Germination temperature
About 21 °C (70 °F); outdoors wait for soil of at least 18 °C (65 °F).
Flower growing guide
Cosmos is one of the easiest annuals to grow from seed: it comes up in about a week in warm soil and flowers 8-12 weeks after sowing. This guide covers sowing depth, why poor soil gives more flowers, and how garden cosmos differs from orange sulphur cosmos.
About 21 °C (70 °F); outdoors wait for soil of at least 18 °C (65 °F).
Shallow: cover lightly, about 6 mm (1/4 in); deeper sowing slows germination.
5-7 days in warm trays; 7-21 days outdoors.
Cover the seed; it germinates in darkness.
None needed.
Both work; direct sowing in warm soil is easiest.
Cosmos seeds are long, thin and easy to handle. Sow them dry, spaced a few centimetres apart so seedlings can be thinned without disturbing their neighbours.
Not needed. Fresh cosmos seed germinates readily without treatment.
Not needed. Cosmos is a warm-season annual with no cold requirement; cold, wet soil only delays it.
Not needed. Sow into moist soil and keep it moist for the first week or so.
In trays about 21 °C (70 °F) gives quick, even germination. For outdoor sowing, NC State Extension advises waiting until the soil is at least 18 °C (65 °F).
Rake the seed into loose soil or cover it with about 6 mm (1/4 in) of compost. Planting too deeply hinders germination.
Light, well-drained soil of average or low fertility. Cosmos tolerates poor, sandy soil well.
Keep the soil moist for the first 5-10 days after sowing, then water less once seedlings are established.
Iowa State University lists cosmos among seeds that need darkness to germinate, so cover the seed rather than leaving it on the surface, but keep the cover shallow.
About 5-7 days at 21 °C (70 °F); outdoors it can take 7-21 days depending on soil temperature.
Sow indoors about 4-6 weeks before planting out. Move young plants before they become root-bound, and harden them off first.
Plant or thin cosmos to about 45 cm (18 in) apart. Tall garden cosmos can reach 0.9-1.8 m (3-6 ft); dwarf varieties stay much lower.
Pinching the growing tip of young plants encourages branching and more flowering stems. The first flowers usually open 8-12 weeks after sowing.
Cause: soil that is too rich or too much fertiliser. Fix: grow cosmos in average or poor soil and do not feed it; extra fertiliser produces foliage at the expense of flowers.
Cause: cosmos has weak stems, especially tall types in wind and storms. Fix: stake tall varieties, plant in a sheltered spot, or choose dwarf varieties.
Cause: powdery mildew or grey mould in humid, crowded plantings. Fix: space plants for air movement and water the soil, not the leaves.
Cause: aphids and leafhoppers feed on soft new growth. Fix: wash off with water or use insecticidal soap; check plants regularly.
Cosmos is a warm-season annual and tolerates heat. In the Indian plains it can be sown when the weather warms after winter and through the warm months; in the hills sow it in late spring after frosts, for flowers in summer and autumn.
Cut off faded flowers, or cut stems for the vase, to keep new buds coming.
Garden cosmos (Cosmos bipinnatus) has fine, thread-like leaves and pink, white, crimson or lavender flowers. Sulphur cosmos (Cosmos sulphureus) has broader, feathery leaves and orange or yellow flowers, and is especially heat and drought tolerant. Both are sown the same way.
Cosmos reseeds readily. Leave a few flowers to ripen if you want self-sown plants next season, or deadhead to prevent them.
Sow cosmos for tall, airy flowers in borders and for cutting.
Yes, but only lightly. Cosmos germinates in darkness, but deep sowing slows it down, so rake it in or cover with about 6 mm (1/4 in) of soil.
Usually because the soil is too rich or the plants were fed. Cosmos flowers best in average or poor soil without extra fertiliser.
Usually 8-12 weeks from sowing, depending on variety and temperature.