Allium giganteum (Ornamental Onion, Allium)

Ornamental Onions are fast-growing perennials that appear for only a few months out of the year. Their enormous purple, pink, or white umbels create a dramatic appearance in gardens.

Allium giganteum (Ornamental Onion, Allium)
Deciduous: Yes, the foliage dies once flowering begins in May.
Hardiness Zones: 5-8
Height: 1.0-1.5 meters (3-5 feet) tall
Diameter: 50-60 centimeters (18-24 inches) across at maturity
Growth Rate: fast
Age: perennial
Root System: likely fibrous
Family: Amaryllidaceae
Subspecies: var. ‘Twinkling Star’

Tolerates: Black Walnut (Juglans nigra) toxins (juglones), herbivores (deer and rabbits)
Problems (major): no major pest issues
Problems (minor): Winds, especially when strong, easily knock over the inflorescences (known as scapes). Bulb rot may kill ornamental onions.
Poisonous: potentially toxic (especially to dogs) if consumed in large quantities

Soil requirements: performs best in moderate/medium/loamy, well-drained soils (tolerates most pH levels)
Air requirements: not sufficiently researched
Watering requirement: moderate
Sun requirement: prefers full sun, survives in some shade

Leaf shape: basal, strap-shaped, light-green to gray-green
Leaf size: less than 12 inches in diameter per leaf
Flower structure: The scapes grow up to 1.5 meters (5 feet) tall with an umbel (globular cluster of individual flowers) consisting of tiny (sometimes white or pink)Ā purple, star-shaped flowers (known as florets).
Flowering frequency: blooms annually in May-June
Bulb/tuber: bulb up to 3 inches in diameter
Monocot/Dicot: monocot
Annual/Biennial/Perennial: perennial

Notable characteristics:
The large cluster of florets (forming an umbel) are large and spectacular. Although visible above-ground for only a short amount of time, these are commonly grown in gardens for their flowers.

Uses:
These are commonly planted as ornamentals.

Sources used:

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Five (5) entire A. giganteum

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A close-up of an umbel with purple, star-shaped florets

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Three (3) entire Ornamental Onions

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Foliage

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A pollinated umbel, seeds maturing

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Two (2) mature purple umbels at the Missouri Botanical Garden

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A giant white allium at the Denver Botanic Gardens

All of the images provided were taken by me. They may be used for educational/informational purposes only, provided that this article/online journal is appropriately cited first.

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