How to Grow Ornamental Alliums

Ornamental alliums are onions grown for their flowers. The genus has roughly 700 species, including garlic, chives and shallots, but the garden types carry their small flowers in round heads that range from the size of a quarter to the size of a volleyball. Depending on species they flower from late spring into midsummer; Rutgers counts them among the last of the spring bulbs, still blooming in late May.

Types worth knowing

Iowa State Extension describes these, and most are widely sold:

• Giant allium (A. giganteum). Pale purple, softball-size heads on 40-inch stems in late spring. NC State notes it can reach 6 feet. Its leaves start fading as it flowers.
• ‘Globemaster’. A hybrid of A. christophii and A. macleanii bred in Holland as a cut flower. Iowa State calls it one of the most reliable alliums, with softball- to volleyball-size pinkish-purple heads on 24- to 36-inch stems in early summer.
• ‘Purple Sensation’ (A. aflatunense). Baseball-size violet heads on 20- to 30-inch stems.
• Star of Persia (A. christophii). Loose amethyst heads 10 to 12 inches across on 12- to 20-inch stems. They last two to three weeks and dry silvery-tan in place.
• A. schubertii. Open heads like exploding fireworks, approaching volleyball size, on 12- to 24-inch stems.
• A. karataviense. One of the shortest, with broad gray-blue leaves and golf-ball heads on 8- to 12-inch stems.
• Drumstick allium (A. sphaerocephalon). Quarter-size heads, maroon at the base fading to purple at the top, in early summer. It reseeds into small colonies.
• ‘Millenium’ and similar hybrids. Clump-forming plants with 2-inch lavender heads in mid- to late summer and glossy leaves that stay green all season, unlike the bulb types.

For the South, Clemson recommends Naples onion (A. neapolitanum), drumstick allium, A. oreophilum and star of Persia, all late-spring bloomers. Around Austin, Travis County Master Gardeners grow the Texas native A. drummondii and rosy garlic (A. roseum), but warn that both reseed aggressively.

Where to plant

Alliums flower best in full sun, ideally six hours or more. Many tolerate part sun, but Iowa State finds that shade produces floppy stems. They want soil that is moist during growth but drains well. Bulbs rot in soggy ground, and overly damp sites also bring fungal problems. Once growth ends they tolerate summer drought.

Put tall types toward the back of a border. NC State plants giant allium in groups of five to nine and pairs it with daylilies and low ornamental grasses. This is a design choice: the neighbors’ foliage hides the allium leaves, which yellow early.

How deep to plant allium bulbs

Plant dry bulbs in fall. Iowa State suggests October; South Dakota State finds alliums less fussy about timing than most bulbs and plants them up to a few weeks before a hard frost. Until then, keep bulbs cool and dark. The dates for your zone above show the window.

Depth depends on bulb size:

• Large bulbs (giant allium, ‘Globemaster’, ‘Purple Sensation’): 6 to 8 inches deep. NC State uses 8 inches deep and 12 inches apart for giant allium, about one bulb per square foot. Nebraska gives 6 inches deep and 12 apart.
• Small bulbs (drumstick, A. moly, A. oreophilum): 3 to 4 inches deep (Iowa State). Colorado State gives 2 to 4 inches. Nebraska lists other allium species at 6 inches deep and 6 to 12 inches apart.

Set the pointed end up, cover, and water in. Colorado State recommends mulching bulbs left in the ground each fall.

Care through the year

Alliums need little. Fertilizer is usually unnecessary; if a soil test shows a need, Iowa State suggests a light all-purpose feeding in early spring as the leaves emerge, every few years at most. Keep soil moist while plants are growing. The tallest stems may need staking.

After bloom, leave the leaves until they are completely brown so the bulb can store food. On large-bulb species the foliage often yellows before or while the flowers open.

The dried heads of the big globe types stay attractive for weeks and make good dried flowers. Deadhead the reseeding types as soon as the flowers fade. Iowa State names drumstick allium, A. caeruleum, A. moly and nodding onion (A. cernuum) as free seeders.

Dividing and propagating

Large-bulb alliums form offsets that can be separated in fall and replanted. Clump-forming types such as ‘Millenium’ are easy to divide in early spring or early fall. Alliums also grow from seed, but many need alternating warm and cold periods to germinate, and Iowa State says seedlings take about three years to reach blooming size.

Problems

• Bulb rot, white rot and black rot. All are favored by wet soil. Improve drainage or choose another site.
• Rust and mildew. Damp, crowded conditions. Give plants air.
• Bulb mites. Discard soft or damaged bulbs.
• Unwanted seedlings. Deadhead reseeding species promptly.
• Ugly foliage at bloom time. Normal for large species. Hide it with neighboring plants rather than cutting it green.

Deer, rabbits and rodents rarely bother alliums, because every part smells of onion when crushed.