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Spring onion

Fastest onion you can grow, ready in 8 weeks and happy in a windowbox.

1. Rake a fine seedbed, it is a small seed and hates clods.
2. Draw a shallow drill 1 cm deep and water the bottom of it before sowing.
3. Sow thickly along the drill, about 3 seeds per centimetre, and cover lightly.
4. Keep damp, seedlings appear in 10 to 14 days.
5. Do not thin. Crowding is what keeps the stems slim and white.
6. Sow a fresh short row every 3 weeks for continuous supply.
7. Pull from 15 cm tall onward, taking the biggest first.

Soil

Any decent soil that drains, pH 6.0 to 7.0. No manure needed.

Sun

Full sun to light shade. Summer shade is an advantage in hot zones.

Water

Little and often. Never let the top 5 cm dry out or growth stalls and the flavour turns harsh.

Spacing

2 to 3 cm apart, rows 15 cm. No thinning needed.

Depth

1 cm deep

Care

['Weed by hand between rows, the seedlings look like grass and are easy to hoe off by mistake.', 'No feeding required in decent soil, nitrogen just makes floppy tops.', 'In Mediterranean summers give afternoon shade or skip the July and August sowings entirely.', 'Autumn-sown hardy varieties sit through winter and give the earliest spring pull.']

Harvesting

Pull when the stem is pencil-thick, usually 8 weeks after sowing. Ease them out with a fork if the soil is dry. Use the green tops as well as the white, they are the same plant.

Storing

Poor keepers. 1 week in the fridge standing in a little water, or chopped and frozen for cooking. Better to leave them in the ground and pull as needed.

Grow it next to

Carrots, beetroot, lettuce and strawberries. Keep away from peas and beans.

Problems

[{'cause': 'Seed sown too deep or the bed dried out at germination', 'fix': 'Sow only 1 cm deep and water the drill before sowing, not after.', 'symptom': 'Seedlings never appear'}, {'cause': 'Left in the ground too long, or a bulbing variety by mistake', 'fix': 'Pull at pencil thickness and buy seed labelled spring onion or bunching onion.', 'symptom': 'Bulbs swell instead of staying slim'}, {'cause': 'Downy mildew in wet weather or drought stress', 'fix': 'Thin the stand by pulling early, and water consistently.', 'symptom': 'Tips go yellow and papery'}, {'cause': 'Onion fly larvae or overwatering in heavy soil', 'fix': 'Cover with fleece after sowing and switch to a raised bed.', 'symptom': 'Whole row flops and rots at soil level'}]

Saving your own seed

Let a few plants run to flower in their second year, dry the seed heads on the plant, rub out the black seed. They cross with any other onion flowering nearby.

Containers

One of the best container crops. A 20 cm deep windowbox grows a dense band of them. Sow a short row every 3 weeks.