Overview
What onions need, when they crop, and whether they suit your garden.
An onion builds its bulb in response to day length, not size: as days lengthen past a certain point, it starts to swell. So the more leaves it has grown by then, the bigger the bulb. That is why onions go in early, why weeds matter so much, and why a variety bred for your latitude makes all the difference.
Quick facts
- Botanical name
- Allium cepa
- Feeding
- Moderate feeder
- Hardiness
- Any zone, as an annual
- Time to harvest
- 80 – 120 days
- Propagation
- Bulb, Seed
Growing details
- Germination
- 14 – 21 days
- Spacing
- 10 cm (4 in)
- Sowing depth
- 13 mm (½ in)
- Frost sensitivity
- Frost hardy
Growing calendar
- Start indoors: January, February, March
- Sow outdoors: March, April
- Plant out: March, April, October, November
- Harvest: June, July, August, September
Mid-winter to early spring, about 8 weeks before planting out. Early to mid-spring, as the soil warms. Spring sets mid-March to mid-April; overwintering sets in autumn. Autumn-planted sets from early summer; spring sets and seed from late summer.
Is this plant right for you?
- Beginner friendly from sets
- Frost hardy
- Stores for months
- Full sun
- Needs weeding
- Day-length sensitive
- Toxic to pets
Toxic to pets
Toxic to dogs, cats and horses: damages red blood cells (ASPCA).
Before You Grow
Choose sets, seed or transplants, and a variety matched to your day length.
Onions are toxic to dogs, cats and horses (ASPCA): keep scraps and stored bulbs out of reach.
What you need
Sets, seed or transplants
Of a variety suited to your latitude. See choose your onions.
A sunny, weed-free bed
With free-draining, fertile soil. See soil.
Compost
A bucketful per square metre, well rotted, not fresh manure.
A string line and trowel or dibber
For straight, evenly spaced rows. See planting.
Fleece
Against birds pulling up sets, and onion fly. See pests.
Racks or nets
For curing and storing the crop. See storing.
Choose your onions
Day length first. Onions start forming bulbs when days reach a set length, and varieties differ. Long-day onions bulb at 15 to 16 hours of daylight and suit northern latitudes, including the UK and northern US; short-day onions bulb at about 12 hours and suit the South (UMD, Illinois). Intermediate types fall between; in mid-latitudes long-day and intermediate types usually do best (UMD). Grow the wrong type and you get small bulbs or none.
Sets are small onions grown the year before: the quickest, most reliable start. Heat-treated sets bolt much less (RHS, Gardeners’ World). In the UK: ‘Sturon’, ‘Red Baron’ and ‘Hercules’.
Seed is cheaper, with more varieties, but needs an early start. Seed doesn’t keep long, so buy fresh (RHS).
Transplants, bundles of young seedlings, give the largest, best-looking bulbs (Illinois), and UMD finds them the best method for most Maryland gardeners.
Overwintering (Japanese) sets, such as ‘Senshyu Yellow’, ‘Autumn Champion’ and ‘Electric’, go in in autumn for an early-summer crop.
Colour and use. Yellow, white and red; mild and sweet ones for salads, strong ones for cooking. Globe-shaped varieties keep longest (UMD).
Ways to grow onions compared
-
Spring sets
- When
- Mid-March to mid-April
- Time to harvest
- About 80 days
- Notes
- Quickest; bolting risk
-
Autumn sets
- When
- October to March
- Time to harvest
- Early summer
- Notes
- Hardy varieties; early crop
-
Seed indoors
- When
- Mid-winter to early spring
- Time to harvest
- 90 to 120 days
- Notes
- Most choice; transplant in spring
-
Seed outdoors
- When
- Early to mid-spring
- Time to harvest
- 100 to 120 days
- Notes
- Thin to spacing; use thinnings
-
Transplants
- When
- Early spring
- Time to harvest
- As seed
- Notes
- Largest, best bulbs
From Gardeners’ World, the RHS, Utah State and Illinois. For a first try, heat-treated sets in spring.
Where onions grow best
Onions are a cool-season crop that likes a warm, sunny, open site (RHS).
