Annual
How to grow cosmos
Cosmos bipinnatus
Cosmos is a half-hardy annual sown indoors in March or April and planted out in late May, once frost has passed, in full sun. It flowers from July until the first frost and is one of the easiest annuals to raise from seed. The key is poor soil: rich ground or fertiliser gives tall, leafy plants with few flowers. Pinch out the tip at 20 cm, then cut or deadhead twice a week.
- Hardiness
- Half-hardy annual
- Position
- Full sun
- Soil
- Light, free-draining; not too rich
- Ideal pH
- 6.0 to 7.5
- Water
- Low to moderate
- Feeding
- Light feeder
Sowing and planting cosmos
WhenSow Mar to Apr indoors; plant out late May; flowers Jul to Oct
Starting off
Sow Mar to Apr indoors at 18 to 21 °C in modules or a tray, covering seed with 5 mm of compost; it germinates in 7 to 10 days. Pot on into 9 cm pots, harden off for 10 days and plant out after the last frost in late May; a direct sowing in May also works on light soil.
Spacing and planting
Tall varieties 45 to 60 cm apart, dwarf 'Sonata' types 30 cm; sow or plant at 0.5 to 1 cm depth. Dwarf types are fine in a 30 cm pot, but tall types topple in containers.
Varieties worth growing
- 'Purity'
- Tall, pure white single flowers on 1.2 m plants. The classic for cutting and for the back of a border; needs support in an exposed spot.
- 'Sonata' series
- Dwarf at 50 to 60 cm in white, pink and carmine. Early to flower, needs no staking and is the usual choice for pots and bedding.
- 'Rubenza'
- Ruby-red flowers that fade to antique rose, on bushy plants about 80 cm tall. Starts flowering early and reliably.
- 'Xanthos'
- Soft primrose yellow, compact at about 60 cm and early. A good container variety and a different colour from the usual pinks.
- Cosmos atrosanguineus
- Chocolate cosmos: a tender, tuberous perennial with small maroon flowers that smell of chocolate. Bought as a plant, not grown from seed, and overwintered frost-free like a dahlia.
Cosmos month by month
Timings are for central England. Allow two weeks later in the north and on cold, heavy soil.
- January
Nothing to do.
- February
Nothing to do. Order seed.
- March
Sow indoors from late in the month at 18 to 21 °C.
- April
Main sowing. Pot on into 9 cm pots and grow on cool and bright. Pinch the tip at 20 cm.
- May
Harden off for 10 days and plant out in late May. Sow direct on light soil.
- June
Put in supports for tall varieties. Water until plants are established, then only in drought.
- July
First flowers. Start cutting and deadheading to the next leaf joint.
- August
Cut or deadhead twice a week. Do not feed.
- September
Keep deadheading. Leave a few heads to ripen if you want to save seed.
- October
Flowers continue until the first frost.
- November
Pull frosted plants and compost them. Lift chocolate cosmos tubers and store frost-free.
- December
Nothing to do.
Feeding cosmos
Cosmos in the ground needs no feed at all, and flowers better without it. The only exception is plants in pots, where the compost runs out by midsummer.
Low, rich soil or too much nitrogen gives lush leaf and few flowers. Grow lean.
Feeding plan
Planting out, in the ground
Nothing
No fertiliser, manure or fresh compost. Choose a bed that was not fed this year.
Pots, from six weeks after planting
3 ml per litre of water, the bottom of the label range, once a month: 27 ml in a 9 litre can. No more often, or the plants turn leafy.
Lower leaves yellowing on thin, dry, sandy soil
1 g per litre of water as a single drench after a thorough watering. Usually the problem is drought, not hunger.
No feed is recommended for cosmos in the ground. The products linked are for pot-grown plants only.
Signs to watch
Masses of feathery foliage and few flowers by Aug
Excess nitrogen from rich soil or manure
No feed, a high-potash liquid only
Yellowing lower leaves on thin, dry sand
Nitrogen and water short
Water well and one dilute seaweed feed
Stems flop
Too much shade or feed
Full sun and lean soil next year
Pests, diseases and problems
Seedlings stretch, flop and collapse at the base indoors
Sown too early in poor light and too much warmth, then damping off in wet compost
Do not sow before late March. Move seedlings somewhere cooler and bright as soon as they are up, water from below, and pot on deeply to bury the bare stem.
Young plants shredded or eaten off in the first weeks after planting out
Slugs and snails
Plant out sturdy plants from 9 cm pots, not small plugs, and check after dark for the first fortnight. Established cosmos is rarely touched.
Newly planted cosmos blackened and limp after a cold night
Frost on a half-hardy plant put out too soon
Wait until late May, harden off for 10 days, and keep fleece to hand. Plants cut by frost seldom recover well; replace them from spare seedlings.
Colonies of green or black insects on shoot tips and under buds
Aphids
Rub off or wash off with water. Predators usually catch up by July.
White powdery coating on the foliage in September
Powdery mildew late in the season, on dry plants
Water during dry spells and keep deadheading. It comes too late to matter much.
Flowers and buds turn brown and mouldy in a wet autumn
Grey mould (botrytis)
Cut off affected heads, thin crowded growth, and pull the plants once they stop producing clean flowers.
Pruning
Pruning and care
Pinch the tip when plants are 20 cm tall for a bushier plant with more stems. Deadhead or cut every spent bloom to the next leaf joint, as seed set in Aug halts flowering. Push in twiggy pea sticks or a ring of canes at planting for tall types; an annual, so pull after the first frost and compost.
Good to knowDeadhead for months of bloom; pinch young plants to bush out.
Growing cosmos in pots
Dwarf varieties such as 'Sonata' and 'Xanthos' are the ones for pots: three plants in a container 30 to 40 cm wide, in peat-free multipurpose compost with no added fertiliser. Tall varieties catch the wind and pull a pot over. Water when the surface is dry, which in July means most days, and deadhead twice a week. Start a dilute liquid feed only once the plants have been in the pot for six weeks, and keep it to once a month. Chocolate cosmos is best grown in a pot so the tuber can be moved under cover for winter.
Cosmos: common questions
Why is my cosmos tall with no flowers?
The soil is too rich, or the plants are in too much shade. Nitrogen from fertiliser, manure or fresh compost makes cosmos grow leaf instead of flower, so stop feeding and avoid enriched beds next year. Some older tall varieties also wait for the shorter days of late summer before flowering; modern ones such as 'Sonata' and 'Rubenza' start earlier.
Can I sow cosmos seed directly outside?
Yes, in May, once the soil has warmed and frost is unlikely. Sow thinly 0.5 to 1 cm deep where the plants are to flower, on light, well-drained soil, and thin seedlings in stages to 30 to 45 cm apart. Direct-sown plants flower a few weeks later than those started indoors, usually from August.
Does cosmos come back every year?
No, Cosmos bipinnatus is an annual and dies with the first frost. It sometimes self-seeds in mild gardens, and seed saved from ripe heads in September can be sown the next spring, though it may not match the parent. Chocolate cosmos is a perennial, but only if the tuber is kept frost-free over winter.
Sources
Growing information on this page was checked against these references in November 2026. Feeding rates are Dr Forest label rates.
- Cosmos bipinnatusRHS
- How to grow cosmosBBC Gardeners' World