When to plant Onion in Temperate Australia
Temperate zone — applies to Sydney, Melbourne, Adelaide, Perth, Newcastle and similar areas. Growing elsewhere? Cool · Subtropical · Tropical · Arid.
Sow onions direct from February to August in temperate Australia, just 5 mm deep in rows 25–40 cm apart, thinning plants to 5–10 cm. This is a slow crop by any measure: germination takes 10–20 days and bulbs need 160–230 days to mature, so seed sown in autumn or winter is harvested the following summer. The pay-off for the wait is that onions ask very little along the way — sun, firm well-drained soil, and diligent weeding while the seedlings are small. Match the variety to your sowing month and the rest mostly takes care of itself.
Sowing calendar — temperate zone
Green months are your sowing window: February to August.
Growing notes
Onions are hardy, so temperate frosts from May onward do them no harm at all — seedlings sit through winter slowly building leaves, then bulb up as days lengthen in spring. The subtlety in the February-to-August window is day length. Onion varieties are triggered to bulb by particular day lengths, so early-season sowings in February and March suit early (short-day) varieties, autumn sowings suit mid-season types, and late winter sowings suit late (long-day) keepers. Sow a late variety too early and it may bolt to seed in spring instead of bulbing; the seed packet's recommended sowing months are worth taking literally.
Give them full sun and soil that was composted for a previous crop rather than freshly enriched — too much nitrogen grows lush tops and soft bulbs that store badly. Firm soil suits them, and because a young onion is a single thin blade for months, weeds are the crop's biggest practical enemy. Hand weed early and often; an onion bed lost to weeds in June rarely recovers.
When the tops soften and flop over naturally in summer, stop watering, let the bulbs cure in the ground for a week or two if the weather is dry, then lift and dry them fully before storing somewhere airy.
Onion at a glance
| Sowing depth | 5 mm |
|---|---|
| Plant spacing | 5-10 cm |
| Row spacing | 25-40 cm |
| Germination | 10-20 days |
| Days to maturity | 160-230 days |
| Frost tolerance | Hardy |
| Method | Raise Seedlings,Sow Direct |
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 do you sow onions in temperate Australia?
- Sow direct from February to August, matching variety to timing — early varieties in late summer and autumn, late keeping varieties toward winter's end. The seed goes only 5 mm deep.
- How long do onions take to grow?
- Onions are one of the slowest vegetables: 10–20 days to germinate and 160–230 days to mature. An autumn sowing is typically harvested in early to mid summer the following year.
- Why are my onions bolting to seed instead of forming bulbs?
- Bolting is usually a variety-timing mismatch — sowing a late variety too early, or seedlings being checked by stress — so the plant flowers instead of bulbing. Sow the variety in its recommended months and keep growth steady.
- Do onions survive frost?
- Yes, onions are hardy and grow through a temperate winter without protection. Frost is not the risk with this crop; weeds smothering the slow, slender seedlings are.
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