The Cosmos genus contains both annual and perennial plants, although most are annuals. The familiar, 3- to 6-foot-tall, pastel-colored annual cosmos, sometimes called Mexican aster (Cosmos bipinnatus), may survive winters in U.S. Department of Agriculture plant hardiness zones 9 and 10, but won't live for long. They they may reseed in mild-winter climates, such as USDA zones 8 through 10.
Annual cosmos flowers come in white, pink and shades of burgundy. They bear daisylike blooms, although the Seashells mix has rolled petals and the Double Click mix has frilly, double flowers. Sonata is a dwarf mix. It prefers sandy, well-drained soil, sunny conditions and frequent watering when condition are dry. Excessive amounts of fertilizer will reduce blooms, according to Floridata. Due to annual cosmos' habit of reseeding in warmer climates, it can become invasive.
Tender Perennial Cosmos
Chocolate cosmos (Cosmos atrosanguineous) bears dark maroon flowers smaller than those of annual cosmos. While sold as a perennial that grows in USDA zones 7 through 11, is a tuberous plant that requires excellent drainage to overwinter and often must be dug in fall and taken indoors in cooler areas that experience hard frosts. Deadhead spent flowers with sanitized pruning shears wiped with alcohol to help support the plant and produce more blooms. Chocolate cosmos works well used in containers that drain and it also smells like chocolate.
Orange cosmos (Cosmos sulphureus) is an annual but may self seed in the warmer climates of USDA zones 5 through 10. It is a shorter, bushier and coarser plant than annual cosmos and produces small yellow, gold and orange blooms. It is drought-tolerant, though severe drought creates stunting and thrives in hot, sunny conditions. Regular water applications during dry conditions produces the best appearance. It reseeds in areas with mild winters and can become invasive.
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