Allium
How to grow onions
Allium cepa
Onions want an open, sunny bed of firm, free-draining soil and very little fuss. Plant sets 10 cm apart from mid March to mid April, or in September and October for an overwintered crop, with the tip just showing. Keep the row weeded, because onions cannot compete, and stop feeding by June and watering once the bulbs have swelled. Lift in August when the tops fall over by themselves and dry the bulbs until the skins rustle.
- Hardiness
- Hardy; autumn sets overwinter outside
- Position
- Full sun, open site
- Soil
- Firm, fertile, free-draining
- Ideal pH
- 6.0 to 7.0
- Water
- Low to moderate, stop watering once the tops begin to fall
- Feeding
- Moderate feeder
Sowing and planting onions
WhenSets Mar to Apr (or Sep to Oct for overwintering); seed sown Feb to Mar; harvest Jul to Sep
Starting off
Sets are easiest: plant mid March to mid April, or heat-treated sets in autumn, into firm soil. Seed goes in late Feb to Mar in modules at 10 to 16 °C, 1 cm deep, germinating in 10 to 14 days, with 5 to 6 seedlings per module planted as a clump; or sow direct in March in drills 1 cm deep.
Spacing and planting
Sets 10 cm apart in rows 25 to 30 cm apart for medium bulbs, 5 cm for small pickling onions; push each set in so the tip just shows. Seed-raised plants go out at the same spacing. Workable in a 30 cm deep trough but yields are small; better in open ground.
Varieties worth growing
- Sturon
- The standard spring set: round, golden, slow to bolt and one of the best keepers, lasting into spring in a cool shed.
- Red Baron
- The most reliable red for sets or seed, with a strong flavour, and it stores far better than most red onions.
- Ailsa Craig
- Grown from seed for very large, mild bulbs. Use them by Christmas, as they do not keep long.
- Radar
- A hardy set for September and October planting that stands the winter and is ready in June, weeks before spring sets.
- Santero F1
- Bred with resistance to onion downy mildew, the variety to choose where the disease has taken a crop before.
Onions month by month
Timings are for central England. Allow two weeks later in the north and on cold, heavy soil.
- January
Check stored bulbs and use any that are sprouting or soft.
- February
Sow seed in modules indoors late in the month. Top-dress overwintered sets with nitrogen at the end of the month.
- March
Rake in the base dressing a week ahead, then plant sets from mid month. Sow seed direct on light soil.
- April
Finish planting sets by mid April; heat-treated sets go in now. Plant out hardened-off module clumps.
- May
Hoe between rows and hand weed in them. One light top-dress. Water only in a dry spell.
- June
Keep weeding. Cut out any flower stalks and mark those bulbs to use first. Lift overwintered onions late in the month. No feed from now on.
- July
Stop watering once the bulbs have swelled. Finish lifting overwintered onions.
- August
Lift maincrop bulbs once the tops have fallen and yellowed. Dry on a rack under cover.
- September
Store dry bulbs in nets or plaits. Plant autumn sets such as Radar from mid month.
- October
Finish planting autumn sets. Mesh over them against allium leaf miner.
- November
Weed the overwintering row. Check stored bulbs.
- December
Check stored bulbs. Order sets and seed.
Feeding onions
A moderate feeder that needs its food early. One base dressing, one light top-dress in May, and nothing after June, or the necks stay thick and the bulbs will not keep.
A balanced feed early in growth, then ease off nitrogen once bulbs begin to swell so they ripen firm and store well.
Feeding plan
Autumn or winter before planting
Garden compost
A 3 to 5 cm layer spread over the bed. No fresh manure: it gives soft bulbs.
A week before planting in March
Fruit & Vegetable Fertiliser 4-5-6
70 g/m² raked into the surface, the lower end of the label. With rows 30 cm apart that is about 20 g per metre of row (70 x 0.3).
Mid to late May
Fruit & Vegetable Fertiliser 4-5-6
50 g/m² scattered between the rows and hoed in: about 15 g per metre of row. Season total 120 g/m², inside the 150 g/m² label limit.
