How to grow Onion

Allium cepa · bulb onion, common onion

A cool-season vegetable grown from sets or seed. It forms a bulb that keeps for months once dried.

Quick facts

Type	Vegetable
Family	Amaryllidaceae
Light	Full sun
Soil pH	6–7
Frost	tolerates light frost
Soil for sowing	from 5 °C
Germination	10 days
Sowing to harvest	100 days
Where it grows	Greenhouse, Open ground, At home

When to plant

It tolerates light frost: sow straight into the bed about three weeks before your last spring frost.

In an unheated greenhouse everything moves about three weeks earlier.

The first harvest comes about 100 days after the shoots appear.

Frost dates · When to plant

Care

Greenhouse

Stage	Watering	Temperature	Feeding
Growth	every 3 days	—	—
Harvest	—	—	—

Open ground

Stage	Watering	Temperature	Feeding
Growth	every 7 days	—	balanced (NPK), 21 days into the stage; nitrogen, 63 days into the stage
Harvest	—	—	—

At home

Stage	Watering	Temperature	Feeding
Growth	every 2 days	—	—
Harvest	every 2 days	—	—

Growing notes

Light

Needs full sun, at least 6 hours of direct light a day.

Soil

Well-drained, fertile soil improved with garden compost or well-rotted manure, pH 6–7. Do not use fresh manure.

Care

Plant sets 2 cm deep with the tip just showing, 10–15 cm apart. Onions have shallow roots, so give about 25 mm of water a week in dry weather. Feed a general fertiliser 4–5 weeks after planting and nitrogen about six weeks later, but not after mid-summer. Do not earth up the bulbs, and snap off any flower stems. Stop watering once the bulbs have swollen and the tops start to fall over; lift them when the foliage yellows and topples, then dry them in the sun for about two weeks. For green leaves, plant sets close together in a pot at least 15 cm deep by a sunny window and keep the compost moist; they are ready in 4–6 weeks.

Pests and diseases of Onion

Sources

Figures checked against these sources.