Bulb · Amaryllidaceae
Giant Ornamental Onion
Allium giganteum
Plant the bulbs in fall and giant allium rewards you with drumstick globes that float above the border by early summer.
Full sun
Every 7–10 days
USDA 4–8
3–6 ft · 0.9–1.8 m
Overview
Giant ornamental onion is grown for one thing above all, the huge purple flower globes that rise on bare stems in early summer. It belongs to the same family as the onion and garlic in your kitchen, but here the draw is the show, not the table. The flowers can reach head height and stand well above everything around them.
It's an easy, hardy bulb that comes back year after year, surviving winters down to -29°F (-34°C). Plant the dormant bulbs in fall, let them push up tall scapes in late spring, and the globes hold their color for two to three weeks before the strappy foliage quietly dies back. After bloom the plant fades into the border and lets its neighbors take over.
One warning runs through everything below. Every part of this plant is toxic to cats, dogs, horses, and livestock, the same way a chewed onion would be. It's a striking, low-effort bulb for an open, sunny bed, but keep it out of reach of animals that might graze it.
Care
The short version: full sun and sharply drained soil, bulbs planted deep in fall, steady moisture through the growing season, and a stake for the tall stems in a windy spot.
Light
Give giant allium full sun for the best, sturdiest flowers. It will take a little afternoon shade, but too much shade leaves you with weak stems that lean and fewer blooms.
Water
Keep the soil evenly moist through the growing season, then let it ride out summer drought once the foliage dies back. Water roughly every 7 to 10 days, and never let the bulbs sit in soggy ground, which rots them.
Soil
Plant in average, well-drained soil, whether that's loam, sand, or even clay, as long as water moves through it. The bulbs tolerate a wide pH range from 5.5 to 8.0, so most garden beds suit them.
Temperature
This is a tough, cold-hardy bulb that handles winters down to -29°F (-34°C) and summer heat up to 86°F (30°C). The bulbs actually need a cold winter to set good flowers.
Humidity
Ordinary outdoor air is fine, so there's no humidity target to hit. What matters more is drainage and airflow, since damp, stagnant conditions are what invite bulb rot.
Feeding
Work a balanced or low-nitrogen bulb fertilizer into the soil at planting in fall, with a second feed in early spring. Skip heavy nitrogen, which pushes floppy leaves at the expense of flowers.
Pruning
Cut the spent flower scapes off after they bloom, but leave the foliage to wither and die back on its own, since it feeds next year's bulb. In an exposed spot, stake the tall stems before they topple.
Repotting
Lift and divide established clumps every 3 to 4 years in fall, once you notice fewer or smaller flowers. Congested, overcrowded bulbs are the sign it's time to split them apart.
Size & spacing
Plants stand 3 to 6 feet (90 to 183 cm) tall, so they belong toward the back of a border. Set the bulbs 9 to 12 inches apart and 5 to 8 inches deep so each clump has room to fill out.
When to plant
Giant allium runs on a fall-to-summer clock. Plant the bulbs in autumn so they root before the ground freezes, wait through winter and spring, and the flower globes open in June and July for a couple of weeks.
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
Most allium trouble starts underground or in a strong wind. Soggy soil rots the bulbs, tiny mites pit them, and tall stems snap if they're left unsupported. Match what you see below and act before a small issue spreads through the planting.
Soft, mushy bulbs and plants that flop over
Wet, slow-draining soil leads to bulb rot, and in a stubbornly soggy spot a black rot fungus can finish the job. Lift a bulb and you'll find it soft and foul-smelling. Pull and bin the rotted ones, ease up on watering, and replant the firm survivors somewhere the water actually drains.
Fix:
- Remove and discard affected bulbs
- Improve soil drainage and reduce watering
- Replant healthy bulbs in well-drained amended soil
Stunted, twisted growth from pitted bulbs
Bulb mites burrow into the bulb scales and feed, which leaves the new growth distorted and the bulb scarred. Check suspect bulbs closely for the tiny mites and their feeding damage. Throw out anything badly infested, plant only firm clean bulbs, and rotate where you grow them.