Late February, overwintered sets only
Organic Nitrogen Fertiliser 13% N
25 g/m², the lowest label rate, once: about 8 g per metre of row. It replaces what winter rain has washed out.
July onwards
Nothing
No feed of any kind once the bulbs are swelling.
Signs to watch
Thick necks and soft bulbs that will not store
Excess nitrogen or feeding after midsummer
Stop feeding by June
Yellow leaf tips and slow early growth
Nitrogen short on light soil
One nitrogen top-dress in May
Bolting with a hard flower stalk
Cold check or large sets, not feeding
Heat-treated sets, planted later
Pests, diseases and problems
Leaves yellow and wilt, bulbs pull out easily with white fluffy mould and tiny black dots on the base
Onion white rot, a soil fungus that survives for many years without a host
Lift affected plants with the soil round them and bin them; never compost. Grow no alliums on that ground, clean boots and tools so the soil is not carried about, and raise future crops in a fresh bed or in containers of bought compost.
Leaf tips turn pale then grey-purple and felty, and the foliage collapses in a wet summer
Onion downy mildew
Space rows at the full 30 cm, keep weeds down so air moves, remove affected plants promptly and do not store their bulbs. Grow Santero F1 and rotate strictly.
Rows of white dots on the leaves in spring or autumn, then twisted leaves and small brown pupae in the neck
Allium leaf miner, which lays eggs in March to April and again from September to November
Cover with fine insect mesh from late February to the end of May and from September to the end of November, with the edges buried.
Young plants yellow and flop in June, white maggots in the base of the bulb
Onion fly
Grow under insect mesh. Sets are attacked less than seed-raised plants. Lift and destroy affected plants before the maggots move into the soil.
Sets found lying on the surface a few days after planting
Birds tugging at the dry tip, or frost lifting sets on loose soil
Plant into firm soil, trim off any long wisp of dead skin first, and cover with fleece or netting for three weeks until the roots have a hold. Replant lifted sets at once.
Stored bulbs go soft and brown at the neck with grey mould under the skin
Neck rot, a botrytis that enters through a thick or damp neck
No feeding after June, let the tops fall naturally, dry for two to three weeks under cover with air all round, and use thick-necked bulbs first rather than storing them.
Harvest and rotation
Harvest and storage
Ready in August when the foliage yellows and flops by itself, 20 to 22 weeks from spring sets; never bend the tops over. Lift with a fork and dry on a rack for 2 to 3 weeks until the skins rustle. A 3 m row gives 30 bulbs of 150 to 250 g. Store plaited or in nets in a cool, airy shed until March.
Rotation and companions
Rotation: allium group with leeks, garlic and shallots, 3 to 4 year gap; follows brassicas or legumes well on firm ground. Keep away from beans and peas, which dislike the allium root exudates and shade the bulbs.
Good to knowStop feeding and watering once the foliage yellows and topples; dry the bulbs thoroughly before storing.
Growing onions in pots
Onions will grow in a trough or large pot at least 30 cm deep, but each plant gives one bulb, so the return for the space is small. Set them 10 cm apart in peat-free multipurpose compost with a fifth of grit added, and mix in the 4-5-6 at 5 g per litre only if the compost is old; fresh compost has enough for the crop. Water when the top few centimetres are dry and stop in July. Containers of fresh compost are the practical way to grow onions where the garden soil carries white rot.
Onions: common questions
Is it better to grow onions from sets or seed?
Sets are easier and quicker, and they are the better choice for most gardens. Seed is cheaper, offers more varieties, bolts less and gives the largest bulbs, but needs sowing under cover in late February and careful weeding while the seedlings are small.
Why are my onions going to seed?
A cold spell after planting has made the plant behave as if it has been through a winter. Large sets and early planting make it worse. Cut the flower stalk out as soon as you see it, use those bulbs first because they will not store, and next year plant heat-treated sets in April.
How long do onions take to grow?
Spring-planted sets take 20 to 22 weeks, from a March planting to an August harvest. Autumn-planted sets are in the ground for about nine months and are ready in June or July. Seed sown in late February is ready in late August or September.
Sources
Growing information on this page was checked against these references in November 2026. Feeding rates are Dr Forest label rates.