Cosmos atrosanguineus (Chocolate Cosmos) is a species of Cosmos, native to Mexico, where it is extinct in the wild. The species was introduced into cultivation in 1902, where it survives as a single clone reproduced by vegetative propagation.
It is a herbaceous perennial plant growing to 40-60 cm tall, with a fleshy tuberous root. The leaves are 7-15 cm long, pinnate, with leaflets 2-5 cm long. The flowers are produced in a capitulum 3-4.5 cm diameter, dark red to maroon-dark brown, with a ring of six to ten (usually eight) broad ray florets and a center of disc florets; they have a light vanillin fragrance (like many chocolates), which becomes more noticeable as the summer day wears on. 
Cultivation and uses
The single surviving clone is a popular ornamental plant, grown for its rich dark red-brown flowers. It is not self-fertile, so no viable seeds are produced, and the plant has to be propagated by division of the tubers, or by tissue culture.
It requires partial sun or full sun, and flowers from mid to late summer. It is frost-sensitive (Zones 6-11); in temperate zones, the tuber has to be dug up and stored in a frost-free store over the winter.
- Huxley, A. (ed.) 1992. New RHS Dictionary of Gardening 1: 739. Macmillan.
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