When to sow cosmos

Cosmos is a half-hardy annual from Mexico: frost kills it, so timing follows your last frost. Sow indoors in March or April. Some seed houses suggest starting in February, but cosmos grows fast, so there is no need to rush.

Plant out once frost is over. In Germany, Austria and Switzerland that means after the Ice Saints in mid-May; in the UK, late May or early June.

You can also sow straight into the bed from May, when the soil has warmed. Direct-sown cosmos is less work, but it starts flowering later, usually from August.

Sow the long, thin seeds into trays or small pots of seed compost and cover them only thinly, about 0.2–0.4 in. Buried deeper, they come up poorly. Keep them warm, ideally around 64–68°F; seedlings usually appear within one to three weeks.

Prick the seedlings out into individual pots once they have their first true leaves, grow them on somewhere bright, and harden them off for a week or two before planting out.

Space plants 12–20 in apart, closer for compact varieties and wider for tall ones. In a 120 × 80 cm raised bed that means two rows of three, or up to eight plants of a compact variety. Tall cosmos will shade the rest of a raised bed, so give it a spot at the north end.

For direct sowing, rake the soil fine, sow in small groups and thin to the final spacing once the seedlings have a few true leaves. Protect them from slugs, which love young cosmos.

Cosmos is one of the few flowers that does better with less. Rich, heavily manured soil gives big, soft, floppy plants that flower late and sparsely. Ordinary garden soil that drains well is ideal, and there is no need to feed. Avoid very wet, heavy ground.

Give it full sun: in shade it hardly flowers. Established plants cope well with dry spells; water seedlings until they are growing strongly, and after that only in long droughts, then deeply.

Tall varieties can reach 59 in and fall over in wind or heavy rain. Push in a few canes or pea sticks early, or plant in a group so the stems hold each other up.

Plants raised indoors usually flower from July; direct-sown ones from August. With regular deadheading they keep going until the first frost, often into October.

Snip off faded flowers every week or so, down to the next bud or side shoot. Cut flowers for the vase in the morning on a dry day; the more you cut, the more new stems the plant makes.

Cosmos is a good plant for hoverflies and bees, and it keeps offering them food late into autumn.

For seed, leave a few flower heads to ripen late in the season and collect the dark, needle-shaped seeds once they are dry. Cosmos also self-seeds readily. Seed from named varieties and hybrids may not come true to colour or height.

Most garden cosmos is Cosmos bipinnatus, with feathery leaves and white, pink or crimson flowers. Heights run from about 20 in for compact varieties to 59 in for tall cut-flower types, so check the packet before choosing a spot.

Yellow and orange cosmos belongs to a separate species, Cosmos sulphureus.

Cosmos is trouble-free for most gardeners. Slugs are the main threat to young plants, especially direct sowings; plants raised in pots are tougher by the time they go out. Aphids sometimes gather on shoot tips and buds.

Powdery mildew and grey mould can appear late in the season or in damp, crowded plantings. Give plants enough space and remove affected growth.

Cosmos is in the daisy family, like lettuce and sunflowers, but as a flower it plays no real part in a vegetable rotation. It is often planted between vegetables to bring in hoverflies, whose larvae eat aphids. That is a reasonable idea; we found no trials showing a measurable effect on nearby crops.