Eating Garden

How to Grow Onions

Allium cepa

A cool-season plant grown into a bulb that is dried, cured and stored after the tops fall.

At a glance

  • Difficulty

    Takes some care

  • Sunlight

    Full sun

  • Water

    Average thirst

  • Plant spacing

    3 to 4 in apart

More about this plant

Choose an onion type suited to your latitude, plant early and keep the shallow roots moist. Stop watering when bulbs are full and tops fall, then cure them before storage.

  • Grows over two seasons.
  • Can grow in raised beds, the open ground.
  • Fine in a small space.

Optional local dates

Planting dates where you live

The growing guide above works anywhere. This section adds local frost dates when we have them.

Optional local dates

Get planting dates for where you live

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Helpful gardening guides

When to plant

Match these relative steps to your local seasons. Where frost applies, use the last freezing night in spring and the first in fall. In frost-free climates, follow trusted local seasonal guidance.

  • Start indoors 10 to 12 weeks before your last spring frostStart seed indoors ten to twelve weeks before planting outside.
  • Sow outdoors 0 to 2 weeks after the start of your growing seasonDirect sow as soon as the soil can be worked in spring.
  • Plant out 0 to 3 weeks before your last spring frostPlant when weather is near 50F and the risk of a heavy freeze has eased. Onion transplants tolerate light frost.

What you need

  • How to start itFrom seed or a young plantYou can sow it straight into the ground, or start it indoors first.
  • How deep to plant0.25 to 0.5 in
    Planting depth: 0.25 to 0.5 in below the soil surface. Drawing is not to scale.
  • How far apart3 to 4 inLeave 12 to 18 in between rows.
    Leave 3 to 4 in between plants and 12 to 18 in between rows. View from above; drawing is not to scale.

Step by step

  1. Choose the right day-length typeUse long-day onions in the north and a locally recommended type farther south.
  2. Plant earlySow as soon as soil can be worked, or set out young plants when heavy freeze risk has eased.
  3. Thin to four inchesGive each bulb about 3 to 4 inches. Keep rows at least 12 inches apart.
  4. Keep shallow roots moistWater well when rain is short, and weed close to the surface.
  5. Stop water as tops fallWhen bulbs are full and tops begin falling, let the bed dry for harvest.
  6. Cure before storageDry lifted bulbs in a warm, airy place for two to four weeks.

Common mistakes

  • Choosing the wrong day-length typeAn onion may stay small if its bulb-forming day length does not fit your latitude.
  • Adding too much nitrogenHeavy late feeding makes soft bulbs with thick necks that cure and store poorly.
  • Storing before the neck driesA damp, open neck invites decay. Cure until the outer skin is dry and the neck feels tight.

Water

Average thirst

Keep soil steadily moist while bulbs grow. Soak well when rain is under one inch a week, then stop watering when bulbs are full and tops begin falling.

  • Too dry
  • About right
  • Too wet
  • If it goes drySuffers quickly when dryGrowth slows and leaf tips can dry before bulbs have filled.
  • If it stays wetDislikes sitting wetWet soil near harvest can leave bulbs soft and more likely to rot.

Looking after it

  • FeedingAn occasional feed helpsFollow a soil test and feed only after roots are established. Stop extra nitrogen well before harvest.
  • NitrogenToo much nitrogen gives leaves instead of food
  • ThinningThin seedlings to 4 in apartCrowded seedlings stay small. Pull the weaker ones and keep the strongest.
  • MulchUse a weed-free organic mulch to hold moisture and reduce shallow hoeing.

Harvest

You will know it is ready when: Harvest when about half the tops have fallen over and dried. The bulb neck should begin to close.

Lift bulbs with a fork on a dry, cool day. Cure them in a warm, dry, airy place for two to four weeks until the outer skin is dry and the neck is tight.

  • Keeping itStore fully cured bulbs in a cool, dry place with air movement. Do not let them freeze, and remove any bulb that softens or rots.

Is this right for you?

Cool spring growth followed by the day length needed for the chosen variety to form a bulb. Northern storage onions are usually long-day types; short-day types suit lower latitudes.

  • LightFull sun
  • ShadeNeeds full light
  • SeasonA cool season cropUsually grown in spring or summer.
  • FrostSurvives a light frost
  • HeatCopes with heat

Soil

  • Soil pH6.0 to 7.0A cheap test kit from a garden centre will tell you yours.
  • Soil it likesLoam, Sandy
  • DrainageWants well drained ground
  • RichnessAverage ground is fine

Use well-drained soil rich in organic matter. Add finished compost or well-rotted manure before planting.

Use a soil test. Do not add fresh manure or extra phosphorus unless the test calls for it.

The year at a glance

Measured from your last spring frost, because seasons differ from garden to garden. In frost-free climates, use trusted local seasonal guidance alongside these relative steps.

  1. Start seeds indoors

    10 to 12 weeks before your last spring frost

  2. Plant out

    0 to 3 weeks before your last spring frost

  3. Sow outdoors

    0 to 2 weeks after the start of your growing season

Questions people ask

What does long-day onion mean?

It starts forming a bulb after long summer days. Northern storage onions often need 14 or more hours of daylight.

Can onions handle frost?

Young plants tolerate light frost, but a heavy freeze can injure them or push large sets toward a seed stalk.

When should I stop watering?

Stop when bulbs are full size and the tops begin falling over.

How do I know an onion is cured?

The outer scales are dry and the neck is tight after two to four weeks in a warm, airy place.

Also filed under: Vegetables

Where this came from

Advice varies with your climate and variety. Check the sources behind each section.

Sources & review dates
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