Bulb · Amaryllidaceae
Giant Ornamental Onion
Allium giganteum
Plant the bulbs in autumn, and by early summer giant allium lifts lilac orbs that hang weightless over the border.
Full sun
Every 7–10 days
USDA 4–8
3–6 ft · 0.9–1.8 m
Overview
Giant ornamental onion earns its place in the garden for a single, sculptural gesture: the great purple spheres that ascend on bare stems as summer arrives. It claims kinship with the onion and garlic of the kitchen, yet here the harvest is purely visual. Reaching head height and clearing every neighbor, each globe becomes a floating point of architecture in the planting.
Beneath that drama sits an unfussy, enduring bulb that reappears each season and bears winters as severe as -29°F (-34°C). Tucked into the ground in fall, the dormant bulbs send up their tall scapes in late spring, and the orbs keep their color for two to three weeks before the strappy leaves slip quietly away. Once the bloom has passed, the plant recedes with composure, ceding the stage to whatever follows it in the bed.
One caveat shapes how it is placed. Every part is toxic to cats, dogs, horses, and livestock, no less than a chewed onion, so it asks to be set beyond the reach of any grazing animal. Given an open, sun-filled border, it rewards the smallest effort with a presence few bulbs can match.
Care
The short version: full sun and quick-draining soil, bulbs settled deep in fall, even moisture across the growing season, and a discreet stake to carry the tall stems through a windy site.
Light
Offer giant allium full sun, and it answers with the sturdiest stems and the fullest, most poised globes. A measure of afternoon shade does no harm, yet deeper shade leaves the scapes thin and leaning, the bloom count diminished.
Water
Hold the soil evenly moist through the growing season, then permit it to weather summer's dry spell once the foliage retreats. Water near every 7 to 10 days, and spare the bulbs any standing wet, which rots them where they sit.
Soil
Plant into average, well-drained soil, be it loam, sand, or clay, so long as water passes through rather than lingering. The bulb is gracious about pH, taking anything from 5.5 to 8.0, which suits most beds without amendment.
Temperature
Here is a cold-hardy bulb at ease through winters as low as -29°F (-34°C) and summer heat to 86°F (30°C). The winter chill is no mere endurance test; it is the very thing that coaxes fine flowers from the bulb.
Humidity
Ordinary outdoor air more than satisfies it, so no humidity figure need concern you. What matters is drainage and air movement, since the damp, motionless pockets are precisely where bulb rot takes hold.
Feeding
Fold a balanced or low-nitrogen bulb fertilizer into the soil at fall planting, then offer a second feeding in early spring. Spare the nitrogen, which favors slack foliage over the flowers you are growing it for.
Pruning
Clip away the spent scapes once the bloom fades, but let the leaves yellow and die where they stand, since they are quietly provisioning next year's bulb. In an exposed border, stake the tall stems before they yield.
Repotting
Lift and divide the established clumps every 3 to 4 years in fall, prompted by flowers grown fewer or smaller. Congested, crowded bulbs are the plain signal that the time to separate them has come.
Size & spacing
Reaching 3 to 6 feet (90 to 183 cm), these are plants for the back of a border, where their height reads as intended. Space the bulbs 9 to 12 inches apart at a depth of 5 to 8 inches, leaving each clump room to compose itself.
When to plant
Giant allium keeps an autumn-to-summer calendar. Set the bulbs into the ground in fall so they root ahead of the freeze, let winter and spring do their quiet turning, and the orbs unfurl across June and July for a fortnight or so.
Northern hemisphere
Southern hemisphere
Typical months for a temperate climate: start earlier in warm zones, later where spring frosts linger.
- Plant: Plant bulbs in fall about 5-8 inches deep and 9-12 inches apart
- Harvest: Late spring to early summer (June-July), blooms lasting 2-3 weeks
Hardiness
USDA hardiness zones: 4–8
Temperature: down to -34°C
Troubleshooting
Whatever unsettles giant allium tends to begin underground or arrive on the wind. Sodden soil undoes the bulbs, minute mites scar them, and the tall stems give way when nothing steadies them. Read your symptom against the notes below and act while the matter is small, before it travels through the planting.
Soft, mushy bulbs and plants that collapse
Wet, slow-draining soil sets bulb rot in motion, and in a spot that stays stubbornly sodden a black rot fungus arrives to complete the ruin. A lifted bulb feels soft and carries a foul smell. Draw out and discard the spoiled ones, lighten the watering, and return the firm survivors to ground where water has somewhere to go.
Fix:
- Remove and discard affected bulbs
- Improve soil drainage and reduce watering
- Replant healthy bulbs in well-drained amended soil
Distorted, stunted growth from pitted bulbs
Bulb mites work into the scales to feed, and their toll shows as warped new growth above a scarred, pitted bulb. Look closely at any suspect bulb for the minute mites and the marks they leave. Cast off whatever is badly overrun, plant only firm clean bulbs, and move the planting to fresh ground.
