When to plant Dahlia in Cool Australia

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

Raise dahlias from seed in September to December, sowing under cover and transplanting once all frost has passed. Dahlias are frost tender, so in cool gardens around Ballarat, Orange and Canberra you must wait for the frost-free gap before planting out. Sow seed 10-15 mm deep in trays; it germinates in 7-21 days with warmth. From seed, plants take around 110-130 days to reach their first flowers, giving a long summer-into-autumn display until the frosts return. Space plants generously, 50-100 cm apart in rows 60-90 cm apart, depending on variety, in a sunny, sheltered, fertile bed.

Sowing calendar — cool zone

Green months are your sowing window: September to December.

Growing notes

Dahlias reward a cool-climate garden with a long, generous flush of colour, but the frost calendar governs everything. In cool districts, frost will blacken the foliage in autumn and can catch tender young plants in spring, so time your planting to your local frost-free window and be ready to protect early plants on cold nights.

The key failure mode over winter is losing the tubers to cold, wet soil. In the coldest highland gardens around Armidale, tubers left in the ground often rot or freeze. The fix is to lift them after the foliage is frosted, cut back the stems, dry the tubers off, and store them somewhere cool, dark and frost-free until spring. In milder cool districts with good drainage you can sometimes leave them in place under a thick, dry mulch.

During the growing season, dahlias are hungry and thirsty. Feed with a balanced or slightly potassium-rich fertiliser, water deeply in dry spells, and stake taller varieties early because the brittle, hollow stems snap in wind and under the weight of the blooms.

Slugs and snails devastate emerging shoots in spring, so protect new growth. Deadhead spent flowers continually to keep the plant producing right up until the first frost ends the season.

Dahlia at a glance

Common questions

when to plant dahlias in a cool climate

Sow seed under cover from September, and only plant seedlings or tubers out once the last frost has passed, often October in cool districts. Dahlias are frost tender, so late spring planting avoids losing young plants to a cold snap.

do you have to lift dahlia tubers in cold areas

In the coldest cool-climate gardens, yes. After frost blackens the foliage, lift the tubers, dry them and store them frost-free until spring to avoid rot and freezing. In milder, well-drained spots a thick dry mulch can let you leave them in the ground.

how long do dahlias take to flower from seed

Around 110-130 days from sowing. A spring sowing under cover therefore flowers through summer and into autumn, until the first frosts end the display.

why are my dahlia stems falling over

The hollow stems are brittle and snap under heavy blooms and wind. Stake taller varieties early, before they get top-heavy, and plant in a sheltered position to reduce wind damage in exposed cool-climate gardens.

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