How to Grow Cosmos

Two kinds of cosmos

Garden cosmos (Cosmos bipinnatus) is the familiar one, with fine, threadlike leaves and pink, white, lavender, red or maroon flowers. It grows anywhere from 1 to 6 feet tall depending on variety and soil.

Orange or yellow cosmos (C. sulphureus) has orange, red or yellow flowers and handles heat better. UF/IFAS expects more summer bloom from it in Florida. Its height varies by strain: UF/IFAS describes it as usually 1 to 3 feet, while Utah State lists the species at 4 to 7 feet. Dwarf strains stay short.

For smaller gardens, look at the dwarf garden cosmos. NC State suggests ‘Sea Shells’, ‘Picotee’, ‘Candy Stripe’, ‘Day Dream’ and ‘Dwarf Sensation Mix’ for North Carolina. Cornell describes the Sonata series as 1 to 2 feet tall and recommends the wiry-stemmed Versailles series for cutting. Utah State says ‘Candy Stripe Mix’, ‘Daydream’ and ‘Gazebo Mix’ grow well in Utah.

Site and soil

Plant cosmos in full sun. Utah State calls for at least 8 hours of light, though NC State says half a day of direct sun is enough, and Cornell lists part shade as tolerated. Expect fewer flowers as shade increases.

Soil is where most people go wrong. Cosmos wants average to poor, well-drained soil and does fine with a neutral to slightly alkaline pH. Skip the compost and fertilizer. Utah State notes that rich soil produces weak, floppy, late plants with sparse flowers. To prepare a bed, just loosen the surface with a rake and clear out weeds.

Sowing

Sow outdoors after frost danger has passed and the soil has reached at least 65°F. Germination is best at 70 to 75°F and drops off in soil below 60°F or above 80°F. The dates for your zone above show the window.

Scatter the seed on the surface and rake it in lightly, no deeper than about 1/8 inch. Buried seed germinates poorly. Keep the bed moist until seedlings appear, which takes about 7 to 14 days. NC State allows up to 21.

For earlier flowers in short-season areas, Cornell suggests starting seed indoors 4 to 6 weeks before the last frost and planting out after frost.

Thinning isn’t strictly necessary, but plants are bigger with room. Utah State recommends 1 to 2 feet between plants, or closer if you want them to hold each other up as a screen or backdrop.

Water and care

Once seedlings emerge, cut watering back to a deep soak about once a week. Cosmos thrives in the drier parts of a garden, and overwatering reduces bloom.

Pinch young plants to make them branch and fill out. Tall varieties in windy or storm-prone spots may need stakes. Planting them close together also helps them prop each other up.

Deadhead to keep flowers coming. When a spring planting is covered in dry seed heads, NC State suggests cutting the plants back to 12 to 18 inches rather than pulling them. They rebloom in about a month.

Cosmos is day-length sensitive and blooms most heavily in late summer as the days shorten. In northern gardens, Cornell expects flowers from July until frost.

Warm-winter regions

Where frost is rare, cosmos shifts toward the cooler months. UF/IFAS lists it for planting from fall through spring in South Florida (September to May) and in spring and early summer in North Florida. In the Arizona low desert, the University of Arizona chart has garden cosmos planted from March through June and blooming from July into November. Yellow cosmos goes in from May to July there and blooms from September to November. Around San Antonio, Texas A&M AgriLife suggests a July sowing for late-summer and early-fall color.

Problems

• Floppy stems: the result of rich soil, fertilizer or overwatering. Stake what you have now and plant in leaner ground next time.
• Too many volunteers: cosmos self-sows heavily. Deadhead before seed ripens, or plant it where reseeding is welcome.
• Aphids: occasional. Check new growth.
• Powdery mildew, gray mold and stem canker: NC State recommends good air flow to prevent them. NC State also lists southern blight.
• Bacterial wilt and leaf spots: occasional problems in Florida, according to UF/IFAS.

Cosmos draws butterflies and bees, and Utah State notes it attracts beneficial insects such as lacewings, hoverflies, tachinid flies and parasitic wasps. Finches eat the seed.

Cutting and drying

Cut stems when the flowers are fully open. Regular cutting also encourages more blooms. To dry flowers, hang them in a well-ventilated place out of sunlight. Utah State says they dry in 1 to 5 days, after which you can store them in a moisture-tight container somewhere cool and dark.