5. Leucanthemum Mill.

About 33 species, Europe, Africa, introduced in the New World.

It has now become widely accepted that the former broad circumscription of the genus Chrysanthemum involved the recognition of an unnatural group. The most recent classifications separate this unwieldy group into about 38 genera (Soreng and Cope, 1991; Bremer and Humphries, 1993). Unfortunately, the characters supporting this reclassification are mostly microscopic and/or anatomical, which has made it difficult to write keys to the segregate genera. Chrysanthemum in the strict sense is now confined to a few annual species native to the Mediterranean region. It and a few other non-Missouri genera are part of a group with florets with two different types of achenes: those of the showy, mostly yellow ray florets strongly 3-ribbed or 3-winged; and those of the disc florets thinner-walled and less strongly angled (sometimes 1 of the angles winged) (Soreng and Cope, 1991). Both types of florets lack a pappus, but this and some other defining characteristics are not individually unique to Chrysanthemum, which can only be separated easily by a combination of different characters. Species of Chrysanthemum in the strict sense are cultivated under names like garland chrysanthemum and crown daisy (C. coronarium L.), corn marigold (C. carinatum Schousb.), and tricolor chrysanthemum (C. segetum L.).

In the other genera, when both disc and ray florets produce fruits, these tend to be similar in size and morphology. The familiar garden and florist’s mums are mostly cultivars and hybrids involving a plant currently known as Dendranthema ×grandiflorum (Ramat.) Kitam. Leucanthemum belongs to a group of about a dozen genera characterized by having resin ducts in the achene wall, among other features. In addition to the species treated below, several other species sometimes are grown as ornamentals. They include the Shasta daisy, L.×superbum (Bergmans ex J.W. Ingram) Soreng & E. Cope, which was developed from a cross between the Portuguese chrysanthemum, L. lacustre (Brot.) &Cope, 1991).