Horticulture Magazine

Allium ‘Ornamental Onions’ Plant Care & Growing Tips

purple allium flowers with greenery in background

Allium Overview

Official Plant NameAlliums
Common Name(s)Ornamental Onions
Plant TypeBulbs
Native AreaEurope, Asia, Africa
Hardiness RatingH6
ToxicityNone
FoliageDeciduous
FlowersSmall flowers on pompom umbels
When To SowOctober, November
Flowering MonthsMay, June
Sunlight

Preferred
Full Sun

Exposure
Sheltered

Size

Height
0.5 – 1M

Spread
0.1 – 0.5M

Bloom Time
May & June

Soil

Preferred
Most Soil Types

Moisture
Moist but well drained

pH
Any

We have umbels loose and densely-packed, florets erect and pendent, and complementary colours of sunny yellow and vibrant purple. That’s the ornamental Allium. These plants produce bunches and clusters of tiny flowers in late spring and summer that sashay and sway in the breeze on their slender stems. They are perfect as companion plants or for mass plantings.

If you like onions, purple, and pom-poms, Allium is your baby. Many Allium flowers exude an oniony smell. Just as many are in some tone of purple. And equally many species’ flowers are in the form of pom-pom-like inflorescences.  The Allium genus includes onion, garlic, leek, shallot, chives, and what we are interested in here: pretty flowering plants that are grown as ornamentals, albeit not as central or showy ones.

Alliums are actually a genus of plants whose bulbs and leaves are flavourful and edible. Some, such as the aforementioned onion and garlic, are considered purely edible plants, though many varieties certainly put out pretty flowers, whereas others are grown as flowering plants, though both the bulbs and the flowers are edible and often tasty and nutritious. Though some species of Allium have been bred purely for the edible bulbs and are not supposed to bolt (run to flower) and which species cannot be considered flowering varieties, the line dividing edible Allium and decorative Allium is neither hard-and-fast nor scientific; it is quite arbitrary. 

Allium stipitatum `Mount Everest' flowers in focus with a buzzing bee
The White ‘Pom-Poms’ of Allium ‘Mount Everest’

Genus Allium, belonging to Family Amaryllidaceae, encompasses 980 to 1000 accepted species. These are perennial bulbous herbs and their native range covers the entirety of the temperate and sub-tropical Northern Hemisphere. Most of them bloom in the late spring to early summer bridge season while some bloom through summer. These plants, like others in the Amaryllis Family, are distinguished by their narrow linear or strap-like foliage or cylindrical, hollow leaves. Usually these are of a deep, viridian colour which is bright on some species, greyish on others, and with a bluish tint on some. 

Allium flowering plants have a lot going for them. They are low-care to no-care, fully hardy, reliable perennials. Their flowers are definitely unusual, being florets in pom-pom-shaped and other types of inflorescences and umbels. While some species’ inflorescences come in white and soft tones of mauve, others provide vivid and vibrant splashes of purple and pink. They attract butterflies but repel most garden pests! And though you certainly are not going to dig up a bulb to consume it, these pretty ornamentals will be a source of stems and leaves which you can use as pot-herbs to lend a leeky, shalloty flavour or that you can use as a substitute for scallions in a salad.

Background and Origins

Humankind’s association with one kind of Alliums, namely onions, dates back through remote Antiquity into Prehistoric Times. Onions are represented or referenced in Egyptian tomb drawings and inscriptions dating to 3200 B.C. The Hindu Brahmin prohibition on onion and garlic comes down from the earliest Vedas which date to 1500 to 1200 B.C. Surely some of those Brahmin ancients took a minute to admire the pretty flowers, at least.

Between 330 and 320 B.C. Alexander of Macedonia introduced onions from Egypt to the Greek Islands from whence onions made their way to other European lands. From the Old World it was further westward to the New World in the Fifteenth and Sixteenth Century for onions and their bulbous kin. 

