How to Grow Spring Onions in Australia

Spring onion is mostly the bunching onion (Allium fistulosum), with a few bulb onion (Allium cepa) varieties sown thickly and pulled young. Both belong to the onion family, Amaryllidaceae, and are grown for green stalks and slim white shanks. Day length does not govern the bunching onion, so a pinch of seed every few weeks can keep the kitchen supplied most of the year. What trips gardeners up is the first month, when thread-thin seedlings vanish under weeds or dry soil.

Three mistakes that matter

1. Letting weeds into the row. Spring onions are poor competitors, and seedlings as fine as grass are smothered within weeks. Weed by hand every few days for the first month, then mulch: see Care by growth stage.
2. Letting the soil dry out. The roots are small, so a dry spell quickly stalls growth. Keep the soil evenly moist from sowing right through harvest.
3. Sowing the whole packet at once. A single big sowing gives a glut, then nothing. Sow a short row or a pot every three to four weeks through your window for a steady supply.

When to plant

Seed comes up in soil from about 10°C to 29°C, and quickest at 18°C to 25°C. Cold soil in a cool-zone winter and the heat of a tropical wet season are the real limits, not the month.

At the edges of a window, sow in trays under cover rather than in the open bed. Plants started late in autumn grow slowly through winter, taking up to 14 weeks rather than 8.

Site, soil and setup

A light hand with nitrogen suits this crop, because soft, lush growth invites rot and rust.

Container growing

Spring onions are shallow rooted, so a pot at least 20 cm across and 15 to 20 cm deep, about 5 L, will do. A styrofoam broccoli box, 20 to 30 cm deep, suits them better. Fill it with a quality vegetable potting mix. Potting mix can carry Legionella bacteria, so wet it down, wear gloves and a P2 mask, and wash your hands afterwards. Clumping 'Evergreen' suits a pot, because it can be cut stalk by stalk.

• Water whenever the top of the mix starts to dry, which can mean daily in hot weather.
• Feed seaweed solution at half strength, 15 mL per 9 L, fortnightly for the first month. Then switch to fish and seaweed blend at 25 mL per 9 L, fortnightly.
• Stand the pot in full sun, out of strong wind.

Sowing Seeds

Sow spring onions straight into the bed or pot, or start them in trays at the cold or hot edges of a window. Choose the type first, because it decides how you grow it.

The bunching types do not form a true bulb, so they can stand in the bed for months. To grow onions for storing, see bulb onions.

Sowing direct

• Draw a shallow drill about 5 mm deep, up to 15 mm in light, sandy soil, with rows 15 to 30 cm apart.
• Sprinkle the seed thinly along it, cover with fine soil and firm gently.
• Water with a fine spray and keep the surface damp until the seedlings show, usually in 7 to 14 days.
• For pots, scatter the seed over the mix and cover it lightly.

Starting in trays

• Sow thinly in seed-raising mix in punnets or a tray, 5 mm deep.
• Grow the seedlings on for 4 to 6 weeks, until they are 10 to 15 cm tall.
• Harden them off (a week or so of longer spells outdoors each day, which toughens them for the open bed).
• Tease the roots apart, lay the seedlings in a shallow trench about 10 cm apart and cover the roots.
• Water in with seaweed solution at half strength, 15 mL per 9 L. Seedlings that lie flat stand up within a few days.

Care by growth stage

Steady moisture and a weed-free row decide the crop; feeding is light. Weeks count from sowing and stretch in winter.

Once the seedlings are up, mulch between the rows with fine sugarcane or straw. Pile it or soil against the stems as they thicken to blanch them (keep out the light), which lengthens the white shank. If growth stalls in poor soil, a light dose of all-purpose controlled release granular (NPK around 14-5-10) scratched in can help.

Thinning

Crowded seedlings hold humid air around the leaves, which favours downy mildew and rust. Thin in two passes, and treat every pull as an early harvest.

