These ever-popular ornamental onions range from giants to pot species, says Miranda Kimberley.

A. giganteum - image: Floramedia
A. giganteum - image: Floramedia

There are some plants that remain in fashion throughout the decades. Used as a cottage garden favourite for many years, alliums remain as popular as ever with the gardening public and grace most RHS Chelsea Flower Show gardens too.

Their upright stems topped with spherical flower heads in vibrant shades of purple, blue, white and yellow make these ornamental onions winners.

It is a huge bulbous genus with around 700 species and they are nearly all found in the northern hemisphere, in dry, rocky habitats across the Mediterranean and temperate regions.

They range from tall species, growing up to 1.5m like Allium giganteum, to diminutive species that suit pots.

The typical image of the allium is a spherical head of densely packed usually purple flowers.

This is not the case for all alliums. Some have other kinds of head shape, such as
A. sphaerocephalon, the drumstick allium with its egg-shaped heads, and looser-headed alliums such as the yellow flowered A. moly and the pendulous A. cernuum.

But many popular alliums bred from central Asian species do have this look. This group includes A. giganteum, A. rosenbachianum, A. cristophii Award of Garden Merit (AGM) and A. ‘Globemaster’ AGM. Many of these species also show off star-shaped flowers, particularly
A. cristophii AGM. Another central Asian species is A. schubertii AGM, the gorgeous "exploding firework" allium that demands to be used but is on the tender side, which may prove to be problematic for those in northern areas.

The foliage of alliums generally dies back as the flower is emerging so they are best planted below plants that will cover the dead leaves, but these plants need to let through enough light to keep the bulbs happy and produce healthy growth. Give them a sunny, open, well-drained site, but A. moly can cope with moister soil.

Plant the bulbs deep — between two to four times their depth — around 10-15cm for smaller bulbs and more than 15cm for larger bulbs. Add grit to the planting hole and as a bed for the bulbs if the soil is heavy. Small to medium-sized varieties look good planted in drifts and can be planted quite close together at around 8-15cm apart. But bear in mind how much space some of the larger bulbs need. Plant giants such as A. ‘Globemaster’ AGM and A. schubertii AGM at least 30cm apart.

Alliums are known to be quite a weedy genus. A. sphaerocephalon produces bulbils in its inflorescence and its main bulb can also split into bulblets so it will spread, but unwanted plants can be pulled out. Some people may want to increase their stock by collecting offsets after flowering and replanting them or potting them up.

What the specialists say

Andy Bone, owner, Clare Bulb Company, Suffolk
"Alliums are a highly attractive genus with plenty to offer. They range from small species such as A. moly at around 4in high to tall varieties such as A. ‘Purple Sensation’ and A. ‘Globemaster’ that can reach 3ft. They also range greatly in price, as with some varieties you can get 20 bulbs for £1 and then sought-after single bulbs can go for £3.90, which is the case with A. ‘Globemaster’.

"Of the small varieties the favourite is probably the drumstick allium, A. sphaerocephalon. It’s so attractive to bees it’s not unusual to see them covering the lovely dark-purple flower heads.

They are good value as they spread and self seed. While some people might be worried about them as a slightly weedy species, like most alliums they are easily controllable and can be pulled out where they are not wanted.

"We promote the alliums that are most reliable, flowering year on year. People like A. ‘Mount Everest’, which is a good white and a great foil to the deep-purple A. giganteum. They are the same height.

"We have found this year has been excellent for alliums, with a hard winter followed by fine weather. Our A. ‘Globemaster’ and A. ‘Mount Everest’ didn’t flower until mid June but were still flowering in August. Alliums do best in a dry, sandy soil and sunshine, though for some early-flowering varieties like A. ‘Purple Sensation’, which flower in early March, light levels are clearly not as vital as for the later, larger-flowering types."
David Chisnall, marketing manager, Spalding Bulb & Plant Company, Lincolnshire
"Alliums are fantastic plants, standing proud in the garden with their long stems and big showy heads. They also attract bees and butterflies, which is a very popular feature, and even when they are dying back they remain attractive and can be used as a decoration
once dried.