Now, the United States is the world’s leading producer of one type of Allium, onions; at the same time, Americans are famously averse to another type of Allium, garlic! They are not at all averse to Allium flowers, however. These are grown and appreciated as much in the U.S.A. as they are in the U.K. and Europe. 

ornamental yellow Allium flavum
The ‘Sundrops’ of Allium Flavum

Varieties

Underneath we present selected ornamental Alliums in a range of heights and a variety of colours including white, yellow, pink, and purple.   

A. tuberosum also called Chinese Chives grows to about 40 centimetres. It bears umbels of small, star-shaped white flowers. They have the bonus of being scented. Flowering in late summer and into autumn, Chinese Chives is a late bloomer among Alliums and will extend the season of interest in a mixed planting. It is native to Tibet and the Himalayas.

white Allium tuberosum flowers
The Pretty White Stars of Chinese Chives

A. neapolitanum also called Naples Garlic, is actually an ornamental onion. It reaches a height of only 25 to 30 centimetres but has a spread twice as big. It too bears umbels of small, star shaped flowers that are pure white. They also have the bonus of being fragrant. It blooms in late spring through early summer.

A. ‘Mount Everest’ is an ornamental onion that is quite tall at about 90 centimetres. It produces pure white pom-pom-like inflorescences which are very large at about 15 centimetres in diameter. This species’ flowers last relatively long on the plant or in a vase. It blooms in late spring through early summer. R.H.S. Award of Garden Merit. 

A. flavum or Yellow-Flowered Garlic is a small variety at only about 30 centimetres high with no spread to speak of. It produces umbels of buttery, sunny yellow bell-shaped ‘sundrops’. The umbels are open or loose, and the florets are pendent. It blooms in late spring through early summer. It is native to the Mediterranean region to West-Central Asia. R.H.S. Award of Garden Merit. 

A. senescens or Ageing Allium is a type of chive. Another small variety, it grows to only about 30 centimetres. It bears somewhat sparse and delicate tufty umbels. Their colouring is equally delicate, being of whitish-pinkish lilac hues. It blooms in early and mid-summer. It is indigenous to Siberia through the Koreas. R.H.S. Award of Garden Merit.

close up of Allium lusitanicum with a hover fly in focus
A Budding Umbel of A. senescens Shows a Delicate Pale Pink Hue

A. stellatum or Prairie Onion rises to 50 to 60 centimetres. It bears big, rounded 8- to 10-centimetre umbels with a profusion of scented star-shaped small flowers. They are of a delightful soft pink shade. Another late bloomer among Alliums, it flowers from mid summer into autumn and extends Alliums’ season of interest. It is native to the United States.

A. ‘Eros’ reaches a height of 40 to 50 centimetres and has a roughly equal spread. It bears big dome-like inflorescences about 10 centimetres wide. These have tiny star-shaped flowers of a lovely light lilac to light purple hue with a darker radial stripe along each petal. It blooms in early to mid summer.

A. oreophilum or Pink Lily Leek grows to only 20 to 30 centimetres with no spread to speak of. It bears sparse, loose umbels of small bright candy-pink flowers. Each petal has a red radial stripe and the visible yellow stamens add further colour. The flowers are fragrant. It is indigenous to South Central Asia from the Caucasus to Turkestan.

candy pink flowers of Allium oreophilum
Allium oreophilum’s Candy Pink Flowers – A Pretty and Tasty Garnish!

A. cernuum or Lady’s Leek is of small-to-medium height at 40 to 45 centimetres. Its umbels bear tiny flowers in nodding, pendent form. These dainty bell-shaped flowers are of a soothing soft pink to soft lilac tone with which the prominent yellow stamens make a fine contrast. This plant is native to the United States.

A. caeruleum or Azure-Flowered Garlic is medium-to-tall height at 60 to 80 centimetres and its spherical inflorescences, comprising of star-shaped florets, are but 3 centimetres across. But this is a very special variety because of its colour: stunning true blue; even azure.  It blooms in late spring to early summer. It is native to a swath of land from Siberia down to Turkestan.

azure blue colours of Allium caeruleum blossoms in focus
The Astonishing True Blue to Azure Colour of Allium caeruleum

A. cyaneum also called Dark Blue Garlic grows to a mere 25 centimetres and has a clumping form. Its leaves are among the narrowest. It produces small clusters of tiny, pendent bell-shaped flowers. The colour ranges from soft lilac to rich violet with which the yellow stamens make a strong contrast. Its native range is from Tibet through Korea. R.H.S. Award of Garden Merit.

