Cosmos

1. 1

   Sow indoors or direct

   Late March to early May indoors in modules at 18°C for transplanting. Late May direct outdoors. Don't sow earlier than this; cosmos seedlings hate cold and grow leggy on a windowsill. 5 mm deep, 2 seeds per cell or per station outdoors.

2. 2

   Don't over-feed the soil

   Cosmos flowers best in average-to-poor soil. Rich soil and high nitrogen push leaves at the expense of flowers — the cosmos in a heap of leaves with no blooms failure mode is almost always over-feeding. Don't dig in manure before planting; standard garden soil is fine.

3. 3

   Plant out after the last frost

   Mid-May in southern England; early June further north. Harden plants off for 7–10 days. Space 45 cm apart in a sunny spot. Stake taller varieties (Sensation, Purity) early — they reach 1.2 m and topple in wind.

4. 4

   Pinch out the tip

   When plants are 30 cm tall, pinch out the growing tip to encourage branching. This doubles the eventual flower count.

5. 5

   Water modestly

   Water in well at planting; then only in genuinely dry weeks. Cosmos prefers slightly dry conditions to bog-rich ones.

6. 6

   Deadhead religiously

   Cut spent flowers back to the next bud or side-shoot. Cosmos stops flowering the instant seed pods form — so deadheading is non-negotiable. Pick for the vase regularly too; it's a cut-and-come-again cut flower.

7. 7

   Save seed in autumn

   Let the last flowers of the season go to seed. Cut the dried seed heads in October; rub the seeds out into a paper envelope; store in a cool dry place. Cosmos comes true from seed (it's not F1) — Sensation, Purity, and other open-pollinated cultivars give the same flowers next year.

Aphids can build up; earwigs may damage blooms.