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Entire healthy mature onion plant in a real garden bed, showing growth habit and scale.Primary identification image. 30–45 cm tall, 8–10 cm wide at maturity.

Onion

Allium cepa L.

Also called: Bulb onion, Scallion (immature)

Family
Amaryllidaceae
Season
Cool-season biennial grown as an annual
Best grown in
Portland, Oregon

Grown from sets, transplants or seed in early spring; bulbing is triggered by day length, so timing and cultivar choice matter.

Safety: Toxic to dogs and cats.

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Life cycle & growth stages

  1. 1. Set or seedlingPlant sets 2 cm deep around last frost.
  2. 2. Leaf growthEach leaf corresponds to one bulb ring.
  3. 3. BulbingTriggered by day length; bulb swells.
  4. 4. MaturingTops yellow and topple.
  5. 5. Harvest readyMost tops down.
  6. 6. Harvest and cureLift, dry in sun or airy shed 2 weeks.

Growth stages are ordered chronologically; timing varies with cultivar, temperature, light and care.

Planting calendar for Portland, Oregon

Start indoors 8–10 weeks before last frost; transplant 0–4 weeks before last frost; direct sow 1–4 weeks before last frost; 90–120 days to maturity from transplant. Windows are derived from the region's average frost dates (last Apr 5, first Nov 5) and are estimates until you confirm your own dates in the calendar.

OnionAllium cepa
PlantStart indoorsDirect sowAutumn sowTransplantTendHarvest
OnionJan 25–Feb 8 (estimate)Mar 8–Mar 29 (estimate)—Mar 8–Apr 5 (estimate)Apr 5–Jun 6 (estimate)Jun 6–Aug 24 (estimate)
  • Start seeds indoors: Jan 25 – Feb 8
  • Transplant outdoors: Mar 8 – Apr 5
  • Direct sow: Mar 8 – Mar 29
  • Tend and monitor: Apr 5 – Jun 6
  • Harvest: Jun 6 – Aug 24

Spacing & support

In-row 8–10 cm; rows 30–40 cm. Planner footprint: 1 cell of 30 cm, 9 plants per cell.

Side-profile diagram of mature onion: canopy 30–45 cm tall, 8–10 cm wide, shallow root zone, support and harvest-access envelope.

Soil & water

Soil
Fertile, well-drained loam
pH
6–7
Fertility
moderate
Water
Steady water until bulbs begin to mature; then stop.
Frost
hardy; tolerates frost

Support & training

No support structure needed.

Onion mid-season before harvest, showing branching, vining, rosette or upright habit and space occupied.

Harvest signs

  • Tops yellow and fall over
  • Neck softens
  • Cure 2 weeks before storing
Onion at harvest readiness, showing the visual cues: tops yellow and fall over; neck softens.

Common problems

Pests: onion fly, thrips, leek moth

Diseases: white rot, downy mildew, neck rot

Appearance alone is not a diagnosis; confirm with a regional extension guide.

Side-by-side: healthy onion leaf or fruit and an example of white rot.

Container growing

  • Minimum 10 L (20 cm deep)
  • Use quality potting mix and water more frequently
  • Better for scallions than large bulbs.
Mature onion in a correctly sized container (10 L or more) with drainage visible.

Companion overview

10 reviewed relationship records (evidence: medium).

Supported or conditional

Carrot

Avoid or separate

None with evidence

Traditional claims only: Beet, Bush beans, Pole beans.

6 popular pairings reviewed with no verified relationship.

Full companion profile

Crop rotation

Avoid planting onion or other onion family crops in the same spot for 3 years. Rotation is about previous seasons, not about live neighbours.

Year 1Year 2Year 3AmaryllidaceaeOther familyOther familyback after 3+ years

Shared risks in Amaryllidaceae: onion white rot, downy mildew, onion fly.

Crop rotation hub

Sources, reviewer & update

Sources

Reviewer: Garden Planner editorial team (pending independent horticultural review)

Last reviewed: August 15, 2026 · version 1

Found an error or have feedback? Let us know.

Growing onion

Onions are planted as sets or transplants around the last frost date (seed is started indoors 8–10 weeks earlier). Leaves grow until day length reaches the cultivar's trigger (long-day, intermediate or short-day), then bulbs swell; the more leaves by then, the bigger the bulb. Plant the cultivar type suited to your latitude.

Onion seed beside a ruler or coin for scale.
Onion seedling at cotyledon and first-true-leaf stage, in soil, distinguishable from common weeds.
Onion mid-season before harvest, showing branching, vining, rosette or upright habit and space occupied.

Onions are shallow-rooted and poor competitors: keep them weeded and do not let taller crops shade them. Harvest when tops fall over, then cure for two weeks in a dry, airy place.

Two or more mature onion plants with a tape measure showing in-row spacing (8–10 cm).
Onion flower detail or bolting stage.
Immature bulb of onion developing on the plant.

The famous onion–carrot pairing has conditional field evidence: carrot fly damage fell only when onions greatly outnumbered carrots and only before bulbing. Onion white rot persists in soil for over a decade, so rotate alliums strictly.

Edible parts and safety

Edible: bulb, leaves. Toxic to dogs and cats.

Hands or tool harvesting onion correctly (cut, pull, twist or pick) without damaging the plant.

Pests and diseases

Common pests: onion fly, thrips, leek moth. Common diseases: white rot, downy mildew, neck rot. These vary by region; a photograph is an example, not a diagnosis. See the pest and disease guides and your local extension service.

onion fly on onion, with a wider damage-context view.

Timeline

Season timeline for onion: sowing, transplanting, flowering, harvest and bed occupancy aligned to frost dates.Text alternative: Start seeds indoors Jan 25–Feb 8; Transplant outdoors Mar 8–Apr 5; Direct sow Mar 8–Mar 29; Tend and monitor Apr 5–Jun 6; Harvest Jun 6–Aug 24.

Related plants

See also: Amaryllidaceae family page · crop rotation · regional calendar · guides

Our Methodology & Sources

We synthesize peer-reviewed research, university extension publications, and trusted horticulture resources. Our evidence levels help you understand how strong the science is behind the guidance.

Evidence tiers

Supported evidenceConsistent results from multiple robust studies
Traditional claimHistorical use reported, limited scientific support
Context dependentMixed or variable results across conditions
No verified relationshipNo reliable evidence identified

Primary source types

  • Peer-reviewed journal articles
  • University extension publications
  • Government & research institute reports
  • Systematic reviews & meta-analyses

Last updated

August 24, 2026

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