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Allium cepa /ˈt͡sɛpa/

Common name: Onion · Bulb onion · Green onion

Allium cepa is a biennial in the amaryllis family (Amaryllidaceae), grown as an annual for its layered underground bulb. It has no confirmed wild populations and is known only from cultivation, where it has been grown for at least 5,000 years. The species also includes shallots, scallions, and potato onions as cultivar groups.

Care summary

Sun Full sun, 6+ hours daily
Water Even moisture early; reduce as bulbs mature
Soil Loose, fertile, well-draining, pH 6.0–7.0
Hardiness Zones 3–9; grown as annual or biennial
Mature height 30–45 cm (12–18 in); flower stalks to 1 m
Pet safe Toxic to cats and dogs if ingested
Difficulty Easy
Propagation Grow from seed, small starter bulbs called sets, or by replanting the rooted base of a kitchen onion. For sets, press them into loose soil just below the surface in full sun and keep evenly moist.

Fun facts

  • Cutting onions releases syn-propanethial-S-oxide, the volatile compound that makes your eyes water.
  • Ancient Egyptian workers building the pyramids were reportedly fed onions for strength and endurance.
  • Onion varieties are classified as short-day, intermediate-day, or long-day based on how much daylight triggers bulbing.
  • All onions are toxic to cats and dogs, damaging their red blood cells even in modest amounts.

Common problems

    • Cover beds with fine insect mesh
    • Rotate allium crops every year
    • Remove and destroy infested bulbs
    • Pull up and bin infected plants
    • Avoid alliums on that ground
    • Do not compost diseased bulbs
    • Space plants for good airflow
    • Water at the base only
    • Remove infected leaves promptly
    • Spray plants with insecticidal soap
    • Keep beds free of weeds
    • Water during dry spells