How to Grow Cosmos
Cosmos At a Glance
Difficulty
Easy
Category
Flower
Sun Exposure
Full Sun, Partial Sun
Frost Tolerance
Frost Tender
Cold Hardiness
Survives to 2°C
Plant Family
Asteraceae
Growing Season
Warm Season
Plant Lifecycle
Annual
How to Start It
★ Recommended for beginners
Cosmos cannot take frost, so sow it about 1 week after your last spring frost, once the soil feels warm. Scatter seeds 0.5 cm deep and cover them lightly, then thin them (pull the extras so each plant has room) to 30 cm apart. They sprout in 7 to 10 days and start flowering about 2 months after sowing.
Cosmos is one of the easiest flowers you can grow from seed, and bees and butterflies love it. The secret is poor soil: rich soil and feeding give tall, floppy plants with lots of leaves and few flowers. It often drops seed and comes back on its own the next year.
When To Start
First chance to plant
7 Days After Last Frost
Last chance to plant
30 Days Before First Frost
When should you plant Cosmos?
Your planting dates depend on your local climate. Sign up and add your location to unlock personalized dates.
Your Cosmos Planting Window
Start planting
May 15, 2026
Last chance
Sep 10, 2026
Your Cosmos Calendar
Set your location to turn these into exact dates and reminders.
Sow cosmos outdoors in plain soil, no compost needed
1 week after your last frost· every year
Snip faded cosmos flowers to keep it blooming until frost
10 weeks after your last frost· every year· optional
Did you water your cosmos seedlings today? Keep the top of the soil damp until they are up and growing.
About 1 days after you plant it
How is your cosmos doing? Pinch the top of each young plant to make it bushier.
About 30 days after you plant it
Cosmos check! Cut flowers often and the plant will keep making more.
About 50 days after you plant it
Picking time may be here: check your cosmos.
About 60 days after you plant it
The Journey Ahead
Cosmos's Lifecycle
Seedling
Mature Plant
Seed Production
Step 2
Planting & Sprouting
Expect sprouts in 7–10 days
Growing Tips
- 1Plant cosmos in full sun, at least 6 hours a day, for the most flowers.
- 2Skip the compost and fertilizer, since plain or even poor soil gives far more blooms than rich soil.
- 3Water seedlings until they are growing well, then only in long dry spells, because cosmos handles drought.
- 4When young plants are about 30 cm tall, pinch off the tip of the main stem just above a pair of leaves so they branch and make more flowers.
- 5Push a stake in beside tall varieties, or plant them close together so they hold each other up in the wind.
- 6Knock aphids (tiny green or black bugs on the buds) off with a strong spray of water.
Seedling Phase
Step 3
Growth & Maturity
~60
Days to Maturity
90 cm
Mature Height
45 cm
Mature Width
Pests to Watch For
Diseases to Watch For
Mature Plant
Step 4
Harvesting
Harvest Window
45 days
When to Pick
Cut flowers when fully bloomed
How to Harvest
- 1For vases, cut cosmos when the buds are just opening, since they last longer than fully open flowers.
- 2Cut a long stem just above a pair of leaves, and new flowering stems will grow from there.
- 3Snip off faded flowers every few days so the plant keeps blooming until frost.
- 4Put cut stems straight into cool water, and change the water every couple of days.
Step 5
Saving Seeds
How to Save Seeds
Cosmos seeds are easy to save. Bees carry pollen between plants, so different cosmos varieties growing together will cross and give a mix of colours next year, which many gardeners enjoy, but if you want a variety to stay true, grow only one. Hybrid F1 seeds will not grow true, so save only from a variety that is not a hybrid. Leave a few flowers on the plant until the centre turns into a brown, spiky ball of long, thin seeds that come away easily. Spread the seeds on a plate to dry for about a week, then store them labelled in a cool, dark, dry place, where they keep about 3 years.
Seed Production
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