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Cosmos bipinnatus, commonly called the garden cosmos or Mexican aster, is a medium-sized flowering herbaceous plant native to Mexico. The species and its varieties and cultivars are popular as an ornamental plant in temperate climate gardens. It can also be found in natural areas in much of North America, where it is a garden escape (introduced species) and in some habitats becoming a weed.
Cosmos bipinnatus is considered a half-hardy annual, although plants may reappear via self-sowing for several years. The plant height varies from 2–4 ft (0.61–1.22 m). The cultivated varieties appear in shades of pink and purple as well as white. Its foliage is finely cut into threadlike segments. When flowering, the plant can become top heavy. This problem is alleviated when grown in groups, as the bipinnate leaves interlock, and the colony supports itself.
Some of the varieties of Cosmos bipinnatus in cultivation today include:
- 'Sensation', also known as 'Early Sensation', is a widely available mix of tall varieties
- 'Sensation Pinkie'agm
- Sonata seriesagm
- 'Versailles', developed for the cut flower trade, are shorter than the species, with heights remaining below three feet
- 'Daydream' features a pink inner ring on a white background
- 'Seashells' features quilled petals
Growth characteristics of this plant include:
- Germination takes between 7 and 10 days at the optimal temperature of 75 degrees Fahrenheit (24 °C); flowering begins between 60 and 90 days after germination
- It prefers a soil pH between 6.0 and 8.5, reflecting its native habitat in the alkaline regions of Central America
- Flowering is best in full sun, although partial shade is tolerated
Excessive rain can cause cultivation problems, due to the delicate nature of the stems. Heavy rain can cause breakage. Cosmos bipinnatus can tolerate heat as long as adequate moisture is provided, however, it does not handle droughts, strong winds or cold temperatures well. Snails, slugs and aphids have a taste for Cosmos bipinnatus. Successfully cultivated plants can mature 2 to 4 feet x 12 to 18 inches.
- "BSBI List 2007" (xls). Botanical Society of Britain and Ireland. Archived from the original on 2015-02-25. Retrieved 2014-10-17.
- RHS Plant Selector Cosmos bipinnatus 'Sensation Pinkie' AGM / RHS Gardening
- RHS Plant Selector Cosmos bipinnatus Sonata Series AGM / RHS Gardening
- Bruce Asakawa; Sharon Asakawa (3 September 2001). California Gardener's Guide. Cool Springs Press. pp. 26–27. ISBN 978-1-930604-47-6. Retrieved 25 November 2011.