Fix:
- Discard heavily infested bulbs
- Plant only firm, healthy, mite-free bulbs
- Practice crop rotation and good sanitation
Tall stems leaning or breaking
Those generous orbs balance atop long scapes, so a gusty day will bow or break them, most readily when the bloom carries its full weight. The cause is simply wind pressing on stems left unsupported. Settle the planting where it is sheltered, and stake the scapes ahead of bloom.
Fix:
- Stake tall stems for support
- Site plantings in locations sheltered from strong winds
Toxicity
The whole plant belongs firmly out of any animal's reach. Bulbs, foliage, and flowers each hold the organosulfur compounds that lend onions and garlic their danger, and these can dismantle red blood cells and bring on hemolytic anemia. Cats prove the most vulnerable, with dogs, horses, and grazing livestock genuinely exposed as well, so choose a spot none of them can visit.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds · People
Cats Severe
Of every creature, the cat is the most acutely affected. A single nibble can stir vomiting and set their red blood cells breaking down, surfacing as weakness, a galloping heart, panting, and dark urine.
Dogs Severe
A dog that chews any portion may vomit, then drift into weakness and torpor as its red blood cells give way. Pale gums, hurried breathing, and discolored urine are the cues to watch, and a vet should be reached without delay.
Horses Severe
Horses meet the same red blood cell damage when they graze the leaves or bulbs, left weakened and dull. The anemia can mount through repeated nibbling, asking no single large meal to do its harm.
Rabbits Toxic
Those same onion-family compounds are toxic to rabbits, so the whole plant belongs well clear of them. Not the smallest amount is safe to offer.
Livestock Toxic
Cattle, sheep, and goats may fall to hemolytic anemia from eating alliums, whether fresh in the field or dried within hay. Fence them from the plantings and clear away any trimmings within reach.
Birds Toxic
Birds, too, are sensitive to onion-family toxins, so chickens and pet birds must never reach the bulbs, leaves, or flowers.
People Caution
This is an ornamental onion raised for its flowers, not a kitchen crop, and it is not meant for the table. Handling does no harm, though hands deserve a wash afterward, and curious children are best kept from the bulbs.
Keep all parts, including bulbs, foliage, and flowers, out of reach of pets and livestock. Store bulbs securely and clean up trimmings; wash hands after handling.
Helpline numbers
United States
ASPCA Animal Poison Control: (888) 426-4435 (fee applies)
Poison Control: (800) 222-1222
Canada
Poison Centres: 1-844-764-7669
Varieties
A handful of large-flowered alliums arrive under the giant-onion name, joined by a few near relations offered in its place. These are the ones worth recognizing on sight.
Globemaster
The most commanding of the group, this hybrid raises dense violet globes of 8 to 10 inches on scapes built to hold them.
Allium hollandicum (aflatunense)
A kindred ornamental onion of smaller rose-purple umbels, often offered as a thriftier stand-in for the true giant allium.
Allium cristophii (Star of Persia)
A lower relation whose broad, open umbels resolve into shimmering pink-violet stars.
Companions
Plant near
Roses: Tall allium scapes rise through and complement roses and other border plants while filling early-summer gaps.
Peonies: Allium bloom coincides with and extends color in the perennial border alongside peonies.
How to propagate
Giant allium lends itself easily to multiplication, whether from fresh bulbs or by dividing an established stand. Nearly all the work belongs to autumn.
Bulbs & cormsEasy
Fall · One growing season to bloom
The most straightforward beginning is a dormant bulb set out in fall. Place it 5 to 8 inches deep and 9 to 12 inches apart, and it will offer its first flowers come the following growing season.
Offsets & pupsEasy
Fall · 1-2 seasons
An established bulb forms small offset bulblets about its base. Lift it in fall, ease the offsets free, and replant them to broaden a stand, mindful that they want a season or two to reach blooming size.
DivisionEasy
Fall · 1-2 seasons
Every few autumns, raise a crowded clump and part it to keep the bulbs in good vigor. Each division roots over a season or two and returns to flower on a rhythm all its own.
SeedModerate
Fall to spring · Several years
Alliums seed themselves with abandon, and seed may go down any time from fall through spring. It is the unhurried path, for seed-grown plants ask several years to gather size before they bloom.
Myths
Ornamental flowering alliums are merely decorative and safe for pets, unlike the onions and garlic of the kitchen.
For all their elegance, ornamental alliums share the very genus of onion and garlic and carry the identical toxic organosulfur compounds. From bulb to flower, any part can break down a pet's red blood cells. They pose the same danger to cats, dogs, and horses as anything resting in the spice rack.