When to plant Cosmos in Temperate Australia
Temperate zone — applies to Sydney, Melbourne, Adelaide, Perth, Newcastle and similar areas. Growing elsewhere? Cool · Subtropical · Tropical · Arid.
Sow cosmos direct from August to December, once the worst of winter has passed and the soil is warming. Drop seed 10–15 mm deep straight where the plants are to flower, spacing them about 50 cm apart with rows 45–60 cm apart. Germination takes 7–14 days in mild soil, and plants reach flowering in 80–100 days, so a spring sowing carries colour right through to autumn. Cosmos thrives on neglect in temperate gardens around Perth, Adelaide and Newcastle, asking only for full sun and free-draining ground rather than rich, pampered beds.
Sowing calendar — temperate zone
Green months are your sowing window: August to December.
Growing notes
Cosmos is close to foolproof in the temperate zone, but the classic mistake is being too kind to it. Sown into rich, well-fed soil it grows tall and leafy, flops over, and produces very few flowers. Give it lean ground and full sun instead, and you will be rewarded with masses of daisy-like blooms from summer into autumn.
Because it is only half hardy, hold direct sowings until frost danger has cleared, roughly from August in warmer temperate pockets. Seed sown into cold, wet soil rots rather than germinates, so patience early in the season pays off.
The taller varieties need support in exposed gardens; a few stakes or a low grid of string keeps them upright through the windy afternoons common around Perth and Newcastle. Pinch out the growing tips when plants are 20–30 cm tall to encourage bushier growth and more flowering stems.
Deadhead regularly to keep the display going, as plants that set seed will slow their flowering. Cosmos self-sows enthusiastically, so unless you want volunteers everywhere, remove spent heads before they scatter. Powdery mildew can appear late in the season during humid spells, but by then the plants have usually given their best and can be pulled.
Cosmos at a glance
| Sowing depth | 10-15 mm |
|---|---|
| Plant spacing | 50 cm |
| Row spacing | 45-60 cm |
| Germination | 7-14 days |
| Days to maturity | 80-100 days |
| Frost tolerance | Half Hardy |
| Method | Raise Seedlings,Sow Direct |
Common questions
- why is my cosmos all leaves and no flowers
- This is almost always caused by soil that is too rich or too much nitrogen fertiliser. Cosmos flowers best in lean, unimproved ground in full sun. Stop feeding, make sure it gets at least six hours of direct sun, and the flowers will follow.
- when can I sow cosmos in Perth
- In temperate Perth, sow cosmos direct from August through to December once the soil has warmed and frost risk has passed. Early spring sowings give the longest flowering season, carrying colour from summer into autumn.
- do I need to stake cosmos
- Tall cosmos varieties often need support, especially in windy gardens. A few stakes or a low grid of string keeps stems upright. Pinching out the tips when plants are 20–30 cm tall also produces sturdier, bushier plants.
- does cosmos come back every year
- Cosmos is an annual and dies off after flowering, but it self-sows readily. In a temperate garden you will often get volunteer seedlings the following spring if you let some flowers set seed.
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