How to Grow: Cosmos

Cosmos bipinnatus

Other Name

Mexican aster

Sun Requirements

full sun to part shade

Bloom Period and Seasonal Color

Mid summer to fall in colors of red, violet, orange, white and pink

Mature Height x Spread

1 to 5 feet x 2 feet

Added Benefits

attracts beneficials, drought tolerant

Cosmos are easy to grow, tall annual flowers that feature airy foliage and cup-shaped flowers that flow in the breeze. The foliage almost resembles asparagus while the flowers can be single or double petaled. Cosmos look great planted in groups, are tolerant of many types of soils, self sow and have few pests. They are a common ingredient in Northeast wildflower and meadow mixes for these reasons, but also can be used in cottage gardens since they’re very attractive to butterflies. The tall plants have sturdy stalks so make excellent cut flowers. They only need staking if grown in high wind areas. Some newer varieties are dwarf and bushy making them perfect for containers.

Where, When and How to Plant

Direct sow cosmos seeds in spring around the last frost date for your area. Even if the soil isn’t warm enough for seed germination, cosmos seed will wait for the ideal conditions to grow. Don’t pamper the seedbed. Other than having well-drained soil, high fertility is not critical for cosmos growth. Plant them in full sun. Plants grown in part sun will flower, but may more prone to blowing over in the wind due to weaker stems. Thin seedlings so plants are spaced 1 foot apart. For faster flowering in summer consider buying transplants from a local garden center or starting seeds indoors, under grow lights 4 to 6 weeks before the last frost dates in your area.

Growing Tips

Pinch the tops of plants when they’re 2 feet tall to encourage branching and more flowers. Pinch the tops of side branches the same way after they develop. Don’t fertilize cosmos unless you have very poor soil conditions. Too much nitrogen fertilizer will encourage lots of leaf growth with little flowering. Water well when germinating, but cut back on watering once established since the plants are drought tolerant.

Regional Advice and Care

Cosmos self sow readily so don’t deadhead flowers if you want more cosmos in that area next year. Birds particularly like the seedpods and it’s fun to watch the finches feast on them in fall.

Companion Planting and Design

Plant cosmos in a meadow area with other wildflowers such as rudbeckia and daisies. In a cottage garden, plant them to add color next to Shasta daisies and salvia.  Plant cosmos in butterfly gardens with butterfly weed and butterfly plants. Grow cosmos in cutting gardens to use as cut flower indoors. Try dwarf varieties in containers to add a splash of color.

Try These

‘Sonata Mix’ feature heavy blooming plants on 2 foot tall stalks with colors such as white, pink, and red. ‘Seashells Mix’ has fluted flower petals in pastel colors. ‘Ladybird Mix’ is a dwarf selection with semi-double flowers that only grows 12 inches tall. ‘Versailles mix’ feature strong stemmed flowers that are perfect for cutting.

Excerpted from my book, New England Getting Started Garden Guide.