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How to grow alliums and best varieties
Alliums (ornamental onions) are the star of the border in late spring. Discover our best allium varieties and tips on how to grow them
Ceri studied horticulture at the University of Reading and RHS Wisley. She edits Which? Gardening magazine and our online gardening content.
Often the star of the gardens at the RHS Chelsea Flower Show in May, 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. They should be planted in autumn at the same as other spring-flowering bulbs. Once planted, they will then return to flower each year as they're perennials.
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
| January | February | March | April | May | June |
| Flowering | Flowering | Flowering | |||
| July | August | September | October | November | December |
| Flowering | Plant | Plant |
Growing alliums
Planting allium bulbs
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.
How late can you plant allium bulbs?
When the Which? Gardening magazine researchers trialled planting allium bulbs at different times, they found that bulbs planted in September emerged in March to give the longest total flowering period of five weeks, and spent two weeks in full flower in May.
Bulbs planted later than that were in flower for a week less, but the displays were still good and the flowering period was only delayed by a week or two so it's not a disaster if you buy bulbs in the sale or find a forgotton packet in your cupboard.
Best allium varieties
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Full testing results for alliums
| Variety name | Overall rating | Height & spread (cm) | Flower impact | Flowering duration | Foliage impact (dieback) | Plant shape - steam strength | Bee/insect attraction | Pests & diseases |
|---|---|---|---|---|---|---|---|---|
| 'Ambassador' | Sign up to reveal Get instant access to this and all our scores and recommendations. Unlock tableFirst month £5, then £11.99 per month, cancel at any time Already a member? Log in | |||||||
| 'Beau Regard' | ||||||||
| A. cristophii | ||||||||
| 'Eros' | ||||||||
| 'Forelock' | ||||||||
| 'Gladiator' | ||||||||
| 'Globemaster' | ||||||||
| 'Jackpot' | ||||||||
| 'Universe' | ||||||||
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Overall rating - the more stars the better. Rating ignores price and is based on: flower impact 35%; duration of flowering 20%; shape 15%; foliage impact 10%; bee/insect attraction 10%; pests and diseases 10%.
Caring for alliums
What to do with allium bulbs after flowering
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. The seed heads can be allowed to empty their ripe seeds onto the border and you'll get tiny seedlings the following year which will eventually flower after a few years. Leave the bulbs in the ground and they will come again the following year.
Learn how to make compost
Supporting flower 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 allium 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.