Cosmos atrosanguineus aka Chocolate Cosmos

Cosmos atrosanguineus has two common names: chocolate cosmos and black cosmos. It belongs to the daisy family, Asteraceae. This tender perennial lives for more than one year. It grows from thick roots called tubers beneath the soil. Its native range lies in Mexico, from San Luis Potosí to Hidalgo. Kew places it in the subtropical climate zone. It is not native to Britain or Ireland. In cold gardens, growers may treat it as an annual.

The plant spreads and branches, with thin stems. These reach about 2–3 feet [0.61–0.91 metres] in gardens. RHS gives an upper height of 3.3 feet [1 metre]. Spread reaches about 2 feet [0.61 metres]. RHS lists a smaller range for its spread. The dark green leaves are compound, with separate parts. They are pinnately divided, with parts along a central axis. These parts range from oval to lance-shaped. The plant sheds its leaves, rather than keeping them year-round. RHS says plants take two to five years to mature.

The flowers give this plant its chocolate name. They smell like chocolate. Their colour ranges from brown-red to maroon-crimson. The flower heads form bowls about 0.13 feet [0.038 metres] across. Thin stems hold them above the split leaves. Missouri Botanical Garden gives June through September as bloom time. RHS places the main show of flowers in late summer. Remove spent flowers to help the bloom season last longer. The flowers draw butterflies and suit beds grown for cutting.

Grow chocolate cosmos in full sun and soil that drains well. It prefers soil with a moderate level of nutrients. It also likes some moisture while it grows. It can cope with drier soil and spells of drought. RHS lists chalk, loam and sand as suitable soils. It grows in acid, neutral and alkaline soils. Avoid wet soil, which can make the rootstocks rot. Its size and flowers suit borders, beds and courtyard gardens. It also fits cottage gardens and loose, informal planting schemes.

Cold weather limits where the tubers can stay outdoors. Missouri Botanical Garden lists USDA zones 7–9. RHS rates it H3. RHS advises an autumn mulch where plants stay in place. In colder areas, lift the tubers before the first frost. Keep them free from frost through winter, as with stored dahlias. The plant has few major pest problems. Still, foot rot and grey mould can occur. Soil that drains well aids growth and helps keep roots healthy.