Sun. Full sun: at least six hours, and UMD prefers eight to ten. Green onions manage with five.
Cold. Onions are hardy and take heavy spring frosts (UMD), but long cold spells soon after planting can make sets bolt (Illinois).
Day length. Leaves grow while days are short; bulbs form as days lengthen. Plants need plenty of leaves by late spring, so plant early (RHS). Planted too late, many grow leaves and never bulb (Utah State).
Heat. High temperatures and low humidity help bulbing and curing (Illinois).
- Sun
- 6 hours or more; 8 to 10 is better
- Long-day onions bulb at
- 15 to 16 hours of daylight
- Short-day onions bulb at
- About 12 hours
Soil
Onions have a limited, shallow root system, so they need fertile, moisture-holding soil that drains well (RHS, Utah State).
Preparation. Weed thoroughly, then dig in a bucketful of garden compost or well-rotted manure per square metre. Avoid fresh manure (RHS).
Fertiliser. Onions have a high need for nutrients (UMD). Test the soil and add fertiliser before planting if needed.
Rotation. Grow onions in a new place each year, especially after rots: Utah State recommends long rotations.
Planting
Plant early in spring, tips just showing, 10 cm (4 in) apart.
Small sets bolt less than large ones. Keep sets under 1.5 cm (⅝ in) across for bulbs.
When to plant onions
Plant as early in spring as the soil can be worked, so plants grow plenty of leaves before bulbing starts.
Sets in spring: mid-March to mid-April in the UK (Gardeners’ World); late March or early April in the US Midwest (Illinois). They need about 100 days of growth before autumn: Gardeners’ World plants as late as mid-May, for smaller bulbs.
Overwintering sets: October to March (Gardeners’ World).
Seed indoors: mid-winter to early spring, about eight weeks before planting out (RHS, UMD, Utah State).
Seed outdoors: early to mid-spring as the soil warms (RHS).
Not too early. A long cold spell after planting can make sets and transplants bolt (UMD, Utah State).
- Spring sets: mid-March to mid-April.
- Seed indoors from mid-winter.
- Early enough to grow leaves before bulbing.
How to grow onions from seed
Seed needs an early start so plants are growing strongly by late spring.
-
Sow in modules
About 1.3 cm (½ in) deep, one seed per module, or three or four together for clumps of slightly smaller bulbs (RHS).
-
Keep warm
In a heated propagator; seedlings appear within a couple of weeks.
-
Trim long tops
Snip leggy tops back if they flop (UMD).
-
Harden off and plant out
In spring, about 10 cm (4 in) apart in rows 25 to 30 cm (10 to 12 in) apart. Plant multi-sown modules as clumps; don’t split them (RHS).
-
Or sow outdoors
In early to mid-spring, 1.3 cm (½ in) deep in rows 20 to 30 cm (8 to 12 in) apart. Thin to 5 cm (2 in), then to 10 cm (4 in), eating the thinnings as green onions.
How to plant onion sets and transplants
One set grows into one onion. Firm, small sets make the best bulbs.
-
Sort the sets
Buy firm, dormant sets early. Use small ones, under about 1.5 cm (⅝ in), for bulbs: larger ones tend to bolt, so use them for green onions (Illinois, Utah State). Discard soft or mouldy ones.
-
Plant tips-up
Push each set into the soil so just the tip shows (Gardeners’ World); Illinois plants 2.5 cm (1 in) deep. No need to soak them first.
-
Space them
10 to 15 cm (4 to 6 in) apart in rows 30 cm (12 in) apart. Closer spacing gives smaller bulbs (RHS, Illinois).
-
Transplants
10 to 12.5 cm (4 to 5 in) apart for large bulbs, or 5 to 6 cm (2 to 2½ in) apart and pull every other one as a green onion (Illinois).
-
Cover against birds
Birds pull up newly planted sets: cover with fleece until they root (Gardeners’ World).
Care
Hand-weed, water as bulbs swell, feed early, and snap off any flower stems.
Never draw soil up around bulb onions: it causes neck rot in store.
Care by growth stage
After planting
Weed: by hand
Fleece against birds; replant any sets they pull up. See planting.
