The Cosmos Year: What to Do from Sowing to First Frost

The shape of the season

Cosmos is a half hardy annual, so the year is bracketed by frost at both ends. It cannot go out until the last spring frost has passed and it is killed outright by the first autumn frost. Everything in between is one long flowering run, provided you keep the seed heads off.

From sowing to first flower takes roughly ten to twelve weeks unpinched, and twelve to fourteen if you pinch. Work backwards from your last frost date rather than fixing on a calendar month, and the same plan reads correctly in either hemisphere.

The great strength of cosmos is how long that run lasts. A plant that is deadheaded properly and never fed will flower for three months or more, right up to the frost that ends it.

Spring: raise a stocky plant, not a tall one

The whole spring stage is a race between how fast cosmos grows and how much light you can give it. Four to six weeks indoors before the last frost is the right window, and on a poor windowsill four weeks beats six every time. A short firm plant with four to six true leaves overtakes a tall pale one within a month of going out.

Sow into a lean seed-starting mix rather than a rich one, pot on at three or four true leaves, and keep the plants cooler once they are up, around 60 to 65F (15 to 18C). Cooler growing slows stem extension without slowing root growth, which is exactly the trade you want. Pinch once the plant is eight to twelve inches (20 to 30 cm) tall.

Harden off properly over seven to ten days, starting with a sheltered hour and building to a full night outside. Cosmos moved straight from a warm windowsill into open ground stalls for two or three weeks and can scorch in strong spring sun.

Early summer: the fortnight that decides the year

Plant out once the last frost has passed and the soil is above 60F (15C). Space the tall types eighteen inches (45 cm) apart, mid height ones twelve to fifteen inches (30 to 38 cm), and dwarfs nine to twelve inches (23 to 30 cm). Do not enrich the planting hole. Water in hard so the rootball itself is soaked, then leave the plant alone.

The two weeks after planting are when most losses happen, and almost all of them are slugs. Clear mulch and weeds a hand's width back from every stem, water in the morning rather than the evening, and go out with a torch an hour after dark on the first two nights. After that the plants look after themselves.

Get the support in as soon as the plants reach a foot tall. Netting or a string grid at eighteen inches (45 cm) is far better than canes for a group, and the plants grow up through it and hide it entirely. This is much easier now than after the first storm.

Note the date you planted out, the date of the first open flower, and how many plants you lost in the first fortnight. Those three numbers tell you next year whether to sow earlier, plant out larger, or take slugs more seriously.

Deadhead to a bud, not to a stub

Deadheading is the whole job from first flower to frost, and how you do it matters more with cosmos than with most annuals. Do not simply snap the spent head off. That leaves a bare stalk that dies back, looks untidy for weeks, and gives gray mold a way into the stem, and it does nothing to encourage the plant to branch.

Instead, follow the flower stem down to the first side shoot or pair of leaves carrying a visible bud, and cut just above that point. New growth breaks from there within days and each new shoot carries flowers of its own. The plant gets bushier every time you deadhead it properly, which is why a well deadheaded cosmos looks better in late summer than it did in early summer.

Every head allowed to set seed shifts the plant towards ripening that seed and away from making new buds. A stand left alone for three weeks in midsummer visibly slows down. Deadheading every three or four days takes a minute or two per plant; doing it once a month becomes a job that fills a bucket.

1. Learn what a spent flower looks like The petals go limp, dull and pull backwards, and the green center swells and firms up. Take it then, before the seed hardens and the plant commits to ripening it.
2. Cut down to a bud Trace the stem back to the first side shoot or leaf pair with a visible bud and cut just above it. Never leave a bare stub, which dies back and can rot into live stem.
3. Keep to a rhythm Every three or four days in high summer. A fixed rhythm is what keeps cosmos flowering until frost, and it is far less work than an occasional heavy tidy up.
4. Mark a few plants for seed From late summer, stop deadheading two or three of the best plants and tie string round them so nobody tidies them by habit. The seed needs three or four weeks on the plant to ripen.

Late summer to frost: keeping it going and finishing up

By late summer a first sowing has usually gone leggy and bare at the base. Cutting every stem back by a third to just above a leaf joint brings a strong flush about three weeks later that carries through to frost. Do it while there is still real growing weather left, not in the last warm week of the season.

This is also where a second direct sowing made in early summer earns its place, coming into full flower as the first batch is being cut back. Between the two you get unbroken flower from midsummer until the frost.

For seed, leave the marked heads on the plant until the seed is dark, hard and dry, three or four weeks after the petals fall. Green seed will not germinate and goes moldy in the envelope. Once frost blackens the plants, pull them, compost anything sound, and bin any mildewed material rather than composting the spores back into the garden. Clearing promptly also removes the wet mat of stems that shelters slugs over winter.