How to grow Onion in Australia

Where to plant

Grow onions in full sun in firm, fertile, free-draining soil improved with compost and aged manure well before sowing. Onions compete poorly with weeds, so start with a clean bed and keep it weeded. Wet soil leads to rot.

Rotate: do not grow onions where onions, garlic or leeks grew recently, and never where white rot has appeared, as it survives in the soil for many years.

• South East Queensland, the Northern Rivers, North Queensland and Darwin: sow short-day varieties in autumn, from about March to June in the subtropics and April to June in the tropics, so they grow through the cool, dry months.
• Sydney and Perth: sow short-day to intermediate varieties in autumn, about March to June.
• Melbourne and Adelaide: stagger varieties. Sow short-day types in autumn, intermediate types in midwinter, and long-day keepers in late winter to early spring.
• Canberra: sow early varieties in April and May, and mid to late varieties in August and September.
• Hobart: sow long-day varieties from July to September; they bulb up in the long summer days and are harvested in late summer.

Choosing a variety

Sow the wrong type at the wrong time and the plant either bolts to seed before forming a bulb or forms only small bulbs. Seed packets state the type and the region it suits. Brown onions are all-purpose, reds are crisp and good raw, and whites are mild.

How to plant

1. Sow seed in punnets or direct into the bed, about 1 to 1.5 cm deep, in rows about 20 to 30 cm apart.
2. Keep the soil moist until seedlings come up in about two weeks.
3. Thin or transplant seedlings to 10 to 15 cm apart when they are pencil thin.
4. When transplanting, plant shallowly so the base of the bulb sits near the surface, and trim very long leaves and roots slightly if needed.
5. Water in and keep weeds down.

Watering and feeding

Keep the soil evenly moist while the leaves are growing. Onions have shallow roots, so regular, moderate watering suits them better than rare heavy soaks. Ease off once the bulbs are swelling and stop when the tops begin to fall over, so the bulbs ripen rather than rot.

Onions are not heavy feeders. Dig in compost and a balanced fertiliser before sowing and give a nitrogen-rich side dressing early in the season. Stop feeding nitrogen once bulbs start to form, as it gives soft bulbs that store poorly.

Pruning and care through the year

• Weed often and by hand, taking care not to damage the shallow roots.
• Do not mound soil over the bulbs; they ripen best sitting on the surface.
• Pull and use any plants that send up a flower stalk, as they will not store.
• Watch for silvery streaks from thrips in dry spring weather.

Harvesting

Onions are ready when the tops yellow and fall over, typically 6 to 9 months after planting. Loosen the soil with a fork and lift the whole plant without bruising the bulb.

Cure the bulbs by laying them in a single layer somewhere dry and airy, in the sun for 2 to 3 days if the weather is dry, until the outer skins are papery and the necks have completely dried. Then trim the roots, cut the tops 3 to 5 cm above the bulb and store them in a cool, dry, well-ventilated place, below about 20°C and out of humidity. Well-cured long-day onions keep for months; short-day types keep less well. Bulbs that were wet in the weeks before harvest, damaged or poorly cured are the ones that rot in storage.

Growing in pots

Onions can be grown in a wide trough or pot at least 30 cm deep, 10 cm apart, but each plant gives only one bulb, so pots are better used for spring onions or a few red salad onions.

Using it

Use cured onions in almost any savoury dish. Red onions are crisp and mild enough for salads. Green tops of young plants can be used like spring onions. Do not refrigerate whole cured onions; keep them cool, dry and dark.