How to Grow Cosmo Flowers

Written by willow sidhe
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How to Grow Cosmo Flowers
Cosmos flowers appear in a variety of colours throughout summer. (Medioimages/Photodisc/Photodisc/Getty Images)

Cosmo flowers (Cosmos bipinnatus), also known as Mexican aster or cosmos, are tender annuals native to Mexico and the southwestern United States. Cosmos produces feathery, threadlike foliage and showy, daisy-like flowers in shades of yellow, white, pink, purple and red. The plant reaches an average height of 1 to 3 feet, although it can reach up to 7 feet tall, depending on the growing conditions and variety. Cosmos thrives in its natural range -- USDA hardiness zones 9 through 10. The plant also adapts to the conditions in zones 5 through 8, but it only reseeds in areas with warm winter temperatures.

Skill level:

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Things you need

  • Rototiller
  • Bloom-booster fertiliser
  • Garden stake or stick
  • Garden twine

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  1. 1

    Prepare a planting location for cosmos that has well-draining, sandy soil of average fertility and that receives full sunlight. Avoid rich, fertile soils, which can cause the plant to grow lanky and produce less flowers. Cosmos does not tolerate shade.

  2. 2

    Sow cosmos seeds directly in the garden in early spring or after the danger of frost has passed in cooler zones. Till the soil with a rototiller to a depth of about 12 inches just before planting for the best results. Plant the seeds about 1/8 inch deep and 12 inches apart.

  3. 3

    Water thoroughly just after planting, soaking the soil to a depth of 3 to 4 inches. Keep the soil slightly moist until germination occurs in seven to 21 days. Once growth begins, cosmos tolerates dry conditions. Water once every two weeks in summer, but only if two weeks pass without any natural rainfall.

  4. 4

    Apply a water-soluble, bloom-booster fertiliser in early summer, before cosmos begins to bloom. Follow the fertiliser label instructions for the best results. Do not feed more than once and do not use a high-nitrogen fertiliser, as this encourages the development of foliage instead of flowers.

  5. 5

    Stake taller varieties of cosmos to prevent them from flopping over under their own weight. Place a garden stake or fairly straight stick into the ground 1 to 2 inches behind the plant. Secure the stem loosely to the stake with garden twine until it stands up straight.

Tips and warnings

  • In areas with cold winters, you can also start cosmos seeds indoors five to six weeks before the last frost. Plant seeds 1/8 inch deep in small pots filled with moistened seed-starting mix. Place the pots in a warm area that receives indirect sunlight. Germination occurs in seven to 21 days. Keep the soil moist until the last frost has passed, and then plant the seedlings in the garden.
  • Remove faded cosmos flowers as soon as possible to encourage additional blooms. When flowering ceases, the plant will die back. Replant the following year. Allow the plant to reseed in zones 9 through 10, if desired, and new plants will appear the next spring.

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