Onion
Allium cepa
Pungent allium that guards carrots but sulks beside beans.
Spacing & sowing
- Plant spacing
- 4.7 in
- In-row spacing
- 3.9 in
- Between rows
- 11.8 in
- Square-foot density
- 9 per square
- Sow depth
- 0.4 in
Calendar (from your frost dates)
- Sow indoors
- 70 days before the last spring frost
- Sow direct
- 28 days before the last spring frost
- Plant out
- 14 days before the last spring frost
- First harvest
- 100 days after the last spring frost
- Harvest ends
- 30 days before the first fall frost
Growing
- Type
- Vegetable · Grown as an annual
- Botanical family
- Allium (onion family)
- Sunlight
- Full sun
- Watering
- Low–moderate
- Soil
- Firm, fertile, well-drained
- Feeding
- Bulbs & roots
- Frost tolerant
- Yes
- Season
- Cool Season
- Harvest type
- Single Harvest
- Start from
- seed
Growth stages
- 0 d Sow seeds
- 14 d Germination
- 140 d Harvest
Harvest
- When
- Sets planted in spring mature in late summer, roughly 20 weeks on; autumn-planted and overwintered onions come 3–4 weeks earlier. Harvest for storage once growth stops, in a spell of dry weather if possible.
- What to look for
- The tops yellow and fall over naturally at the neck — the definitive signal that bulbing is complete. Skins at the shoulder turn papery and colored; don't bend the tops over by hand, just wait for most of the row to lodge on its own.
- How to harvest
- Ease each bulb loose with a fork, lifting from below rather than pulling by the fragile neck, and leave the roots and tops attached. Lay bulbs on the soil surface, a rack, or in a greenhouse for two weeks to cure until the outer skins rustle.
- How often
- A once-a-season harvest, though you can pull individual bulbs green for cooking any time from early summer. Lift the whole crop before prolonged wet weather, which stains skins and starts neck rot.
- Storing the crop
- Fully cured bulbs with tight, dry necks keep 4–8 months plaited into strings or in net bags, somewhere cool, dry and airy — never in plastic or the fridge. Use thick-necked or soft bulbs first; they won't keep.
Pests & problems
- Onion fly & leek moth Maggots burrow into the base of onions, leeks, and garlic; leaves yellow and droop. Grow alliums under insect mesh and rotate to fresh ground each year.
- Thrips Minute slender insects that leave silvery flecking and distortion on leaves and flowers in hot, dry spells. Keep plants watered and use blue sticky traps under glass.
- Root & crown rot Roots or crown turn brown and mushy in waterlogged soil or compost; the plant wilts even when wet. Improve drainage, water less, and pot up only into free-draining mixes.
- Rust Orange-brown powdery pustules on leaf undersides, weakening the plant over the season. Remove affected leaves, avoid overhead watering, and clear all debris in autumn.
Garden allies
Beneficial predators that hunt this plant's pests — grow their favourite flowers nearby to recruit them. companion plant
- Beneficial nematodes Steinernema & Heterorhabditis spp.Hunts Onion fly & leek moth
Microscopic worms watered onto damp soil that hunt down soil-dwelling grubs and release a bacterium that kills them within days. Timing is the whole thing: warm, damp soil and a small target grub, which means late summer for most of what they are used against.
- Big-eyed bug Geocoris spp.Hunts Thrips
A stubby 3 mm bug with enormous wrap-around eyes, working the soil surface and the low foliage for aphids, mites, thrips and insect eggs. It is the main natural check on chinch bugs in a lawn and it is killed outright by insecticide.
- Damsel bug NabidaeHunts Thrips
A slender, mantis-armed bug that grabs aphids, small caterpillars, and thrips in low crops and meadowy edges. Leave a strip of unmown grasses and flowers for it to overwinter in.
- Green lacewing Chrysoperla carneaHunts Thrips
Its bristly larvae — "aphid lions" — impale aphids, mealybugs, thrips, and mite colonies, eating hundreds a week. Adults feed on nectar and pollen from small open flowers and overwinter in hollow stems left standing.
- Hoverfly SyrphidaeHunts Thrips
Bee-mimicking flies whose legless larvae graze aphid colonies clean while the adults pollinate. A ribbon of shallow, nectar-rich flowers kept in bloom all season recruits wave after wave of them.
- Minute pirate bug Orius spp.Hunts Thrips
A pinhead-sized true bug that is the best natural check on thrips, and happily spears aphids, mites, and insect eggs too. It needs pollen when prey is scarce, so keep something flowering nearby.
- Predatory mites Phytoseiulus & AmblyseiusHunts Thrips
Fast orange mites that hunt spider mites egg-by-egg, and cousins that clear thrips and fungus-gnat larvae. Sold as a biological control for greenhouses; outdoors they follow humidity — mist and mulch, don’t dust with sulfur.
- Rove beetle StaphylinidaeHunts Onion fly & leek moth
Slim, quick beetles with short wing cases that hunt in soil and litter, taking root-fly eggs and larvae, fungus-gnat larvae, and mites. Mulch and undisturbed ground keep them resident year-round.
Germination & growth
- Germination temp
- 50–77°F
- Min soil temp
- 43 °F
- Photoperiod
- long day
Companion planting
Good neighbors
- Carrot Repels pests
- Kohlrabi Repels pests
- Rutabaga Repels pests
- Winter savory Repels pests
- Salsify Repels pests
- Broccoli
- Cabbage
- Cauliflower
Keep apart
Did you know?
Onions make you cry by releasing a sulfur gas that turns to a mild acid in your eyes.
The Cottage Garden Companion — the gardener's herbarium at cottagegardencompanion.com