How to grow Cosmos

Cosmos bipinnatus · garden cosmos, Mexican aster

Fast-growing annual with feathery leaves and daisy-like flowers. It flowers from midsummer until frost if deadheaded and blooms best in late summer, as the days shorten.

Quick facts

Type	Flower
Family	Asteraceae
Light	Full sun
Soil pH	6.5–7.5
Frost	does not tolerate frost
Soil for sowing	from 16 °C
Germination	10 days
Seedlings ready	35 days
Where it grows	Greenhouse, Open ground, At home

When to plant

Sow for seedlings about 6 weeks before planting out.

Plant out in open ground about a week after your last spring frost, once the soil is warm.

In an unheated greenhouse everything moves about three weeks earlier.

Frost dates · When to plant

Care

Greenhouse

Stage	Watering	Temperature	Feeding
Seedlings	every 3 days	—	—
Growth	every 5 days	—	—
Flowering	every 5 days	—	—

Open ground

Stage	Watering	Temperature	Feeding
Seedlings	every 3 days	—	—
Growth	every 7 days	—	—
Flowering	every 7 days	—	—

At home

Stage	Watering	Temperature	Feeding
Seedlings	every 3 days	—	—
Growth	every 4 days	—	—
Flowering	every 4 days	—	—

Growing notes

Light

Full sun; it does well in the sunnier, drier parts of the garden.

Soil

Well-drained soil of average to poor fertility, neutral to slightly alkaline (pH 6.5–7.5). Rich soil gives floppy stems and late, sparse flowers.

Care

Sow after the last frost once the soil is above 16 °C, about 3 mm deep, or start in cells 5–7 weeks before planting out. Seedlings appear in 7–14 days. Space plants 30–60 cm apart; wider spacing gives sturdier stems, and tall varieties may need support. Keep the soil moist until the seedlings are up, then water deeply once a week; overwatering reduces flowering. Do not fertilise. Deadhead to keep it blooming. Few pests or diseases trouble it.

Sources

Figures checked against these sources.