When to plant Onion in Tropical Australia

Tropical zone — applies to Darwin, Cairns, Townsville, Broome, Mackay and similar areas. Growing elsewhere? Temperate · Cool · Subtropical · Arid.

Sow onion seed directly from March to July, covering it just 5 mm deep, as the dry season opens. Onions are a marathon crop in the tropics, needing anywhere from 160 to 230 days to mature, so an early sowing is essential to finish before the wet returns. Seed germinates in 10 to 20 days. The critical factor is day length: onions bulb in response to daylight hours, and only short-day varieties will form heads at tropical latitudes like Cairns and Broome. Long-day types sold for cooler states grow lush tops and never bulb. Thin seedlings to 5 to 10 cm apart.

Sowing calendar — tropical zone

Green months are your sowing window: March to July.

Growing notes

Choosing the right variety decides an onion crop in the tropics before a seed goes in the ground. Onions bulb according to the length of the day, and at the low latitudes of Townsville and Darwin only short-day varieties receive the signal to form a head. Plant a long-day southern variety and you will harvest nothing but leaves.

Timing is the second half of the puzzle. With 160 to 230 days to maturity, seed must go in early in the dry season so bulbs can finish and cure before monsoon humidity rots them in the ground. Sow thinly into a firm, weed-free, well-drained bed and keep the shallow seed consistently moist through the slow 10-to-20-day germination.

Onions have shallow roots and hate competition, so relentless weeding matters more here than for most crops. Water steadily while the tops grow, then withhold water once the bulbs swell and the tops flop, letting the skins dry. The frequent failures are bolting — plants running to flower instead of bulbing, usually from stressed or overly mature seedlings — and thick necks that will not store, caused by too much late nitrogen or water. Feed early, ease off as bulbs form, and lift promptly once the tops fall over.

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 tropical Australia?

Sow direct from March to July, early in the dry season, so the long 160-to-230-day crop can mature and cure before the wet returns.

Why do my onions grow leaves but never form a bulb?

Almost always the wrong variety. At tropical latitudes you need short-day onions; long-day types bred for southern states will not bulb here.

Why did my onions run to flower?

Bolting is usually triggered by stressed or over-mature seedlings. Sow at the right time, keep growth steady, and thin so plants are not crowded.

How long do onions take to grow in the tropics?

Seed germinates in 10 to 20 days, and bulbs need a long 160 to 230 days to mature, so plant as early in the dry season as possible.

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