Alliums: how to grow and when to plant them

By Abigail Whyte

Published: Friday, 13 May 2022 at 12:00 am

Onions, leeks, chives and garlic are edible members of the Allium family, a genus distinguished by the simple flower umbels that are usually arranged in the shape of a dome or a ball.

Alliums are mainly native to the northern hemisphere with a few species found in South America. How many allium species there are remains unclear. In her authoritative 1992 book, Alliums (Batsford Ltd), Dilys Davies writes that there are between 800 and 1,000 species, but subsequent research suggests that there may be no more than 750.

In the garden, the most popular alliums are those with tennis-ball flowers on the top of tall stems. Drifting on a cloud of grasses or punctuating a parterre, they are at home both in formal gardens and naturalistic planting schemes. Allium hollandicum ‘Purple Sensation’ is the flag-bearer for this group, but there are forms with larger flowers in a range of purples and mauves.

IN BRIEF

What Genus of about 750 flowering bulbous plants – informally known as the onion family and characterised by a distinct smell that varies in intensity between species.

Origins Most come from the temperate climates of the northern hemisphere, particularly Europe; fewer from Central Asia and South America.

Size From 10cm to 1.2m tall with flowers up to 45cm in diameter.

Conditions Species for the cut flower trade are more needy, but most will flourish in well-drained soil in full sun.

Not all alliums have dense, spherical flowers. Allium cernuum has loose flowers made of pinky-mauve bells that dangle from dark pedicles in June.

With so many species in the wild it is not surprising that new ones are still being introduced into cultivation. Allium litvinovii has recently been found growing in Uzbekistan and Kyrgystan, and is starting to be available in specialist nurseries. The intense blue flowers make it very appealing.

How to grow alliums

When do alliums flower?

Alliums flowers mainly in early summer, with a few species flowering in late summer. Seedheads can last into autumn.

Where to plant alliums

Most alliums need a well-drained soil and prefer a sunny site. If you have heavy clay, dig in plenty of compost and horticultural grit before planting them.

When to plant alliums

Alliums bought in pots can be planted at any time but you will get more for your money if you buy them as dry bulbs in the autumn.

How to care for alliums

Using plant supports or bamboo canes to hold up tall varieties will always look ugly, so grow them through other plants to keep upright.

The leaves of many alliums, particularly the tall forms, are lax and long and quickly become tatty. By the time the plant is flowering they are often very unsightly. If you have the time (and can be bothered) you can trim off the dead parts, but it is often easier to plant the bulb so that the foliage is hidden by other plants.

Growing alliums in pots

Some of the smaller alliums grow well in pots and many grow well in larger containers. Allium caeruleum, which has bright blue flowers on 50cm- tall stems in June and July, can be difficult in the garden but thrives on the good drainage and baking heat of a pot in full sun.

My favourite allium for growing in containers is the short (20cm tall) Allium senescens subsp. glaucum. The dusky-pink flowers, the size of a ping-pong ball, last for about six weeks in August and September and are surrounded by blue-grey leaves that corkscrew around the stems.

Alliums to avoid

Not all alliums are welcome in the garden; some are invasive weeds that can be difficult to get rid of.

Allium ursinum Also known as wild garlic or damsons, they may look pretty in the dappled light of woodland but in the garden the plant will spread rapidly by seed and bulbil and smother less vigorous plants. It is tempting to grow them because the leaves are tasty in soups and salads, but only plant them if you have acres of woodland for them to romp around in.
Allium paradoxum This, too, will quickly swamp other plants, as will the white-flowered Allium triquetrum, a Mediterranean plant that has naturalised in parts of Cornwall and Devon.
Allium roseum This, plus the quirky Allium vineale ‘Hair’, has tiny bulbils at the centre of the flower, making them disastrously prolific. If you already have them in the garden never put them on the compost heap, as this may spread them even further.