• AGM plants

    AGM plants have been through a rigorous trial and assessment programme. They are:

    • Excellent for ordinary use in appropriate conditions
    • Available to buy
    • Of good constitution
    • Essentially stable in form & colour
    • Reasonably resistant to pests & diseases

Allium giganteum

giant onion

  • Other common names giant onion
    giant allium

  • Family Alliaceae

  • Genus Allium are bulbous herbaceous perennials with a strong onion or garlic scent, linear, strap-shaped or cylindrical basal leaves and star-shaped or bell-shaped flowers in an umbel on a leafless stem

  • Details A. giganteum is a perennial with spreading, glossy, strap-shaped basal leaves which die down before the flowers. Dense globose umbels of bright purple flowers are borne on tall stems in summer

  • Plant range C Asia to NE Iran


  • Foliage Deciduous

  • Habit Columnar/Upright

  • Fragrance Leaf

  • Toxicity

  • Hardiness

    Hardiness ratings

    All ratings refer to the UK growing conditions unless otherwise stated. Minimum temperature ranges (in degrees C) are shown in brackets

    • H1a - Under glass all year (>15C)
    • H1b - Can be grown outside in the summer (10 - 15)
    • H1c - Can be grown outside in the summer (5 - 10)
    • H2 - Tolerant of low temperatures, but not surviving being frozen (1 to 5)
    • H3 - Hardy in coastal and relatively mild parts of the UK (-5 to 1)
    • H4 - Hardy through most of the UK (-10 to -5)
    • H5 - Hardy in most places throughout the UK even in severe winters (-15 to -10)
    • H6 - Hardy in all of UK and northern Europe (-20 to -15)
    • H7 - Hardy in the severest European continental climates (< -20)




  • Full Sun

  • Aspect South-facing or North-facing or West-facing or East-facing

  • Exposure Sheltered


  • Chalk


  • Chalk


  • Chalk


  • Chalk


  • MoistureWell-drained, Moist but well-drained

  • SoilChalk, Clay, Sand, Loam

  • pHAcid, Alkaline, Neutral


  • Ultimate height 1-1.5 metres

  • Ultimate spread 0.1-0.5 metres

  • Time to ultimate height 2-5 years

How to grow

Cultivation Easy to grow in fertile well-drained soil. Add grit when grown in clay soils to improve drainage

Propagation Propagate by seed, sowing in containers in a cold frame when just ripe or in the spring. Alternatively, remove offsets in autumn

Suggested planting locations and garden types Flower borders and beds Cut Flowers Cottage & Informal Garden

How to care

Pruning No pruning required

Pests Generally pest free

Diseases Generally trouble-free: Onion white rot and a downy mildew may occur

Advertise here

Did you find the information you were looking for?

RHS members can get exclusive individual advice from the RHS Gardening Advice team.

Join the RHS now