Giant onion

Bulb · Perennial

Allium giganteum

Giant onion

A dramatic bulb with perfect purple globes of starry flowers held on tall, bare stems in early summer.

  • Full sun
  • 1.5–2 m (5–7 ft)
  • 20–30 cm (8–12 in)
  • H5 · Zone 7
  • June–July
  • Deciduous

Overview

From Central Asia, this is one of the tallest ornamental onions. In early summer, sturdy, leafless stems up to 2m carry dense spheres 10-15cm across made of hundreds of small, star-shaped, lilac-purple flowers, a magnet for bees. The broad, grey-green leaves start to wither around flowering time, so hide them behind other plants. The papery seedheads stay decorative for weeks. It is hardy to about -15°C (5°F) and needs full sun and well-drained soil.

At a Glance

Sun Full sun
Height 1.5–2 m (5–7 ft)
Spread 20–30 cm (8–12 in)
Hardiness H5 · Zone 7
Flowers June–July
Soil Chalk, Loam, Sand
Moisture Moist well drained
Habit Upright
Water Low
Maintenance Easy

Growing Conditions

Min. Temperature -15°C (5°F)
pH Range 6–8
Fertility Average
Drainage Free Draining

Aspect Suitability

  • South-facing
  • West-facing

Site Tolerance

  • Drought

Soil Tolerance

  • Chalk
  • Sandy
  • Loam
  • Acid
  • Neutral pH
  • Alkaline

Foliage

Persistence Deciduous
Colour Grey-green
Leaf shape / texture strap_shaped

Foliage interest: March–June

Seasonal Timeline

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Flowering
Foliage
Fruiting
Propagate
  • Flowering Jun Jul
  • Foliage Mar Apr May Jun
  • Fruiting Jul Aug Sep
  • Propagate Sep Oct

Maintenance Calendar

  • Apr Ignore the leaves turning yellow at the tips, which is normal before flowering
  • Jul Leave flower heads standing after flowering, as the seedheads stay decorative for weeks
  • Sep Cut down the dry stems once they look untidy, and dry some for indoor arrangements
  • Oct Plant bulbs 15-20cm deep in autumn, which anchors the tall stems and protects bulbs from frost

Garden Design

Works well in: Cottage, Modern / Contemporary, Prairie / Meadow, Wildlife-friendly

Plant behind mid-height perennials like hardy geraniums or catmint, which hide the fading leaves while the globes float above.

Companion planting: Grasses, Salvias, Geraniums, Nepeta

Wildlife Value

  • Bees
  • Butterflies
  • Other pollinators
  • Beneficial insects
  • RHS Plants for Pollinators

Safety

  • Toxic to dogs
  • Toxic to cats
  • Toxic to horses

Like all onions and garlic, alliums are toxic to dogs and cats (and horses) if eaten; ornamental bulbs are not edible.