When to plant Aster in Cool Australia

Cool zone — applies to Hobart, Canberra, Ballarat, Orange, Armidale and similar areas. Growing elsewhere? Temperate · Subtropical · Tropical · Arid.

Raise aster seedlings from September to December, sowing into trays and transplanting out once frost has passed, as these are half-hardy plants that resent cold checks when young. Seed germinates in 14–28 days at 5 mm deep, and plants flower in 100–110 days, giving a display from late summer into autumn. Space them 20–30 cm apart with matching row spacing for good airflow. In cool districts like Hobart, Canberra and Orange, a spring sowing after the last frost gives asters the warm run they need to bud up before the cooler weather returns.

Sowing calendar — cool zone

Green months are your sowing window: September to December.

Growing notes

Asters bring late-season colour just as many other annuals fade, which makes them valuable in a cool climate where the flowering window is short. They are half-hardy, so mature plants shrug off light frost, but young seedlings are vulnerable and should not be planted out until the last spring frost has passed in districts like Ballarat and Armidale.

Raise them in trays, sow at 5 mm, and prick out into a sunny, open bed with free-draining soil. Good airflow matters, which is why the 20–30 cm spacing should not be crowded.

Common failure mode: the classic aster problem is fungal disease. In cool, damp conditions, or where plants are crowded and watered overhead in the evening, powdery mildew and aster wilt take hold and spoil the display. The fix is generous spacing for airflow, watering at the base rather than over the leaves, and rotating planting position each year rather than returning asters to the same soil.

Stake taller varieties, which flop after rain in exposed Canberra or Orange gardens, and deadhead regularly to keep the flush going. Asters are a good source of late nectar, so leaving a few final flowers benefits pollinators heading into autumn.

Aster at a glance

Common questions

when to plant asters in a cool climate

Raise seedlings from September and transplant out once the last frost has passed, continuing sowing through to December. Young asters are vulnerable to frost, so wait for settled conditions before planting them out in districts like Canberra.

why do my asters get mildew

Powdery mildew thrives in cool, damp, still conditions and where plants are crowded. Space them 20–30 cm apart for airflow, water at the base rather than over the foliage, and avoid replanting asters in the same soil each year.

how long do asters take to flower

Around 100–110 days from sowing. A spring sowing after frost gives flowers from late summer into autumn, filling the gap as earlier annuals fade.

can asters survive frost

Established, half-hardy asters tolerate light frost, which is part of why they flower so late in the season. Young seedlings are more tender, so raise them under cover and plant out only after the last spring frost.

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