Fix:
- Discard heavily infested bulbs
- Plant only firm, healthy, mite-free bulbs
- Practice crop rotation and good sanitation
Tall stems leaning over or snapping
Those big globes sit on long scapes, so a gusty day can bend or break them, most often when the flowers are at full weight. Strong wind on unsupported stems is the cause. Plant in a sheltered spot and stake the stems before they bloom.
Fix:
- Stake tall stems for support
- Site plantings in locations sheltered from strong winds
Toxicity
Treat the whole plant as off limits to animals. Bulbs, leaves, and flowers all carry the organosulfur compounds that make onions and garlic dangerous to pets, and they can break down red blood cells and cause hemolytic anemia. Cats are the most sensitive, but dogs, horses, and grazing livestock are all at real risk, so site this plant where they can't reach it.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds · People
Cats Severe
Cats are hit hardest of all. Even a small nibble can trigger vomiting and start breaking down their red blood cells, which shows up as weakness, a racing heart, panting, and dark urine.
Dogs Severe
A dog that chews any part of the plant can vomit and then grow weak and listless as its red blood cells break down. Watch for pale gums, fast breathing, and discolored urine, and get to a vet quickly.
Horses Severe
Horses suffer the same red blood cell damage if they graze the foliage or bulbs, leaving them weak and dull. Anemia can build up over repeated nibbling, not just one big meal.
Rabbits Toxic
The same onion-family compounds are toxic to rabbits, so keep the whole plant well away from them. There is no safe amount to offer.
Livestock Toxic
Cattle, sheep, and goats can develop hemolytic anemia from eating alliums, whether fresh in the field or dried in hay. Fence them off from plantings and clear any trimmings they could reach.
Birds Toxic
Birds are sensitive to onion-family toxins too, so chickens and pet birds should never get at the bulbs, leaves, or flowers.
People Caution
This is an ornamental onion grown for its flowers, not a kitchen crop, so it isn't meant to be eaten. Handling is fine, but wash your hands afterward and keep curious children away 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
Several big-flowered alliums turn up under the giant-onion name, and a few close relatives stand in for it. These are the ones worth knowing by name.
Globemaster
This hybrid carries the show-stopping flowers, with dense violet globes of 8 to 10 inches held up on sturdy scapes.
Allium hollandicum (aflatunense)
A related ornamental onion with smaller rose-purple umbels, often sold as a cheaper stand-in for the true giant allium.
Allium cristophii (Star of Persia)
A lower-growing relative whose wide, open umbels open into metallic 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 is easy to increase, whether you buy fresh bulbs or split what you already grow. Most of the work happens in fall.
Bulbs & cormsEasy
Fall · One growing season to bloom
The simplest way to start is from dormant bulbs in fall. Set them about 5 to 8 inches deep and 9 to 12 inches apart, and they'll send up their first flowers the following growing season.
Offsets & pupsEasy
Fall · 1-2 seasons
Established bulbs make small offset bulblets around their base. Lift them in fall, snap the offsets free, and replant them to widen a planting, though they take a season or two to reach blooming size.
DivisionEasy
Fall · 1-2 seasons
Dig up a crowded clump in fall every few years and split it apart to keep the bulbs vigorous. Each division settles in over a season or two and blooms again on its own schedule.
SeedModerate
Fall to spring · Several years
Alliums self-seed freely, and you can sow them anytime from fall through spring. It's the slow route, since seed-grown plants need several years to bulk up before they flower.
Myths
Ornamental flowering alliums are just for show and safe for pets, unlike kitchen onions and garlic.
Ornamental alliums sit in the very same genus as onion and garlic and carry the same toxic organosulfur compounds. Every part, from bulb to flower, can break down a pet's red blood cells. They are just as dangerous to cats, dogs, and horses as anything in the spice rack.