Cosmos

Sun

Full sun; shade produces fewer flowers and weaker stems.

Water

Water regularly while seedlings establish, then during prolonged dry spells. Plants in containers need more frequent watering.

Soil

Well-drained soil of low to moderate fertility; tolerates sandy and chalky soils. Avoid waterlogged ground and heavily enriched beds.

Height

2-6 ft (0.6-1.8 m), depending on variety

Spread

1-3 ft (0.3-0.9 m)

Flowers

Summer to the first autumn frost; white, pink, rose and crimson, with yellow centres. Some varieties have pale yellow flowers.

Foliage

Soft green leaves divided into fine, thread-like segments.

Growth

Fast; flowers in its first growing season.

Care needed

Low

• Plant out after the last frost, spacing plants about 12-18 in (30-45 cm) apart; allow more room for large varieties.
• Water young plants until established, then water deeply during extended dry weather rather than keeping the soil constantly wet.
• Avoid high-nitrogen fertilisers and heavy manure applications, which encourage leaves at the expense of flowers.
• Support tall varieties with twiggy branches or stakes before wind and rain bend the stems.
• Remove faded flowers regularly to prolong flowering and limit unwanted seedlings.

Pruning. Pinch out the growing tip when young plants are about 8-12 in (20-30 cm) tall to encourage branching. During flowering, cut spent stems back to a leaf joint; remove plants after frost kills them.

Early spring

Sow indoors about four to six weeks before your expected last frost, using good light and avoiding overly warm conditions that produce leggy seedlings.

Spring

Harden off indoor-grown seedlings gradually before planting outside.

Late spring

After frost danger has passed, plant out seedlings or sow directly into prepared, well-drained soil.

Early summer

Thin crowded seedlings, pinch growing tips if bushier plants are wanted, and support tall varieties.

Summer

Deadhead or cut flowers regularly, and water during prolonged dry spells.

Autumn

Collect ripe seed from open-pollinated plants if wanted, then remove plants once frost has killed them. Named varieties may not come true from saved seed.

• Thread tall varieties through sunny mixed borders for light height without a dense block of foliage.
• Combine pink and white flowers with salvias and ornamental grasses for a loose, meadow-like planting.
• Grow in cutting beds with zinnias and dahlias for a steady supply of summer stems.
• Use compact varieties in large containers or along the front of sunny borders.

Garden styles it suits: Cottage garden, English country, Potager (kitchen garden), Pollinator garden, Pastel border and Modern rustic.

Plant it with: Salvia, Zinnia, Dahlia, Verbena, Purple coneflower and Ornamental grasses.

Wildlife: Open-centred flowers provide pollen and nectar for bees, hoverflies and butterflies. Double-flowered varieties generally offer less accessible food; ripe seed may attract small seed-eating birds.

• Aphids can cluster on soft shoot tips; wash them off with water and encourage ladybirds and other predators.
• Slugs and snails may eat seedlings; protect young plants with barriers or use a locally approved iron-phosphate bait according to its label.
• Powdery mildew can develop on crowded plants; improve spacing, water the soil rather than the leaves and remove badly affected growth.
• Tall, leafy plants with few flowers usually indicate excessive feeding or insufficient sun; stop nitrogen feeding and choose a sunnier, less fertile site next season.

Cosmos bipinnatus is not known to be toxic to cats, dogs, horses or people, and no plant part is recognised as poisonous. It is not intended as pet feed; eating large amounts of any garden plant can cause digestive upset.

Where it spreads. Self-seeds readily and is regarded as an environmental weed in parts of eastern Australia, including south-eastern Queensland and New South Wales. Prevent seed escape near bushland and check local weed advice before planting.

• Sulphur cosmos (Cosmos sulphureus)

  Usually has yellow or orange flowers and broader, more coarsely divided leaf segments rather than the fine, thread-like foliage of garden cosmos.

• Chocolate cosmos (Cosmos atrosanguineus)

  Has dark reddish-brown, chocolate-scented flowers and broader leaflets; it is a tuberous perennial rather than a seed-grown annual.

• Love-in-a-mist (Nigella damascena)

  Also has finely divided foliage, but its flowers sit within a conspicuous ruff of thread-like bracts and are followed by inflated seed pods.

Cosmos comes from the Greek word for order or ornament, while bipinnatus refers to the twice-divided leaves.

Propagation. Grow from seed sown indoors in spring or directly outdoors after the last frost. Cover lightly and keep moist until germination. Self-sown seedlings are common, but seedlings from named varieties may differ from their parents.

Accepted botanical name

Cosmos bipinnatus

In the US and Canada

Introduced in Canada, the lower 48 states and Pacific Basin.

Native range

Mexico Central, Mexico Gulf, Mexico Northeast, Mexico Northwest and Mexico Southwest.

Measured

Soil pH 7 to 7.5. Average height 97 cm.

Sources: GBIF (CC BY 4.0), USDA PLANTS and Trefle (CC BY 4.0). The care and design notes are LandscapeGPT's own.