The genus once included many more species, but was split several decades ago into several genera; the naming of the genera has been contentious, but a ruling of the International Code of Botanical Nomenclature in 1999 resulted in the defining species of the genus being changed to Chrysanthemum indicum, thereby restoring the economically important florist's chrysanthemum to the genus Chrysanthemum. These species were, after the splitting of the genus but before the ICBN ruling, commonly treated under the genus name Dendranthema.
The other species previously treated in the narrow view of the genus Chrysanthemum are now transferred to the genus Glebionis. The other genera split off from Chrysanthemum include Argyranthemum, Leucanthemopsis, Leucanthemum, Rhodanthemum, and Tanacetum.
Today’s flowers are not as bright or large as ‘show’ varieties.
Chrysanthemums were cultivated in China as a flowering herb as far back as the 15th century BC. An ancient Chinese city was named Ju-Xian, meaning "chrysanthemum city". The plant is particularly significant during the Double Ninth Festival. The flower was introduced into Japan probably in the 8th century AD, and the Emperor adopted the flower as his official seal. There is a "Festival of Happiness" in Japan that celebrates the flower.
Chrysanthemums are broken into two basic groups, Garden Hardy and Exhibition. Garden hardy mums are perennials capable of being wintered over in the ground in most northern latitudes. Exhibition varieties are not usually as sturdy. Garden hardies are defined by their ability to produce an abundance of small blooms with little if any mechanical assistance (ie. staking) and withstanding wind and rain. Exhibition varieties on the other hand require staking, over-wintering in a relatively dry cool environment, sometimes with the addition of night lights.
The Exhibition varieties can be used to create many amazing plant forms; Large disbudded blooms, spray forms, as well as many artistically trained forms, such as: Thousand Bloom, Standard (trees), Fans, Hanging Baskets, Topiary, Bonsai, and Cascades.
Chrysanthemum blooms are divided into 13 different bloom forms by the US National Chrysanthemum Society, Inc., which is in keeping with the international classification system. The bloom forms are defined by the way in which the ray and disk florets are arranged.
Chrysanthemum blooms are composed of many individual flowers (florets), each one capable of producing a seed. The disk florets are in the center of the bloom head, and the ray florets are on the perimeter. The ray florets are considered imperfect flowers, as they only possess the female productive organs, while the disk florets are considered perfect flowers as they possess both male and female reproductive organs.
Irregular Incurve: These are the giants of the chrysanthemum world. Quite often disbudded to create a single giant bloom (ogiku), the disk florets are completely concealed, while the ray florets curve inwardly to conceal the disk and also hang down to create a 'skirt'.
Reflex: The disk florets are concealed and the ray florets reflex outwards to create a mop like appearance.
Regular Incurve: Similar to the irregular incurves, only usually smaller blooms, with nearly perfect globular form. Disk florets are completely concealed. They used to be called 'Chinese'.
Decorative: Similar to reflex blooms without the mop like appearance. Disk florets are completely concealed, ray florets usually don't radiate at more than a 90 degree angle to the stem.
Intermediate Incurve: These blooms are in-between the Irregular and Regular incurves in both size and form. They usually have broader florets and a more loosely composed bloom. Again, the disk florets are completely concealed.
Pompon: *Note the spelling, it is not pompom. The blooms are fully double, of small size, and almost completely globular in form.
Single/Semi-Double: These blooms have completely exposed disk florets, with between 1 and 7 rows of ray florets, usually radiating at not more than a 90 degree angle to the stem.
Anemone: The disk florets are prominently featured, quite often raised and overshadowing the ray florets.
Spoon: The disk florets are visible, and the ray florets are spatulate.
Quill: The disk florets are completely concealed, and the ray florets are tube like.
Spider: The disk florets are completely concealed, and the ray florets are tube liked with hooked or barbed ends, hanging loosely around the stem.
Brush & Thistle: The disk florets may be visible. The ray florets are often tube like, and project all around the flower head, or project parallel to the stem.
Exotic: These blooms defy classification as they possess the attributes of more than one of the other twelve bloom types.
Chrysanthemum leaves resemble its close cousin, the mugwort weed — so much so that mugwort is sometimes called wild chrysanthemum — making them not always the first choice for professional gardeners.
Chrysanthemum leaves are steamed or boiled and used as greens, especially in Chinese cuisine.
Pyrethrum (Chrysanthemum [or Tanacetum] cinerariaefolium) is economically important as a natural source of insecticide. The flowers are pulverized, and the active components called pyrethrins, contained in the seed cases, are extracted and sold in the form of an oleoresin. This is applied as a suspension in water or oil, or as a powder. Pyrethrins attack the nervous systems of all insects, and inhibit female mosquitoes from biting. When not present in amounts fatal to insects, they still appear to have an insect repellent effect. They are harmful to fish, but are far less toxic to mammals and birds than many synthetic insecticides and are non-persistent, being biodegradable and also breaking down easily on exposure to light. They are considered to be amongst the safest insecticides for use around food. (Pyrethroids are synthetic insecticides based on natural pyrethrum, e.g., permethrin.)