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Home & garden.

14 Jan 2022

How to grow alliums

Alliums (ornamental onions) are the star of the border in late spring. Discover our best allium varieties and tips on how to grow them
CT
Ceri Thomas
Alliums

Often the star of the gardens at the RHS Chelsea Flower Show, alliums (ornamental onions) with their purple globes are a stunning sign of summer's imminent arrival. Bees adore them, too. There has been lots of breeding in alliums over the years, so there's a surprising number of varieties available, and not all of them bear the purple globe flower heads we're all familiar with.

Which? Gardening magazine grew a range of popular varieties to see which would give us the best display.


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Key facts

Plant type Summer-flowering bulb

Position Full sun

Soil Free-draining

How to grow alliums: month by month

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Best allium varieties

Which members can log in now to see the full results and which are our Best Buy varieties. If you're not a member, join Which? to get instant access.

Best Buy alliums
What it looks likeVariety nameHeight x spread
60 x 20cm
This variety offers something a little different from the statuesque purple globes so often seen in show gardens. It's an elegant variety, with soft mauve, almost bell-like flowers and whippy green foliage. The flowers in our trial opened in May and were immediately popular with honey bees and other smaller bees. The foliage was a little floppy, but provided a soft-green foil to set off the delicate blooms. Peak flowering: May-June
What it looks likeVariety nameHeight x spread
80 x 40cm
Forming perfect globes of dense, mid-purple florets, this variety well deserves its Best Buy along with its RHS AGM. The flowers in our trial started to open in mid-April, and by May the round heads, held on strong, upright stems, were busy with bees. It kept flowering for seven weeks before leaving star-like seedheads that stood for another month before the wind knocked them down. The foliage, never that attractive for alliums, died down neatly at the base of the stems. Peak flowering: April-May
What it looks likeVariety nameHeight x spread

How we test alliums

We tested the alliums over a two-year period. In autumn, we bought 28 varieties, including some previous Best Buys, holders of the RHS Award of Garden Merit (AGM), some garden favourites and some more unusual varieties. The bulbs were planted in the ground at the Which? Gardening magazine trial gardens at Capel Manor, north London. In spring, we recorded how many bulbs produced flowers, how long the flowers lasted, how impressive they looked, which pollinators were attracted to them and how well they stood up to the weather. We also looked at the foliage to see if it died back neatly or ended in a tangled mess at the base of the plant. We repeated this assessment in the second year, looking at health, flowering and overall attractiveness.

Caring for your plants

Planting

Plant bulbs at three times their own depth in the soil in autumn, spacing them about 20cm apart (or 10cm apart in the case of smaller bulbs, such as ‘Eros’). 

Some alliums have rhizomes (underground stems) instead of bulbs; these look more like spring onions than dry bulbs on arrival. Plant these in autumn just below the soil surface and 10cm apart.

Alliums aren’t too fussy, but a sheltered spot with well-drained soil in full sun is ideal. You can also grow them in pots filled with a Best Buy compost for containers as long as you plant them at the necessary depth.

Cutting back

To keep alliums tidy, gather up the dead leaves in early summer and remove any stems that become detached at their bases in late summer. 

Supporting stems

Although most of the stems stood up reasonably well in our trial, in a very exposed site you might need to provide support for them.

Common growing problems

Alliums are generally trouble-free in borders, but watch out for the orange spots of rust, and cut back diseased foliage. Pests to keep an eye out for include slugs, snails and allium leaf miner. Alliums are hardy in the UK and can be left in the ground all year.