How to grow Cosmos in Australia
Cosmos is a fast, easy summer annual with feathery leaves and daisy flowers in white, pink, crimson, orange and yellow. It grows from seed to flower in about two months, flowers right through the warm season if you keep picking, and its open flowers feed bees, hoverflies and other helpful insects in the vegie patch. Lean soil and little water suit it best.
📖 Our full guide: How to Grow Cosmos in Australia
At a glance
When to sow by region
| Region | When to sow | Regional note |
|---|---|---|
| South East Queensland | September to February | |
| Northern Rivers NSW | September to February | |
| Sydney & Coastal NSW | September to February | |
| Melbourne & Victoria | October to December | Frost tender. Sow Oct to Dec after frost in lean soil. Self seeds, and seedlings appear once frosts finish. |
| Adelaide & SA | September to December | Sow Sep to Dec direct. Flowers through summer on little water; rich soil gives leaf and few flowers. |
| Perth & WA | September to December | Sow Sep to Dec direct. Flowers through summer on little water; rich soil gives leaf and few flowers. |
| Tasmania | October to December | Sow Oct to Dec after frost. Flowers until the first autumn frost. |
| Canberra & ACT | November and December | Frost tender. Sow Nov to Dec after the last frost in early November. Self seeds; seedlings emerge once frosts finish. |
| North Queensland | April to October | Dry season Apr to Oct. Wet season storms flatten the tall stems and the plants rot. |
| Darwin & Top End | April to October | Dry season Apr to Oct. Wet season storms flatten the tall stems and the plants rot. |
Where to plant
Sow cosmos in full sun in light, well-drained soil. It grows in almost any soil, and flowers best in soil that is not too rich: fertile soil, heavy feeding or too much water give tall, leafy plants with few flowers. It is frost tender, so sow it after the last frost.
Garden cosmos (Cosmos bipinnatus) is naturalised in parts of Queensland and New South Wales and is listed as an environmental weed in Queensland. Deadhead plants near bushland, roadsides and paddocks so they do not seed beyond the garden.
- Adelaide and Perth: sow direct from September to December; cosmos flowers through summer on little water.
- Melbourne and Hobart: sow from October to December after frost; it flowers until the first autumn frost.
- Canberra: sow in November and December after the last frost.
- Sydney, South East Queensland and the Northern Rivers: sow from September to February.
- North Queensland and Darwin: sow in the dry season, April to October; wet-season storms flatten tall stems and plants rot.
Choosing a variety
- Sensation Mix is a classic tall garden cosmos in white, pink and crimson, about 1 to 1.5 m tall.
- Double Click Mix has double, ruffled flowers that are good for picking.
- Cosmic Orange is a compact Cosmos sulphureus, with orange flowers on bushy plants that suit hot, dry spots and pots.
How to plant
- Sow seed directly where it is to flower, about 5 mm deep, once the soil is warm and frost has passed.
- Keep the soil just moist; seedlings appear within about a week.
- Thin seedlings to about 30 cm apart, wider for tall varieties.
- When plants are about 20 to 30 cm tall, pinch out the growing tips to make them branch and give more flowers on shorter, sturdier plants.
Watering and feeding
Water seedlings until they are established; after that cosmos handles dry conditions well and needs water only in long dry spells. Do not feed: overwatering and fertiliser reduce flowering.
Pruning and care through the year
- Deadhead or pick the flowers often to keep new ones coming.
- Stake tall varieties in windy, exposed or storm-prone spots, or grow them against a fence.
- Let some flowers set seed at the end of the season if you want self-sown seedlings, away from bushland.
Picking flowers
Cosmos is an excellent cut flower. Cut stems when the first flower on the stem has petals just opening but not yet flat, early in the morning, and put them straight into water. Regular cutting is the best way to keep plants flowering.
Growing in pots
Compact varieties grow well in a pot at least 25 cm wide with an ordinary potting mix. Keep the pot in full sun, water when the top of the mix dries, and do not feed.
Companion planting
Cosmos is one of the best flowers for the summer vegie patch. Its open flowers feed bees that pollinate fruiting crops and hoverflies and tiny wasps whose young eat aphids and caterpillars.
Good companions
- Tomato Brings bees and hoverflies to the summer bed.
- Cucumber Draws bees to pollinate the separate male and female flowers.
- Beans Feeds helpful insects without competing for the same space.
- Capsicum Brings in pollinators and pest predators.
- Marigold Another easy summer flower that suits the same lean, sunny conditions.
Pests and diseases
Cosmos is tough and has few problems. Aphids and thrips in warm weather and powdery mildew late in the season are the main ones.
Pests to watch for
- Aphids Clusters of small insects on new shoots and flower buds.
- Thrips Silvery streaks on leaves and distorted, marked petals.
Diseases to watch for
- Powdery mildew White, powdery patches on leaves late in summer. Give plants space and pull out tired plants.
Common questions
Why is my cosmos tall with few flowers?
The soil is too rich, or the plants were fed or watered too much. Cosmos flowers best in lean soil with little water. Pinch out the tips while young to make plants branch.
Does cosmos come back every year?
Cosmos is an annual, but it self-seeds readily, so new plants often come up the next season once the soil warms.
Should I deadhead cosmos?
Yes. Removing spent flowers, or picking them for the vase, keeps plants flowering for longer.
When should I plant cosmos?
After the last frost, once the soil is warm, from spring into early summer. In the tropics, sow in the dry season. Pick your region at the top of the page for the months.
Is cosmos a weed?
Garden cosmos has naturalised in parts of Queensland and New South Wales and is listed as an environmental weed in Queensland. Deadhead plants growing near bushland or roadsides so they do not spread.
Is cosmos good for the vegie garden?
Yes. Its open flowers feed bees and helpful insects such as hoverflies, which help pollinate crops and control aphids.
Sources we checked: Queensland Government: Species profile, Cosmos bipinnatus; NC State Extension Gardener Plant Toolbox: Cosmos bipinnatus; Johnny's Selected Seeds: Growing cosmos from seed; RHS: Cosmos bipinnatus Sensation Series.