Cosmos

Cosmos bipinnatus

This tall, airy plant lives for one season. It has feathery leaves and daisy-like flowers from midsummer to the first frost and they're good for cutting. It flowers best in poor to average soil. Rich soil gives tall, leafy plants with few flowers. Dwarf types suit pots.

Looking after it

Water: Once it's settled, cosmos copes with dry spells. Water seedlings, new plants and pots. Too much water, like too much feed, gives you more leaves than flowers. Feeding: Cosmos needs very little. Don't feed it in the ground. Plants in pots can have a high-potassium liquid feed, such as tomato feed, once a month.

Cutting and after flowering

For the vase, cut stems when the buds are just opening, back to a side shoot. Pick off faded flowers every few days, or the plant sets seed and stops flowering. Cut flowers last from a few days to a week in a vase. Cut them in the cool of the morning.

What can go wrong

Slugs and snails. They eat seedlings and young plants overnight. Only plant out sturdy plants and protect them at night until they are well grown.
All leaves, few flowers. The plants grow tall and lush but flower late and sparsely, because of rich soil, feeding or shade. Stop feeding and keep picking off faded flowers. Next year, choose a sunnier spot with poorer soil.
Falling over. Tall plants flop over in wind and rain. Pinch out the tip at about 30 cm so the plants grow bushier. Support them with canes or twiggy sticks.

Growing facts: RHS, UC ANR and seed-company sowing guides. Cosmos care: RHS. pH: University of Connecticut soil testing lab.