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.

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At a glance

  • Difficulty

    Takes some care

    3 of 5 · More shovels, more care

  • Sunlight

    Full sun

  • Water

    Average thirst

  • First harvest

    100 to 120 days

    from sowing

  • 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.

Make it your plan

Add your location for local planting datesAdd place

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What are you starting with?
Where will it grow?

Your plan, starting Nov 10, 2026

Estimated harvest: Feb 18, 2027 – Mar 10, 2027. Add your location to compare this date with local planting windows. Your chosen date stays the same.

Add your location to compare this date with local planting windows. Your chosen date stays the same.

Estimated first harvest

Feb 18, 2027 – Mar 10, 2027

Counted from sowing. Variety, plant age, weather, and care can change this estimate.

Harvest when about half the tops have fallen over and dried. The bulb neck should begin to close.

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.

First, gather a few things

  • Onion seeds
  • A growing spot with 3 to 4 in between plants
  • A prepared garden bed
  • A watering can or hose

Get ready

Do these before you put the plant or seeds in place.

  1. Start here

    1. Choose the right day-length type

    Use long-day onions in the north and a locally recommended type farther south.

Start or plant it

Follow the steps for the way you chose to start.

  1. 1. Plant early

    Sow as soon as soil can be worked, or set out young plants when heavy freeze risk has eased.

Water

Check the soil first. Heat, rain, and pots can change how often this plant needs water.

  1. 1. Check before you water

    Push a finger about 1 inch into the soil. Water when it feels dry there.

  2. 2. Water the roots

    Water slowly at the base until the soil feels moist a few inches down. For a pot, water until a little runs out of the drainage holes.

    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.

Need help judging moisture? Read how to check the soil and water the roots.

If the plant looks unhappy

  • Too dry

    Growth slows and leaf tips can dry before bulbs have filled.

    If the soil is dry, water deeply and check it again tomorrow.

  • Too wet

    Wet soil near harvest can leave bulbs soft and more likely to rot.

    If the soil is wet, wait before watering again and check that excess water can drain away.

While it grows

Check on your plant and do these jobs as they come up.

  1. 1. Thin to four inches

    Give each bulb about 3 to 4 inches. Keep rows at least 12 inches apart.

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.

Pick and store it

Use the plant's signs of readiness, then harvest gently.

  1. 1. Check if it is ready

    Harvest when about half the tops have fallen over and dried. The bulb neck should begin to close.

  2. 2. Pick it

    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.

  3. 3. Keep it fresh

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

More picking tips

  1. 1. Cure before storage

    Dry lifted bulbs in a warm, airy place for two to four weeks.

Common problems

    Common mistakes

    • Choosing the wrong day-length type

      An onion may stay small if its bulb-forming day length does not fit your latitude.

    • Adding too much nitrogen

      Heavy late feeding makes soft bulbs with thick necks that cure and store poorly.

    • Storing before the neck dries

      A damp, open neck invites decay. Cure until the outer skin is dry and the neck feels tight.

    Companion plants for onions

    Some of this is measured in trials and some is long-standing garden practice. Each note below says which it is.

    Good together

    Trials show a real benefit:

    • BroccoliIn an Iowa study, onions planted with broccoli helped reduce cabbage looper and imported cabbageworm damage.Measured in trials
    • CabbageA study found onions planted with cabbage helped reduce cabbage looper and imported cabbageworm.Measured in trials

    Extension programs recommend this pairing:

    • CarrotShallow-rooted onions and deep-rooted carrots share a bed without shading each other much.Recommended by extension programs
    How to choose companion plants

    Questions about onions

    Tap a question to see the answer.

    Will onions grow where I live?

    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

    What soil is best for onions?

    • 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.

    Before changing your soil, learn how to test soil acidity and understand the result.

    When and how deep do I plant onions?

    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.
    • Ready to pick about 100 to 120 days from sowing.

    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.

    What happens to onions through the year?

    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

    How do I give onions a better start?

    Simple ways to help your onion grow, from the soil up.

    • Start with good soil

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

    • Give the soil a cover

      Use a weed-free organic mulch to hold moisture and reduce shallow hoeing.

    • Make room to grow

      Thin seedlings to 4 inches apart.

    • Feed with care

      Follow a soil test and feed only after roots are established. Stop extra nitrogen well before harvest.

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

    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.

    Quick answers about onion

    One question each, answered first, with the source named.

    • When to Plant Onions

      When to plant onions is early spring in cold-winter areas, close to your last frost. In Florida, short-day onions go in from September to December.

    Helpful gardening guides

    Dig into the next thing you need, from starting a garden to getting watering right.

    Starting your first garden? Choose your space and get it ready, then plan local planting times.

    Browse all gardening guides

    Also filed under: Vegetables

    Where this came from

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

    Sources & review dates
    Timing
    University of Minnesota Extension, Growing onions in home gardens · checked 2026-09-15
    Utah State University Extension, Onions in the Garden · checked 2026-09-21 · Days to maturity for a dry bulb: bulb onions require 100 to 120 days to mature, counted from seeding.
    University of California Agriculture and Natural Resources, UC Master Gardener Program, Time of planting · checked 2026-09-24 · Where frost is rare, cool-season crops are timed to months averaging 55°F to 75°F.
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