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How to sow cosmos Cosmos are easy-to-grow, long-flowering annuals (Cosmos bipinnatus, Cosmos sulphureus) ideal for borders, beds and pollinator-friendly gardens. Follow these steps for reliable results. When to sow - Sow under cover: 4–6 weeks before last expected frost. - Sow outdoors: after all risk of frost has passed and soil has warmed. - In mild climates: sow in autumn for early spring display. - For continuous flowering: sow every 2–3 weeks until early summer. Seed preparation and soil - No special pre-treatment required; seeds can be sown dry. - Choose well-drained, moderately fertile soil. Cosmos prefer lean to average soil; very rich soil encourages floppy growth. - Position: full sun for best flowering. How to sow - Surface-sow or cover very lightly: press seeds into soil or cover with about 2–5 mm of compost or soil. - Spacing: thin/plant out to 20–30 cm (8–12 in) for tall C. bipinnatus; 10–15 cm (4–6 in) for compact C. sulphureus. - Germination: 7–21 days at around 18–24 °C; keep soil just moist until seedlings establish. Practical care - Watering: keep moist while seedlings establish, then water sparingly—cosmos tolerate drought and too much water reduces flowering. - Feeding: little or no fertiliser; a light low-nitrogen feed encourages blooms without excessive foliage. - Pinching: optional. Pinch growing tips when plants reach about 10–15 cm to encourage bushiness and more flowers, but note pinching delays first blooms slightly. - Support: stake taller varieties in exposed sites to prevent flopping. Maintenance and problems - Deadhead spent flowers regularly to prolong bloom period. - Pests/diseases: generally trouble-free. Watch for slugs on young seedlings and powdery mildew in humid, overcrowded conditions. - To save seed: let heads dry on plant until brown, then collect seeds and store in a cool dry place. Quick tips - Sow directly where plants will flower to avoid root disturbance. - Use succession sowing for long season of colour. - Mix varieties for different heights and colours to create natural, airy displays. With minimal attention, cosmos will reward garden with abundant, long-lasting flowers attractive to bees and butterflies.
by Marion
the 4 September 2025 4 min.
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