Cosmos are one of the easiest flowers to grow from seed in Australia. Sow them into warm soil after the last frost, or through the dry season in the tropics, give them full sun and lean, free-draining soil, and resist the urge to feed them, because rich soil tends to push leaves at the expense of flowers. Pick or deadhead often and they can bloom for months, then quietly drop seed for next year.
Few flowers give back as much for as little fuss as cosmos. A packet of seed, a sunny corner and a little patience, and by mid-summer you can have waist-high clouds of daisy-like flowers nodding over the fence.
They are also wonderfully forgiving, which makes them a lovely first flower to raise from seed. The main skill is knowing when to sow and, oddly, knowing when to leave well enough alone.
Why cosmos are such a forgiving flower from seed
Cosmos are easy because they come from open, sunny, low-fertility ground. They are frost-tender annuals in the daisy family, with roots in Mexico and Central America, and that explains how they grow: heat, sun and lean soil, which most of us can offer.
The seed is long, thin and easy to handle. It germinates quickly in warm soil, and the seedlings grow away fast. You can raise them in punnets or sow them straight where they are to flower.
Sun, and soil that is not too good
Give cosmos full sun and free-draining soil, and feed them as little as possible. This is the one piece of advice that catches people out. Rich, manured beds and nitrogen feeds push soft, ferny foliage and weak stems, and far fewer flowers, since the plant feels no need to hurry into bloom.
In very hot inland gardens some afternoon shade helps, but elsewhere an open, sunny spot is ideal. Once established, cosmos are fairly drought-tolerant and cope with short dry spells, though seedlings and pots still need steadier moisture.
- Plant in: full sun, ordinary or even poor soil, with room to sway.
- Skip: heavy feeding, compost-laden beds, and a shady, sheltered corner.
When to sow cosmos, zone by zone
Sow cosmos once the frosts have passed and the soil has warmed, because they are frost-tender and will sulk or collapse in the cold. The exact window depends on where you garden:
- Tropical (Darwin, Cairns, Townsville): the dry season, roughly May to October, is the comfortable window, since the tropics stay warm year-round and wet-season humidity can bring fungal trouble.
- Subtropical (Brisbane, Sunshine Coast, Coffs Harbour): a long run from spring through summer and into early autumn.
- Arid and semi-arid (Alice Springs, Mildura, Kalgoorlie): spring once frost has passed, through summer, with steady water until the plants take hold.
- Temperate (Sydney, Melbourne, Canberra): after the last frost in spring, often from September in milder spots and a little later in colder inland gardens like Canberra, then on into early summer for autumn flowers.
- Mediterranean (Perth and southern WA, and Adelaide): spring through summer, where the dry summers suit them well with the occasional deep soak.
- Cool and alpine (Tasmanian highlands, Ballarat, Orange, and Stanthorpe, which stays cool despite being in Queensland): the season is short, so start seed under cover and plant out only after the last frost. Quicker, lower-growing types are the safer bet. Our guide to starting seeds indoors helps you get a head start.
The two cosmos worth knowing
The two cosmos species you are most likely to grow are Cosmos bipinnatus and Cosmos sulphureus, and it helps to know which you have, because they behave a little differently.
Cosmos bipinnatus is the tall, airy one, with feathery foliage and large flowers in white, pink and crimson. Tall forms can reach about 1.5 metres, sometimes more in rich soil, so they suit the back of a bed and may want a little support in a windy garden.
Cosmos sulphureus, the sulphur cosmos, carries flowers in hot oranges, golds and yellows. Many garden strains are more compact, and it copes better with heat and humidity, which makes it a favourite of bees and hoverflies. It self-seeds freely in warm climates, so keep an eye on spent flowers near bushland. Whichever colour tempts you, browse our flower seeds to compare types.
Keeping the flowers coming, and the plant in bounds
Pick cosmos often and they can keep flowering for months. Every stem you cut, and every spent bloom you snip off, pushes the plant to make fresh buds rather than seed, so a cosmos patch and a full jar on the kitchen bench tend to go hand in hand. They are one of the great cut flowers: light, long-stemmed and cheerful.
Left alone, cosmos will set seed and sow themselves for next year, which is part of their charm. It does pay to keep half an eye on them, though.
Cosmos self-seed freely, and have naturalised in parts of warmer Australia, including Queensland and Western Australia, so in warm, wet areas and near bushland it is worth deadheading before too much seed drops. If you are unsure, your state or territory weed authority keeps current lists, such as Queensland's weed information. A few minutes with the secateurs keeps a generous self-sower from turning into a nuisance.
For a flower that asks so little, cosmos give an enormous amount back: months of bloom, armfuls for the vase, and a quiet hum of bees from morning to dusk. Plant a patch where you can see it, and it may become one of the first things you sow each year. If you would like the rest of the garden working as hard for the bees, our guide to planting a pollinator garden pairs beautifully with a cosmos patch.
Frequently asked questions
How long do cosmos take to flower from seed?
In warm conditions cosmos usually flower about eight to twelve weeks after sowing, sometimes sooner. Sow in late spring and you can expect blooms through summer and into autumn. Successive sowings a few weeks apart stretch the display.
Why are my cosmos all leaves and no flowers?
In a sunny bed, too much food is the usual culprit. Rich soil and nitrogen feeds can push leafy growth at the expense of flowers, so stop feeding and be patient. If the spot is shady or stays wet, move your next sowing somewhere brighter and free-draining.
Do cosmos come back every year?
Cosmos are annuals, so the original plants die off after they set seed. They self-seed readily, however, so a patch often returns on its own the following warm season. Leave a few flowers to go to seed if you want volunteers.
Can I grow cosmos in pots?
Yes, especially the shorter, dwarf types. Use a free-draining mix, a decent pot and a sunny spot, and go easy on feeding. Taller types suit large containers but may need staking.
Are cosmos good for bees?
Very much so. The open, daisy-like flowers are easy for honey bees, native bees, hoverflies and other beneficial insects, and sulphur cosmos in particular is a magnet for them. Choose open, single-flowered types where you can, since very double blooms are harder for insects to use.
Do cosmos need staking?
Tall cosmos bipinnatus can flop in wind or in rich soil, so a few stakes or a low support help in exposed spots. Growing them in lean soil keeps the stems sturdier. Dwarf types rarely need any support.
Will cosmos become a weed in my garden?
They can self-seed enthusiastically, and some cosmos have naturalised in warmer parts of Australia. Deadheading spent flowers and pulling out unwanted seedlings keeps them in check. Check your state or territory weed information if you garden near bushland.
Related growing guides
- Self-Seeding Flowers That Come Back Every Year
- Eight Easy Cut Flowers to Grow From Seed in Australia
- A Beginner's Guide to Growing Annual Flowers from Seed
Ready to plant? Browse our Flower Seeds.