Growing Cosmos

Cosmos is a sun loving annual that does not produce as many blooms if grown in shade, so plant it in a location that receives at least a half day of direct sunlight. Cosmos perform best if grown in light, well-drained soil. Excess fertilization will cause it to produce leaf growth at the expense of flower production, so only apply a 5-10-5 fertilizer at the rate of 4 pounds per 100 square feet prior to setting out transplants or as a side dressing four to six weeks after sowing seeds.

Cosmos needs only basic care to provide an abundance of colorful blooms all summer. You may need to water during an extremely long dry period. If so, a deep watering, meaning the soil is wet to a depth of six inches afterward, is more beneficial to the plant than frequent, shallow sprinklings. During a deep watering, moisture is provided to the roots where it is most needed. Drip or soaker irrigation is more efficient than overhead sprinkling as less water is lost through evaporation. Overhead watering wets the foliage, encouraging the spread of disease producing organisms. Using a mulch helps conserve moisture, retard weed growth, and gives a neat appearance to the garden bed. Apply a 2- to 3-inch layer of an organic mulch, such as dried grass, leaves, shredded bark, or wood chips.

The taller species, Cosmos bipinnatus, performs best if given support as the slender, wiry stems break during a wind or rain storm. You can use stakes of metal, bamboo, or even a slender tree branch. Loosely wrap string or green floral tape around the plant and tie to the stake for support. Staking is best done when the plants are young since mature plants become too full and unruly to tie up. Also, as the plant grows, the stake is soon hidden among its foliage.