• Thin rows grown for slim stalks to about 2 cm apart once the seedlings are big enough to grip.
• For thicker stems, thin again to 5 to 10 cm apart a few weeks later.
• Eat the thinnings in salads and stir-fries.
• Leave 'White Lisbon' sown thickly and pulled young, because crowding keeps the stalks fine.
• Split an old clump of a bunching type and replant single shoots to start a new one.

Companion planting

Spring onions are slim and quick, so they tuck in beside other low crops and move on before the space is needed.

Grows well with

• Carrot, whose fine foliage casts little shade on the onion row.
• Lettuce, low and quick, harvested over the same weeks.
• Beetroot and silverbeet, which share the moist, well-fed bed.

Keep apart

• Leeks, garlic, chives and bulb onions, which share thrips, rust and white rot.
• Climbing peas and beans, which shade the low row and which planners list as poor neighbours.

Harvesting Stalks

Start pulling once plants reach 15 to 20 cm, usually 8 to 14 weeks from sowing. Full size is 30 to 40 cm tall and about pencil thick.

• Pull whole plants or whole clumps, gripping low on the stem.
• In heavy soil, loosen the ground beside the row with a fork first.
• Cut clumps 3 to 5 cm above the soil instead, and they regrow a couple of times.
• Pull 'White Lisbon' before the base starts to swell into a bulb.

Keep spring onions in a plastic bag with a damp paper towel in the fridge, where they hold for about a week. Discard shrivelled or yellow leaves. Slice them raw into salads or scatter them over a stir-fry at the end. Keep scraps away from dogs and cats, because the onion family is toxic to them.

Common problems

Patchy germination. Onion seed loses vigour fast, and seed more than a couple of years old can be unreliable; cold, wet soil slows it further. Read more

Yellow youngest leaves. On alkaline soil, spring onions often run short of iron, and the newest leaves turn yellow first. Bring the pH down, as in Site, soil and setup.

Burnt, yellowing tips in summer. Hot, dry weather scorches the leaf tips; keep the soil moist and choose a heat-tolerant type such as 'Natsuyo'.

Plants lifting out of the soil. Onions push themselves towards the surface as they grow, which is normal. Draw soil or mulch back around the stems.

Flower stalks. Bunching onions left in the ground send up flower stalks in their second spring. Cut the clump back to about 5 cm and it regrows tender stalks.

Pests and diseases

Most trouble comes from one sap-sucker and three leaf and root diseases. Use only sprays labelled for spring onions, and keep to any withholding period on the label.

Onion thrips (Thrips tabaci). Insects about 1.5 mm long suck the leaf cells empty, leaving silvery-white flecks, mostly in warm weather. Spray horticultural oil or a soap solution several times, late in the afternoon, and pull the weeds that shelter them. Read more

Downy mildew (Peronospora destructor). Pale green to yellowish patches form partway down the outer leaves, with a violet-grey down in humid weather. It is worst when dew sits on the leaves (see Prevention first), and a copper fungicide may help. Read more

Onion rust (Puccinia allii). Bright orange or dark brown bumps form along the leaf veins, spread by wind and rain splash. It is not known to occur in Western Australia, and any rust on the onion family there must be reported.

White rot (Sclerotium cepivorum). Plants yellow and wilt, with white fluffy growth at the base and tiny black specks that survive in soil for up to 20 years. It is a major disease in south Queensland. Bin affected plants and keep that bed free of the onion family for at least ten years.

Prevention first

• Rotate beds, leaving two to three years between onion-family crops.
• Water in the morning, not the evening, so leaves dry before night.
• Pull weeds and self-sown onions, which harbour thrips and hold damp air.
• Sow thinly and go easy on nitrogen, because crowded, soft plants invite rust.

Tips

1. Replant the white bases from a bought bunch in a pot or bed and they reshoot; see regrowing from kitchen scraps.
2. Let a few bunching onions flower. The flowers draw bees, and the ripe seed gives you a fresh supply for the next sowing.
3. In some parts of Australia these are sold as shallots. True shallots are a clustering bulb onion, grown from bulbs rather than seed.