Giant onion flower clusters are formed by scores of blossoms arranged to form a sphere.
Giganteum is a good name for this ornamental member of the onion genus. The flowering stalks (scapes, to be technical) can get 6 ft (2 m) tall, the grayish, strap shaped leaves to 36 in (90 cm) long and up to 4 in (10 cm) wide; the bulbs to 4 in (10 cm) across, and the flower clusters (technically umbels) 4 in (10 cm) across. Those softball sized umbels are packed densely with 100 or more star shaped purplish flowers and appear after the leaves have begun to wither in mid to late summer. Giant onion, as do all members of the genus Allium, emits an onion or garlicky smell when crushed, but only when crushed. The flowers have a pleasant, almost violet-like fragrance.
Giant onion (Allium giganteum)is native to grasslands in central Asia.
Closeup of the giant onion flowers that comprise the spherical clusters.
Culture Light: Giant onion does best in full sun. Moisture: Giant onion should be grown in a fertile, well drained soil and given supplemental water during prolonged dry periods that occur while the plant is growing. Once it has gone dormant in late summer it needs no watering. Hardiness: USDA Zones 5-9. Giant onion can survive Zone 4 winters if mulched well. Provide for some afternoon shade in Zones 8 and 9. Propagation: Sow seeds in containers for later positioning in the garden, or sow in situ. After a couple years of growth, the bulbs usually split and may be divided in fall. Bulbs should be replanted in fall, 2-4 in (5-10 cm) deep. (The general rule of thumb for the planting depth of allium bulbs is 2-3 times the bulb’s diameter.)
Tolerant of heat and humidity, giant onion adds a striking accent to mixed borders and beds. The big purple globes stand above lesser flowers and draw the eye to the flower bed. Giant onion goes dormant in summer and the flowers don’t fully open until the leaves are already withering. Because the leaves die back, the ornamental alliums should be planted with other flowers so you don’t have gaps appearing. Many gardeners like to plant giant onion in groups of three, five or seven near the back of the flower bed. Planted at the base of an open, airy shrub, the dying leaves will be hidden while the extravagant flower heads compete for attention. Giant onions may need protection from strong wind. Fresh or dried, the umbels of make excellent cut flowers, and are widely used by florists.
Sometimes cultivars of ornamental leek (Allium ampeloprasum) are sold as A. giganteum. These are not quite as large and spectacular as the real thing.
By the end of May the giant onion is the superstar of the spring garden. Planted at the back of the flower bed, its huge flowerheads held atop tall stems adds height and drama.
The Alliums are among the most ubiquitous and useful of cultivated plants. Among the 700 or so species in the genus, more than a dozen are grown as vegetables or herbs. Among these are onions (A. cepa), leeks (A. ampeloprasum), scallions (A. fistulosum), garlic (A. sativum), chives (A. schoenoprasum), and garlic chives (A. tuberosum). Some of the more popular ornamental alliums are Persian onion (A. aflatunense) with lilac flowers; star of Persia (A. christophii) with violet flowers; blue allium (A. caeruleum), with blue flowers; golden garlic (A. moly) with yellow flowers; and Allium ‘Globemaster’, which is a hybrid between A. christophii and A. macleanii, has large lavender flower heads and is a widely available favorite among gardeners.
The alliums were formerly placed in the family Liliaceae, and some authors recognize a separate family for them, the Alliaceae.