"This autumn, the best-selling allium by some distance for our company has been the Fantasia
Mix. This selection of allium varieties has a wide range of several appealing features.

"The Fantasia Mix provides three varieties of colour, which means that the plants are decorative in borders as well as beds, and they are low-maintenance plants.

Another added bonus is that they take up relatively little ground space. If required, they even work as a cut flower and can last a long time in the vase."

In practice

David Anderson, manager, Seven Hills Garden Centre, Surrey
"Alliums are one of my favourite bulbs. I have a good selection in my garden, ranging from A. ‘Purple Sensation’ at about 3ft tall to dainty alpine varieties. They are like the fireworks of the border in spring and summer, with their spherical heads creating an explosion of colour, and I love the upright stems that add a strong vertical accent to the borders. Then the dried seed heads create a fine autumn display and later on the seed heads glisten in the winter sun.

"At the Garden Centre Group this year we have promoted an alliums collection, a combination of four fantastic bulb varieties — A. ‘Purple Sensation’ has dense balls of purple flowers in May; A. nigrum produces dense white pom-pom blooms and naturalises quickly; A. caeruleum has stunning small blue flower heads; and A. roseum has loose heads of large pale-pink star-shaped flowers. The collection is doing very well and exceeding all our expectations.

"It’s hard to merchandise a packet of bulbs and even harder to make a display. I keep things clean and crisp but backed up with good pictured point of sale showing just how amazing the flower heads can be."

Species and varieties

A. caeruleum AGM (H5) is a lovely species that produces small, dense globes of star-shaped, bright-blue flowers on stiff stems between May and June. The narrow mid-green leaves die back before the flowers emerge so they are best planted under medium-sized herbaceous plants. It is not reliably hardy so in colder regions mulch over or plant
in pots and move into a frost-free position over winter. Height: 60cm.

A. cernuum are a delicate-looking species with nodding umbels of purplish-pink flowers in midsummer, above slender stems. They increase very easily so find the right spot for them where they can take over. Height: 50cm.

A. cristophii (syn. A. albopilosum) AGM (H7) is a much sought-after variety that produces a very large head of silvery lilac stars. More open than most heads so each star can be more easily made out. A low-growing variety featuring grey-green leaves. Height: 50cm.

A. ‘Gladiator’ AGM (H7) is a strong-growing hybrid of
A. aflatunense and A. elatum that arises from a large bulb. It produces a large, dense head of pinkish-purple flowers in June. Height: 90cm.

A. ‘Globemaster’ AGM (H7) bears enormous heads of violet-purple flowers measuring up to 15cm across. The strap-like leaves will have faded by the time the flowers appear in early-to-mid June and it will flower year on year. Height: 80-100cm.

A. hollandicum ‘Purple Sensation’ AGM (H7) is an incredibly popular variety, featuring a rich purple head on a tall, elegant stem. It is one of the first alliums to flower in mid spring. It has bright-green leaves and densely packed flowers. Height: 90cm.

A. karataviense is a striking, low-growing allium with broad, paired glaucous leaves tinged with purple and short-stalked clusters of star-shaped pinkish-white flowers in spring. Height: 25cm.

A. moly ‘Jeannine’ is a select form of the yellow-flowered, low-growing species. It is a vigorous clump former with lance-shaped, grey-green leaves. Each bulb can produce pairs of stems bearing an umbel of bright-yellow, star-shaped flowers in early summer. It is happy in full sun but also partial shade, which makes it a valuable bulb for planting under deciduous trees and shrubs. Height: 40cm.

? A. schubertii AGM (H4) is a fabulous allium with a head made up of lots of florets at different heights, making it resemble an exploding firework. It forms great dried heads, which can be used as a decoration, either left natural or sprayed silver
or gold for Christmas. Flowers from May to June. Height: 60cm.

? A. sphaerocephalon is known as the drumstick allium because it has tall, slender stems topped with small egg-shaped, deep-purple flower heads between July and August. Height: 60cm.

? A. stipitatum ‘Mount Everest’ is a tall variety that produces tightly packed white flower heads at the top of upright ribbed stems in June. Height: 1.2m.

Thank you to Floramedia, which supplied the images for this article from its photo library

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