A. ‘His Excellency’ is an architectural variety made so by the height of its stalks and the size of its inflorescences. It attains a height of up to 1.2 metres and its large pom-pom-like globular inflorescences are about 12 centimetres wide. These are tightly packed with countless tiny star-like florets in a pleasing mauve-purple colour. It blooms from mid-spring to midsummer.

A. ‘Ambassador’ is a spectacular architectural plant that reaches a height of up to 1.2 metres. It produces massive spherical inflorescences that would put any pom-pom to shame. These are up to 20 centimetres across and are tightly packed with innumerable tiny star-like florets. The colour is an eye-catching and gorgeous deep, rich purple. It blooms from mid-spring to midsummer. R.H.S. Award of Garden Merit.

Allium ‘Ambassador’ Inflorescence
Tall, Huge, and Rich Purple – That’s the Allium ‘Ambassador’ Inflorescence

A. ‘Universe’ and A. hollandicum ‘Purple Sensation’ are two more options that bear spherical inflorescences in shades of purple. Both are recipients of the R.H.S. Award of Garden Merit.

Habitat and Growing Conditions

As Alliums are native to or naturalised over a vast range of habitats, from the Northern reaches of Russia and Canada down to Northern Africa, their habitats and growing conditions vary considerably. Some species grow wild in dry conditions on the steppes and plateaus of Central Asia. Others grow in moister habitats in open grasslands and shady woodlands of the United States. Still others grow in meadows and fields, and are even found in fens, in Europe.

Likewise, Allium species’ USDA Hardiness Zones also have a wide variance. Allium cernuum is cold-hardy down to Zone 3 while Allium flavum is heat-tolerant up to Zone 10. Both of these species are both more cold-hardy and heat-tolerant than other species with a narrower USDA Hardiness Zone, for example, ‘His Excellency’ and ‘Mount Everest.’ As a general rule, the species are more cold-hardy and heat-tolerant than the ornamental cultivars. 

white Neapolitan Garlic flowers
The Pure White Blossoms of A. neapolitanum or Naples Garlic

Where to Plant Allium

If there ever was a companion plant or a ‘co-star plant,’ Allium is it. Not showy enough to be the star of the show, Alliums play the classic Bob Hope role to one or another Bing Crosby plant. They make for superb fillers, interesting and colourful in their own rights, to be planted between, behind, and around, late spring to early summer’s leading man flowering stems such as some Amaryllis, many Irises, Columbines, and Poppies. 

On the other hand, some Allium species, for example ‘His Excellency’ and A. hollandicum ‘Purple Sensation’, are excellent choices for mass plantings in view of the shape and colour of their blooms, and also the movement they bring to a garden as they sway and nod in the breeze.

Other varieties that produce appealing clusters in pleasing and gentle hues are just as good for borders and edging, and for growing along the side of a porch or patio. These include A. stellatum and A. flavum.

Some varieties, for example, A. cyaneum, are as if made to order for rock gardens. At the other end of the spectrum, a few varieties are unquestionably suited to architectural plantings, for example, ‘Mount Everest’ and ‘His Excellency.’

It is easy to grow Allium with such plants whose foliage and flowers will conceal Allium’s fading, yellowing and dying foliage through the summer, because – as for other bulbous plants – the leaves must be left untouched so they can generate energy stores for the bulbs to regenerate after the dormant winter season. 

Alliums should not be grown in a windy spot, let alone a blustery one, because strong winds will tear apart the delicate inflorescences. On the other hand, a gentle wind will greatly heighten the joy of these plants as they will bob and sway merrily in the breeze.