Leaf growth
Weed: first two months matter most
Weed often and feed with nitrogen. See feeding.
Bulbing
Water: most important now · Feed: stop by mid-July
Water well as bulbs swell; remove flower stems. See watering.
Tops falling over
Water: stop
Stop watering and let them ripen. See harvesting.
After harvest
Rotate: next year
Cure, then store cool and dry. See storing.
Watering onions
Onions have shallow roots, so they need regular water, and more as the bulbs swell: UMD finds bulb size is directly proportional to the water they get.
How. Moisten the soil thoroughly, about 30 cm (12 in) deep (Utah State), ideally with drip irrigation. Water in dry spells (Gardeners’ World).
Drought during growth cuts the yield and bulb size (Utah State).
At the end. Stop watering once the tops start to fall, so the bulbs ripen and store well.
Mulch with compost, grass clippings or leaves to hold moisture and smother weeds.
Feeding onions
Onions are hungry: dig in compost before planting, and add a general fertiliser if a soil test shows the need.
Nitrogen boosts. Gardeners’ World feeds autumn-planted onions with a nitrogen-rich fertiliser in spring. Utah State side-dresses spring onions in mid-May and again in late June; UMD once bulbs start to swell.
Stop by mid-July. Late nitrogen keeps the leaves growing and reduces storage quality (Utah State).
Weeding and flower stems
Weed, weed, weed. Onions compete poorly with weeds because of their shallow roots and thin leaves (UMD, Illinois). Hand-pull weeds or hoe very shallowly, especially in the first two months when plants grow slowly (Utah State). Gardeners’ World hand-weeds rather than hoes.
Flower stems. Remove any flower stalks as soon as they appear: flowering diverts energy from the bulb (Gardeners’ World). Bolted onions won’t store: use them straight away (Illinois).
Don’t earth up. Never draw soil up around bulb onions: bulbs rise above the soil as they swell, and hilling encourages neck rot (UMD, Illinois). Only green onions are hilled, to blanch their stems.
Don’t bend the tops. Let them fall over naturally. Breaking them early gives smaller bulbs that keep less well (Illinois).
Onions in containers
Onions can be grown in large containers (RHS), though you’ll get fewer bulbs than from a bed.
Container. At least 45 cm (18 in) wide and deep, with drainage holes, in a sunny spot, filled with peat-free soil-based compost (RHS).
Spacing. As in the ground, about 10 cm (4 in) apart.
Care. Containers dry quickly: water regularly, especially as bulbs swell.
Green onions suit pots well: plant sets close together and pull as needed.
Problems
Bolting, onion fly, leaf miner, mildew and rots are the usual problems.
Common onion problems
Many onion problems start at planting: the wrong variety, the wrong time or oversized sets.
Flower stalks appearing
- Bolting
Large sets, planting too early, or a cold spell after planting. Use small, heat-treated sets, remove stalks at once, and eat those onions first.
No bulbs, just leaves
- Day length
- Late planting
The wrong day-length type for your latitude, or planted too late. Choose long-day onions in the north and plant early.
Small bulbs
- Spacing
- Water
- Weeds
Crowding, drought, weeds or a short season. Space 10 cm (4 in), water well and weed often.
Soft necks, rotting in store
- Curing
- Hilling
Bulbs not dried properly, harvested wet, bruised, or hilled up. Cure thoroughly before storing.
Sets pulled up
- Birds
Birds tug at the dry tips. Cover with fleece until rooted and replant any pulled out.
Common pests
Allium leaf miner
In the UK, widespread in England and parts of Wales since 2002: the larvae bore into the stems (RHS).
Onion fly (onion maggot)
Drooping, yellow leaves are the first sign; by then the maggots are eating the bulb. Grow under fleece the following year, rotate, and don’t overload the soil with organic matter (Gardeners’ World, Utah State). Destroy affected plants.
Thrips
Tiny insects that make leaves silvery or grey, twisted and dying, hiding where leaf meets bulb. Mulch, compost and a stiff spray of water help (Utah State).
More problems with onions
Common diseases
Neck rot
Brown, soft necks and fluffy grey mould in store, worst after wet summers. Don’t overcrowd, and dry bulbs thoroughly before storing (Gardeners’ World, Utah State).
