How to Grow Onions in Australia

Onion (Allium cepa) belongs to the onion family, Amaryllidaceae, and is grown for its brown, white or red bulb, round, long or flat. It is a biennial (a plant that flowers in its second year) grown as an annual. Commercial crops grow from the Lockyer Valley in Queensland to the Riverina in New South Wales. The catch is day length: each variety starts its bulb only when days reach a set length.

Three mistakes that matter

1. Sowing a type that does not suit the month. A variety sown out of step with its day length can bolt (run to flower), grow a thick neck or make no real bulb. Match the type to your sowing month: see Sowing Seeds.
2. Letting weeds crowd the rows. Young onions have thin, upright leaves and compete poorly, and weeds that get ahead in the first months stunt them for good. Weed by hand every week while the plants are small: see Care by growth stage.
3. Feeding nitrogen late, or digging in fresh manure. Both give soft bulbs with thick necks that let rot in and keep poorly. Feed early, stop nitrogen as the bulbs start to swell, and use only well-rotted manure: see Site, soil and setup.

When to plant

Sow so each plant grows plenty of leaf through the cool months before its bulbing day length arrives, because bigger plants make bigger bulbs. Seed germinates in soil from about 8°C to 30°C, so soil warmth seldom limits sowing.

Sow short-day types early in a window and long-day types late in it. Sowing too early risks bolting, and sowing too late gives small bulbs (see Common problems).

Site, soil and setup

Onions sit on the surface with shallow roots, so they want an open, sunny bed that drains well and is free of weeds.

A bed that has just grown leafy greens or brassicas suits onions well, because those crops use up much of the nitrogen. Never sow where onions or garlic have shown white rot (see Pests and diseases).

Container growing

A pot or trough at least 30 cm deep and 30 cm across, holding 20 L or more, grows a small crop. Fill it with vegetable potting mix and grow small types. For flat bulbs 2 to 5 cm across, sow 'Cipollini Brown'; 'Gladalan White' can be pulled young as a spring onion. Space plants about 5 cm apart for small bulbs.

• Water whenever the top of the mix starts to dry, which can mean daily in warm, windy weather.
• Feed seaweed solution at half strength, 15 mL per 9 L, fortnightly from the first true leaf. Switch to fish and seaweed blend at 25 mL per 9 L fortnightly, and stop once bulbs swell. Or mix a controlled release granular for containers into the mix at sowing.
• Stand the pot in full sun, six hours or more a day.

Sowing Seeds

Raise onions in trays for transplanting or sow them straight into the row; trays keep the fiddly early weeding out of the bed.

Some packets name no type, such as 'Rio Red Rocks', 'Early White Grano', 'Long Tropea Red' and the 'Odourless' onions. Follow the sowing notes that come with them. Some seed is treated with a fungicide (thiram) and may be dyed. Never eat it or feed it to animals, wear gloves, wash your hands afterwards, and keep it from children. Use fresh seed, because onion seed loses viability within a year or two.

Starting in trays

• Sow 5 mm deep in seed-raising mix, or drop 6 or 7 seeds into a 1 cm hole in each cell to grow a clump.
• Keep the mix moist in morning sun and afternoon shade; seedlings show in 7 to 14 days.
• Plant out 6 to 8 weeks after sowing, when seedlings are 10 to 15 cm tall and no thicker than a pencil.
• Harden them off first (a week or two of longer spells outdoors each day, which toughens them for the open bed).

Sowing straight into the bed skips transplanting, but the seedlings look like blades of grass and are easily lost among weeds.

Sowing direct

• Draw a furrow about 5 mm deep, with rows 30 cm apart.
• Mix the fine seed with dry sand so it runs out evenly (see sowing tiny seeds), cover it with fine soil and water gently.
• Keep the surface damp until the seedlings show.
• Thin early to 2 cm apart, then later to 10 cm, and eat the thinnings as spring onions.

Planting out

• Trim the tops and roots slightly, which makes the seedlings easier to handle and settle.
• Lay them along the side of a trench about 5 cm deep, roots at the bottom, and backfill. They stand up again within a week or so.
• Space plants 10 cm apart in rows 30 cm apart, or multi-sown clumps 25 cm apart.
• Thin each clump as it grows until three or four bulbs remain.
• Keep the base at soil level, because bulbs swell on the surface and planting too deep invites neck rot.
• Water in with seaweed solution at half strength, 15 mL per 9 L.

