Cosmos are carefree annuals with airy foliage and daisy-like flowers in pink, white, orange and yellow. They grow quickly from seed, thrive in sun and average to poor soil, and bloom as days shorten in late summer. They are one of the easiest flowers to start from seed. This page covers sowing, spacing, care, cutting back and self-seeding.
Cosmos at a glance
| Type | Flower |
|---|---|
| Season | Warm-season |
| Sun | Full sun |
| Plant spacing | 18 in |
| Row spacing | 18 in |
| Per square foot | 1 |
| Seed depth | 0.125 in |
| Days to maturity | Blooms in late summer |
| How to start | Sow cosmos seeds directly outdoors after the last frost, once the soil reaches about 65°F (18°C), according to Utah State University Extension. |
| Water | 1 in per week |
These are the same numbers our plant spacing, square foot and harvest date calculators use. See every crop in the plant database.
When to plant cosmos
Sow cosmos seeds directly outdoors after the last frost, once the soil reaches about 65°F (18°C), according to Utah State University Extension. Germination takes 7 to 14 days and is fastest at 70 to 75°F soil. Use the frost dates by ZIP code tool and the planting calendar to time sowing.
How to plant cosmos
- Give at least 8 hours of light. Full sun is best.
- Use average to poor, well-drained soil. Neutral to slightly alkaline is fine. Loosening the soil and removing weeds is enough.
- Sow shallowly. Scatter seeds on the surface and rake them in lightly, about ⅛ inch deep.
- Space 1 to 2 feet apart. Closer planting lets plants support each other. Use the plant spacing calculator.
- Do not fertilize. Rich soil produces weak, floppy stems, late blooms and fewer flowers.
Caring for cosmos
Water deeply once a week after seedlings emerge; established plants need little water. The garden watering calculator can help new beds.
To encourage a second flush, trim plants back to 12 to 18 inches; rebloom returns in about a month. Heights vary: Cosmos bipinnatus reaches 1 to 6 feet, while yellow cosmos (C. sulphureus) reaches 4 to 7 feet. Cosmos reseed heavily and are sometimes treated as weeds, and deadheading can reduce self-sowing.
Companion plants
| Grow nearby | Zinnias, sunflowers, marigolds, salvia and vegetables, to draw pollinators |
|---|---|
| Keep apart | Rich, heavily fertilized beds, which make floppy plants |
Cosmos fill the back of a border or a vegetable garden with airy color, and their flowers attract bees and other pollinators. Check other pairings with the companion plant finder.
Common problems
| Problem | What you see | What to do |
|---|---|---|
| Floppy stems | Tall plants flopping over | Usually caused by rich soil or fertilizer. Space plants closer, skip feeding and support tall types. |
| Late or few flowers | Lots of foliage but little bloom | Rich soil and too much fertilizer delay bloom; cosmos bloom as days shorten in late summer. |
| Weedy self-seeding | Seedlings appear everywhere next year | Cosmos reseed heavily. Deadhead before seeds ripen to reduce volunteers. |
| Few pests | Few insects or diseases affect cosmos | If you see sap-suckers, rinse them off. See the pest identifier. |
Not sure what you are looking at? The plant problem identifier ranks likely causes from the symptoms you see.
When and how to harvest
Cosmos are grown for flowers, which also cut well for a vase. Picking and deadheading keep plants blooming. Let a few flowers set seed at the end of the season if you want to save seeds or want them to return.
To save seed, wait until the seed heads turn brown and dry, then store the seed in a paper envelope. Seed saving tips are in our seed saving guide.
Plan your cosmos
- Plant spacing calculator: how many plants fit your bed
- Planting calendar: sowing and transplant dates for your frost dates
- Frost dates by ZIP code: your last spring and first fall frost
- Harvest date calculator: when your crop will be ready
Frequently asked questions
How far apart should I plant cosmos plants?
Space cosmos plants 18 inches apart in rows 18 inches apart. In a square foot garden, plant 1 per square foot.
How deep do I plant cosmos seeds?
Sow cosmos seeds about 1/8 inch deep: scatter them on the surface and rake in lightly.
How long do cosmos plants take to grow?
Cosmos seeds germinate in 7 to 14 days and the plants bloom in late summer as days shorten.
How much water do cosmos plants need?
About 1 inch of water a week, from rain and watering combined. Water deeply once a week after seedlings emerge; established plants need little water.
When should I sow cosmos?
Sow outdoors after the last frost, once the soil is about 65°F (18°C). Seeds sprout in 7 to 14 days.
Do cosmos need fertilizer?
No. Rich soil gives weak, floppy stems, late blooms and fewer flowers. Average to poor soil is best.
Do cosmos come back every year?
They are annuals but reseed heavily. Deadhead if you want to limit volunteers.