In the United Kingdom Alliums are best planted in full sun.

mixed flower border with Verbascum, Iris and Peonies in a cottage garden
Alliums Impart Their Own Unusual Charm to a Cottage Garden

Feeding, Care and Growing Tips

Alliums are not fussy about soil or, indeed, anything else. All Alliums will be happy in a good all-round loam mix. The soil should should drain well. A Slightly Acidic soil pH is preferred.

The ‘best’ time to plant these late spring- and early summer-blooming bulbs is early-to-mid November.

Different species’ bulbs vary quite widely in size and even within a species, bulb sizes will differ. You will read some gardening how-tos advising you to plant Allium bulbs at twice the depth of the bulb’s height, and others telling you to plant at thrice the depth. Well, both are correct; the ‘right’ depth is implied by a few factors, most importantly the soil. The more loose, dry, and sand-based your soil is, the deeper you should plant a bulb, say at a depth that is thrice a given bulb’s height. The more compacted, moist, or clayey your soil is, the shallower you should plant a bulb, say twice a given bulb’s height.

Water bulbs after planting and do so lightly through the winter though rainfall may well provide sufficient water. 

grasshopper sat on flowers of Allium stellatum Umbel
A Profusion of Tiny Soft Pink Flowers in an Allium stellatum Umbel

The characteristic pungent odour and taste of garlic and onions, which is also present to a lesser degree in ornamental Alliums, depends upon the amounts and concentrations of sulphates in the soil. No sulphates, no pungency; high sulphates, high pungency. You can chemically control your soil’s sulphate content, though it can be a tricky business. 

Alliums may be fertilized just when the leaves being to show. A light feeding with 5-5-10 fertilizer or a bulb fertilizer would be beneficial. (They can be fed similarly once again after the flowering season is over.) At this time the plant’s water needs will increase and watering should be more regular.

Though blooms may be deadheaded throughout the flowering season, do not cut off the foliage (unless you wish to enjoy fresh greens with your supper!). Let them wither and die.

In autumn the plants can be cut down to the ground. 

Once every three years or so you may want to dig up the bulbs in late autumn so as to divide them. You can plant new ones in pots and then gift potted Alliums in spring. Some Alliums are not bulbous plants but grow from rhizomes or rhizomatous roots. These types should be dug up in spring for division which is done by carefully separating the rhizome clump with a sharp blade.

Before winter sets in, though most Allium bulbs are hardy, you could cover the soil with a thin layer of leaf mulch. 

Common Diseases and Problems

Almost all Allium species that are naturalised to Europe and almost all garlics and leeks, are virtually impervious to pests. The only exceptions are onions and shallots which may fall victim to slugs.

All Alliums repel rodents, deer, and harmful insects including Japanese beetle, while attracting butterflies.

However, Alliums are susceptible to a couple of diseases, namely downy mildew and onion white rot. The latter is a fungal disease that is nearly impossible to treat. The harmful organisms can lie dormant and persist in the soil for many years.

Where to Buy Allium

Alliums have been on a slow but steady rise in popularity in the United Kingdom. As a result, the number of nurseries and garden centres which stock Alliums keeps increasing every year, and the number of varieties that are available is also on the rise. 

Allium bulbs are available in brick-and-mortar nurseries and also from online retailers, including the R.H.S. Plant Shop.

Note that once you buy a few bulbs, plant them, and take care of them, you will automatically get new ones every few years!

Health Benefits

All parts of Allium plants from the bulb upwards are of medicinal value.

The health benefits of garlic and onion are all too well known. They are heart-healthy, control blood pressure, and reduce the chances of stroke. And that’s just for starters. Moreover, garlic and onion have been used by herbalists for centuries to – correctly, as it turned out – treat heart disease, pulmonary problems, and coughs and colds.

blue magenta wild onion flowers
The Rich Violet Pendent Flowers of Allium cyaneum

Many academic papers have been written (and are available online) about both these benefits, as well as about the curative properties, of garlic and onions.

You can derive at least modest health benefits by consuming leaves, stems, and flowers of the ornamental Alliums in your garden.

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