In this guide
Quick answer
Sow cosmos seeds 1/4 inch deep directly in the garden after your last frost, in full sun and ordinary (even poor) soil. They sprout in 5-10 days, bloom in about 60-75 days, and keep flowering until frost if you cut or deadhead them. Don't fertilize: rich soil makes tall leafy plants with few flowers.
Cosmos is the easiest tall flower you can grow from seed. It thrives on neglect, handles heat and dry spells, and pulls in bees and butterflies all summer.
Two kinds of cosmos
| Type | Varieties | Height | Colors | Notes |
|---|---|---|---|---|
| Garden cosmos (Cosmos bipinnatus) | Sensation Mix, Purity, Candystripe | 3-5 ft | White, pink, magenta, striped | Feathery leaves, long stems for cutting |
| Sulphur cosmos (Cosmos sulphureus) | Bright Lights | 2-3 ft | Yellow, orange, red-orange | Loves heat, a butterfly magnet |
Purity is pure white, the one florists love. Candystripe has white petals edged and streaked with crimson. Sensation is a classic mix of pinks, white and magenta.
When to plant cosmos seeds
Sow cosmos outdoors after your last frost, once the soil has warmed. You can start them indoors 4 weeks before last frost for earlier flowers, but direct sowing is easier and the plants catch up quickly.
For flowers into fall, sow a second batch about a month after the first.
How to sow
- Pick a spot with full sun (6+ hours). Cosmos stretches and flops in shade.
- Don't add compost or fertilizer unless your soil is pure sand or clay. Lean soil gives more flowers.
- Sow seeds 1/4 inch deep, a few inches apart, and keep the soil damp until they sprout (5-10 days).
- Thin to 12 inches apart for garden cosmos and 9-12 inches for sulphur cosmos.
Getting more flowers
- Pinch garden cosmos once when it's about 12 inches tall: snip the top few inches just above a leaf pair. It branches out and gives many more stems.
- Cut or deadhead constantly. Cosmos makes seed fast, and a plant covered in seed heads slows down. Picking a bouquet every few days keeps it blooming.
- Water during long dry spells, but otherwise leave it alone. Established cosmos is drought tolerant.
Keeping tall cosmos upright
Varieties that reach 4-5 feet can lean after summer storms. Plant them in a block so they hold each other up, run a string or netting at about 2 feet, or put them along a fence. Pinching also makes sturdier, bushier plants.
Cutting cosmos for bouquets
Cut stems when the first flower on the stem is just opening and the buds below are colored. Cut in the morning and put them straight into water. Cosmos lasts about 5-7 days in a vase.
Does cosmos reseed?
Often, yes. Leave a few flowers to go to seed at the end of the season, and you may find seedlings in the same spot next spring. Seedlings from mixed colors will come up in a range of colors.
Problems and fixes
- Huge plants, no flowers: too much fertilizer or rich soil, or too much shade. Stop feeding and cut the plant back by a third.
- Plants falling over: see "keeping tall cosmos upright" above.
- Aphids on new growth: knock them off with a strong spray of water. Ladybugs usually arrive soon after.
Frequently asked questions
Is cosmos an annual or a perennial? Garden and sulphur cosmos are annuals. They die at frost but often reseed.
Can I grow cosmos in a pot? Yes, in a pot at least 12 inches wide. Sulphur cosmos and shorter varieties are easier in containers than 5-foot garden cosmos.
Is cosmos good for bees? Yes. Its open, daisy-like flowers make pollen and nectar easy to reach, and bees, hoverflies and butterflies visit all summer.
