pink to burnt orange.
Chrysanthemum rubellum is another group of hardy chrysanthemums. They tend to reach 2 to 3 feet in height, spread to form open and loose clumps which often require staking and are hardy to zone 4. They are sturdy and easy to grow. If you would like to avoid staking them in September and October, try pinching them in May to June, or cut them back by ½ in June. Some of the popular rubellums are Chrysanthemum 'Clara Curtis', Chrysanthemum 'Mary Stoker', and Chrysanthemum 'Duchess of Edinburgh'.
Although the classic Chrysanthemum 'Sheffield Pink', also known as 'Hillside Sheffield Pink' or just 'Sheffield', is generally classified as a rubellum, it is worth growing in your late-season perennial border matched with the burgundy-leaved Sedum 'Matrona' or a wispy ornamental grass.
The chrysanthemums that are widely available in the fall tend to be annuals. They are dwarf varieties that have been hybridized for compact growth and plenty of flowers. A few of them may be hardy but most will not survive the winter. They offer an easy, inexpensive way to add color to your fall garden. Use them in containers or mass them along walks or in flower borders where you can enjoy their cheerful colors.
These days you may get lucky and pick up a chrysanthemum that has been bred in Minnesota. These mums, as you can imagine, are cold hardy. They are often easily recognized by their names; many of them begin with 'minn' or end with 'sota', such as 'Minnautumn' or 'Lemonsota'.
Belgian hardy mums are another find. They are compact growers that bloom prolifically and do not require pinching to maintain their nice, compact shape. They are also cold hardy. Buy them early in the season, and try to get them into the ground in September so they have a chance to establish before winter sets in.