Allium giganteum (AL-ee-um jy-GAN-tee-um) is a most unusual plant, from its iconic size and umbels to the fact that this plant is such a big secret.
One of a very few Asian plants to come from the southwestern portion of the continent (most hail from the southeast and Indian Subcontinent), this perennial member of the Amaryllidaceae family is every bit as beneficial as it is beautiful.
Better known as giant allium or giant ornamental onion, this is one of the tallest alliums to be cultivated worldwide and is the parent of the famous cultivar Allium ‘Globemaster.’
But for its beauty and exoticness, this is a very easy plant to grow in gardens and pots, making its obscurity outside Allium enthusiasts even more curious.
Allium Giganteum Care
Size And Growth
Reaching an impressive height of 4.9′ feet, Allium Giganteum is a relatively fast-growing plant that might need a support stake, especially if your garden is known to get high winds.
The spread is a far more modest 18″ inches, which makes it easier to integrate into your garden when companion planting.
Even before blooming, the basal, glossy grayish-green, strap-shaped leaves provide a modest yet interesting ground cover.
Flowering And Fragrance
Of course, the leaves of Allium Giganteum aren’t the true appeal of this plant, which comes into full effect from late spring through midsummer.
At this time, large umbels up to 10″ inches across replace the shorter-lived foliage, bursting into a display of violet to purple star-shaped flowers numbering in the hundreds.
But the rounded flower heads aren’t the end of the show.
These will dry out long before they lose color, and the resulting seeds from pollinated blooms are still attractive and often attract wildlife.
Light And Temperature
Alliums love the sun, although they can be a little sensitive in harsher climates.
For the best effect, ensure they get 6 to 8 hours of full sun daily.
This can be at midday in cooler climates but is better provided in the morning or evening with some light midday shade in subtropical or arid environments.
Giant onions can tolerate partial shade, but the size of the umbel and overall height will suffer.
Something else to remember is that high humidity can increase the risk of alliums contracting fungal infections.
These plants do well in normal household humidity levels and tolerate low humidity during droughts.
As a general rule, giant onions grow best in USDA hardiness zones 4 to 9, although some have reported success in zone 3 when the bulb is properly insulated or overwintered indoors.
Zones warmer than 9 may be too harsh in the summer, although potted specimens should do fine.
Be wary of frequent temperature fluctuations in spring, as this can weaken the flower stem, and you should protect the plant from high winds.
Watering And Feeding
Most alliums – including the giant onion – do best when watered using the soak and dry method.
The size of this plant means it needs to be watered a little more often, so give it a drink when the soil feels dry 1″ inch down.
Note that this plant becomes drought tolerant once established.
As with many allium flowers, the giant onion doesn’t require feeding, although a bit of compost in early spring can give it a little boost.
Soil And Transplanting
You can plant the giant onion in almost any soil or potting mix.
Loamy soil is best, but it will also thrive in poor soils.
Any soil pH from 5.5 to 6.5 is perfect, but it can tolerate more neutral levels.
The only exception is that the plant needs well-drained soil, so you may need to amend clay soils with perlite or coarse sand and some organic material.
As mentioned, growing Allium giganteum in containers is possible, and an 8″ to 10″ inch deep pot is usually sufficient.
When choosing a container for your allium, you must remember that the bulbs need to be planted in your container soil at the same depth as they would in your garden.
Of course, this will change by variety, but choosing an 8″ to 10″-inch deep pot should ensure that you won’t have any issues.
You will need to ensure that there are at least 2″ inches of soil beneath the pot for the roots to grow.
Ensure the container has good drainage and at least 2″ inches of soil beneath the bulb to facilitate root growth.
Divide in the fall if it begins to outgrow its container.
Grooming And Maintenance
You don’t need to prune Allium giganteum unless you need to remove a diseased leaf.
Instead, allowing the leaves to die back naturally is usually best to maximize photosynthesis.
Cutting the plant back to soil level at the end of autumn in colder climates will help this plant overwinter, especially younger plants.
However, there’s one important choice you’ll have to make when this allium flowers.
As it takes years for the giant onion to fully mature, you will need to decide if you want the plant to put all of its energy into the seeds or the bulb.
If you choose to deadhead, the allium will put more energy into beefing up its bulb, which makes division easier and also allows the plant to naturalize.
Conversely, if you let those purple flowers go to seed, they will attract birds, provide visual appeal throughout late summer until near winter, and your allium will self-seed.
How To Propagate Giant Onion?
The most popular method of propagating giant onions is through division.
However, you can also grow it from seeds once the parent has matured, and it will self-seed if the flower heads are left alone.
Giant Ornamental Onion Pests Or Diseases
As with all alliums, the giant onion gives off a slight oniony scent that repels a wide range of pests, including deer and rabbits.
They’re also drought-tolerant once established.
Despite these benefits, the plant can still be infested with onion flies or thrips.
Rot and fungal infections are the only real disease risks, and these only occur if the plant is overwatered.
All alliums are toxic to cats and dogs, and the giant onion has a lot of sulfates which can cause digestive issues in people when consumed.
These symptoms include mild symptoms such as diarrhea, nausea, and vomiting.
However, the bulbs and leaves are used for cooking in some cultures, and proper preparation may make these parts more edible.
Allium Giganteum Uses
Due to the long-lived umbels, Allium Giganteum is a perfect plant for flower arrangements or as wedding flowers.
They look best when grouped and are an excellent background for borders but also look great when potted.
Most importantly, this plant attracts bees, butterflies, and beneficial insects while repelling pests, making it a perfect companion plant for most garden settings.