Care by growth stage

Clean rows and steady water in the leafy months, then potash and even moisture as the bulbs swell, decide both size and keeping. Weeks count from sowing and stretch in cold districts.

Mulch lightly between the rows once plants are established, keeping it clear of the bulbs so they do not rot. In frosty districts a little straw over the rows protects young plants in hard cold. Rake it back in spring so the soil can warm. Hand-weed rather than hoe close to the plants.

Companion planting

Onions have narrow leaves, so good neighbours stay low and do not share their diseases.

Grows well with

• Lettuce, quick and low, which is cut before the bulbs need the room.
• Carrot, whose fine leaves cast little shade on the onion row.
• Spinach, low and leafy, which shares the cool season without shading the row.

Keep apart

• Garlic, leeks and shallots, which share white rot, thrips, downy mildew and rust.
• Sweet corn and climbing beans, tall crops that can shade the short onion leaves.
• Pumpkin and other sprawling vines, which smother the rows.

Harvesting Bulbs

Lift the bulbs once about half the tops have yellowed and fallen over, usually 24 to 34 weeks from sowing.

• Bend over any tops still standing once most have fallen.
• Choose a dry day, loosen the soil with a fork and ease the bulbs up rather than pulling on the leaves.
• Handle them gently, because a bruised bulb is the first to rot.
• Eat short-day types, red types and 'Long Tropea Red' early, and keep brown long-day types for storing.

Curing

Without curing, even a long keeper is likely to rot early. Roast brown onions whole, slice red ones raw into salads, and cook cipollini whole.

Common problems

Flower stalks. Bolting usually follows a sowing too early, so plants thicker than a pencil meet winter cold, or drought and heat stress. Bolted bulbs will not store, so use them first.

Tiny bulbs or none. A short-day type sown too late bulbs before it has grown, making pickling-sized onions. A long-day type in the north may never meet its day length. See When to plant for the timing.

Thick necks. Late nitrogen and fresh manure are the usual cause (see Three mistakes), and an early sowing can do the same.

Split or double bulbs. Irregular watering while the bulbs swell splits them and shortens their storage life.

Pests and diseases

Most onion troubles in Australian gardens are sap-sucking thrips and a few soil and leaf diseases that rotation and hygiene help keep down. Use only sprays labelled for the pest on onions, and keep to any withholding period before harvest.

Onion thrips (Thrips tabaci). Slender insects about 1.5 mm long rasp the leaves, leaving silvery streaks, and large numbers can kill seedlings. Plants suffer most as bulbs begin to form. Hose them off or spray insecticidal soap, and encourage ladybirds and lacewings. Read more

Downy mildew (Peronospora destructor). Pale green-yellow patches carry a purplish-grey growth, spreading in cool, humid spells around 10°C to 15°C. Pull volunteer onions that carry it over, and use a copper fungicide if it keeps returning. Read more

White rot (Sclerotium cepivorum). Plants yellow, wilt and pull free, with white fluff and black specks like poppy seeds on the base. The specks survive in soil for 20 years or more. Bin sick plants with the soil around their roots, never compost them, and grow no alliums there again. It travels in soil, so clean boots and tools after working that bed. Western Australia keeps it out with strict movement controls.

Neck rot (Botrytis allii). The neck softens and rots once the bulbs are in storage. It carries on seed, volunteer plants and debris, and thick necks and bruised bulbs let it in, so cure every bulb thoroughly.

Black mould (Aspergillus niger). Sooty black spore masses form under the outer scales of stored bulbs. Heat and humidity at harvest favour it, so dry the bulbs quickly and store them cool.

Cutworm (Agrotis species). Soil caterpillars chew seedlings at ground level at night and hide 1 to 10 cm down by day, most in cool, wet weather. Patrol with a torch after dark and pick them off. Read more

Prevention first

• Raise your own seedlings rather than buying them, so white rot does not arrive with them.
• Keep onion, garlic and leek scraps out of the compost, and clear old onion debris after harvest.
• Avoid crowding and water the soil, not the leaves, so the foliage dries quickly.
• Pick off dead leaves as they appear, since they harbour disease.

Tips

1. In southern gardens, sow three types for a longer harvest. Sow a short-day type around Anzac Day, an intermediate type in June and a long-day type in August.
2. As bulbs near maturity, brush the soil off the top third of each one. The sun and air on the shoulders help it ripen and cure.
3. Eat the thick-necked and oddly shaped bulbs first. Thin-necked bulbs with a flat base usually keep longest.