How to grow onions, king of the alliums
It’s easy to overlook the humble onion, yet this bulbous king of the alliums appears in almost every savoury vegetable dish we eat. This crop stays in the ground for a long time though, so you’ll need a large garden if you want to grow your whole supply. If you lack space, focus on smaller, quicker varieties or have fun with fancy gourmet coloured cultivars.
SOW AND GROW
When to sow: March to July in warmer areas and April to September in cooler areas.
When to transplant: July to September in warmer areas and August to October in cooler areas.
Position: Full sun.
Harvest: 24-32 weeks.
GET STARTED
Onions are classified as early, mid or late, and the classifications affect when you sow and plant.
In temperate regions, you can sow onions from early autumn to midwinter (starting with earlies, then mids, then lates) and in cooler regions in the same sequence from mid-autumn to early spring.
Plant them (in the same order) in warm regions from midwinter to early spring; and cool regions late winter to mid-spring.
Onions are naturally biennial and if sown or planted at the wrong time, will bolt to seed and form a flower rather than the edible bulb you want. So read the seed packet or plant label carefully to ensure you time your crop right.
STEP BY STEP
Slow-growing onions often have to compete with speedy weeds that shoot up around them, so if starting from seed it’s best to start them off in trays.
Sow seed shallowly or directly onto the surface of seed trays containing moist seed-raising mix and thin them out to 5cm as they grow.
Seed should germinate in 10-14 days. Plant them into the garden bed when they have four to six true leaves.
Plant onion seedlings no more than 2-3cm deep to avoid them becoming long and misshapen. In the garden, space plants 15cm apart and space rows 20cm apart.
GROWING TIPS
Onions need full sun to develop fat bulbs.
Prepare the soil before planting by digging in compost, aged manure and lime. Onions require good drainage, so add gypsum if you have a heavy, waterlogged soil.
They also need sufficient moisture to produce juicy onions, so water weekly in summer.
Mulching will help keep water in the soil too. Feed with liquid blood and bone, fish emulsion or worm tea every three weeks.
Harvest onions when the tops wither and turn brown. Loosen the soil with a fork and lift out the bulbs, being careful not to damage the skins.
Leave the stalks intact and hang in a warm, dry place for a couple of weeks.
STANDOUT VARIETIES
Pukekohe Longkeeper is a long-standing Kiwi favourite due to its resistance to downy mildew and resilience in wet weather as well as its good storing qualities (it will keep for six to eight months).
Good red onions include Red Brunswick and Sweet Red or, for pickling, choose smaller varieties such as white Pearl Drop or red-skinned Purplette, which can be harvested as baby bunching onions if left unthinned, meaning you can harvest small bulbs from the side of the plant and leave the rest of the crop to keep growing.
If you want to grow a super-size onion, Walla Walla is a large, white American 19th century heirloom onion, the bulbs of which swell to between 450g and 900g each.
TROUBLESHOOTING
Downy mildew presents as grey or purple spots on the leaves and can affect larger alliums especially after prolonged rain, and when nights are warm and days are cool. Avoid overcrowding and keep up the weeding to increase airflow. Mulching with compost will help keep weeds down and increase the bacteria that keep fungal diseases at bay.
Thrips can cause silvering of the leaves and aphids can also cluster on leaves in warm weather. Both are more likely in dry conditions, so keep an eye on irrigation. Blast them off with the hose regularly or squash with your fingers.