When to plant Onion in Subtropical Australia

Subtropical zone — applies to Brisbane, Gold Coast, Sunshine Coast, Coffs Harbour, Lismore and similar areas. Growing elsewhere? Temperate · Cool · Tropical · Arid.

Sow onion seed direct from February to August in the subtropical zone, barely covered at 5 mm deep. This is the patient gardener's crop: germination takes 10–20 days and bulbs need 160–230 days, so a March sowing near Lismore won't be lifted until spring. Variety choice matters more here than almost anywhere in the garden — bulbing is triggered by day length, and subtropical latitudes need short-day or intermediate varieties; southern long-day types will grow lush green tops and never form a bulb. Thin seedlings to 5–10 cm and keep rows scrupulously weeded, because onions compete poorly.

Sowing calendar — subtropical zone

Green months are your sowing window: February to August.

Growing notes

The onion's 160–230-day haul is the defining fact of growing it in the subtropics: whatever month you sow between February and August, the crop spends most of its life in the benign, low-humidity winter — exactly where you want it. Fully hardy, it ignores any light frost the region can produce. Early sowings from February to April get the longest run and the biggest bulbs; July and August sowings still work but finish smaller, because lengthening spring days trigger bulbing regardless of plant size. The whole schedule is really about the exit: bulbs need to be lifted and cured in the dry spring air of Brisbane or Coffs Harbour, since onions curing in December humidity grow mould instead of papery skins.

Grow them in full sun in firm, fertile, well-drained soil, and resist the urge to hill soil around them — bulbs form best sitting almost on the surface. Ease off nitrogen from mid-season, or you'll get thick soft necks that never store.

• Grey-purple fuzz on leaves in a humid spell is downy mildew: water mornings only, at the base, and widen spacing
• Bulbs refusing to form is almost always a long-day variety at the wrong latitude
• Thick necks and poor storage point to late nitrogen or overwatering near maturity

Onion at a glance

Companion planting

Grows well with

Carrot: Scent of each deters the other's primary pest (carrot fly, onion fly)

Chamomile: Improves onion growth and flavour

Beet: Efficient space use; onions deter leaf miners that attack beet

Keep away from

Bean (Bush): Alliums inhibit nitrogen-fixing bacteria essential to beans

Bean (Climbing): Alliums inhibit nitrogen-fixing bacteria

Peas: Alliums inhibit legume growth

Common questions

When should I sow onions in Brisbane?

Sow direct from February to August. February–April sowings produce the biggest bulbs, and the crop is lifted in spring after 160–230 days.

Why won't my onions form bulbs?

Almost always the wrong variety — bulbing is day-length driven, and long-day types bred for southern Australia never trigger at subtropical latitudes. Choose short-day or intermediate varieties.

How long do onions take from seed?

Expect 10–20 days for germination and 160–230 days from sowing to a mature bulb, depending on variety and sowing month.

Can I grow onions over summer in the subtropics?

No — the window closes in August so bulbs mature and cure in dry spring weather. Onions curing through the humid December–February wet tend to mould rather than dry.

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