White rot
Yellowing, wilting leaves and white fluffy fungus on the base of the bulb; it persists in soil for years.
Basal rot and pink root
Rotting where roots meet the bulb, or roots turning pink and dying (Utah State). Use long rotations and avoid overwatering.
Harvest & Use
Lift when the tops fall over, then cure the bulbs dry before storing.
Harvesting onions
When. Spring sets are ready in about 80 days, seed-grown onions in 90 to 120 (Gardeners’ World, Utah State). Autumn-planted onions crop from early summer; spring-planted from late summer.
Signs. Leave bulbs in the ground until the tops yellow and fall over; UMD lifts when about two-thirds of the tops have fallen. Never lift before the leaves have yellowed.
How. Loosen the soil with a fork and lift gently. Don’t bruise them: damaged onions don’t store.
Green onions can be pulled once the tops are 15 cm (6 in) tall (Illinois), from about 50 days after sowing (Utah State).
- Lift when the tops fall over.
- Loosen with a fork; don’t bruise.
- Use bolted or damaged bulbs first.
Curing and storing onions
Onions keep for months if they are cured properly first.
Cure. Leave lifted bulbs on the soil to dry for a few days in fine weather, or on racks or newspaper, then somewhere airy and out of direct sun for one to two weeks with the tops on (UMD). Utah State lets the tops shade the bulbs from sunburn. They are ready when the outer skins rustle (Gardeners’ World).
Store. In nets, plaits or ventilated boxes somewhere cool, dry and airy. The RHS keeps them in the light, as darkness encourages sprouting; Gardeners’ World stores them dark. Near 0 °C (32 °F) and 65 to 70% humidity they keep six to seven months; in a cool garage or basement, two months or more (UMD). Warmth makes them sprout; frost damages them.
Keeping quality varies: globe-shaped varieties keep longest; many sweet, mild onions keep poorly (Illinois); Utah State gives two to four months for its sweet varieties.
Propagation
Shallots and perennial onions that multiply from year to year.
Shallots and perennial onions
Bulb onions are grown fresh each year from sets, seed or transplants, one plant per onion. Some onions multiply themselves instead (UMD).
Shallots and multiplier (potato) onions (Allium cepa var. aggregatum) form a cluster of bulbs from each one planted. Plant single bulbs pointed end up in late September to mid-October, under 2.5 to 5 cm (1 to 2 in) of soil, or in late March to April under about 1 cm (½ in). Autumn plantings give bigger bulbs; spring ones bolt less. Space 10 to 15 cm (4 to 6 in) apart. Lift when the tops die back, and keep some bulbs to replant.
Egyptian walking onions (A. cepa var. proliferum) grow small bulbils at the top of the stalk. The stalk flops, the bulbils root where they land, and the patch walks across the bed.
More to Explore
Quick answers, guides that pair well with onions, and the sources behind this one.
Frequently asked questions
Should I grow onions from sets or seed?
Sets are quicker and more reliable; seed is cheaper with more choice. See choose your onions.
How deep do I plant onion sets?
Just deep enough that the tips show. See planting.
Why are my onions bolting?
Large sets, early planting or cold spells. See common problems.
Why didn’t my onions form bulbs?
Usually the wrong day-length type for your latitude, or late planting.
When are onions ready to harvest?
When the tops yellow and fall over. See harvesting.
Should I bend over onion tops?
No: let them fall naturally, or bulbs are smaller and keep less well (Illinois).
How long do onions keep?
Cured well, two to seven months depending on variety and conditions. See storing.
Are onions toxic to dogs?
Yes: onions are toxic to dogs, cats and horses (ASPCA).
Related guides
Sources
- How to grow onions (opens in a new tab) Royal Horticultural Society
- Onion (opens in a new tab) ASPCA Animal Poison Control
- Growing onions in a home garden (opens in a new tab) University of Maryland Extension
- Onions in the garden (opens in a new tab) Utah State University Extension
- Onion (opens in a new tab) University of Illinois Extension
- How to grow onions (opens in a new tab) BBC Gardeners’ World