Notes on Alliums
An ornamental onion with ideas above its station. When to plant allium bulbs, how little they ask of you in return, and why the show carries on long after the flowers have gone over.
There’s a moment every May when I catch myself admiring an onion.
Because that’s what an allium is, underneath it all. A member of the onion family that decided, somewhere along the way, to stop being dinner and start being architecture. Snap a stem and you’ll get the faint kitchen smell — a quiet reminder of its relatives — but everything above ground says otherwise. One straight stem, sometimes waist height, holding up a perfect sphere made of hundreds of tiny star-shaped flowers. Usually purple. Occasionally white, pink or blue. It shouldn’t work. It completely does.
And here’s the thing: for all that swagger, alliums ask almost nothing of you. They’re drought tolerant. They’re happy in pots. They give you height in a border without taking up any real room at ground level, which is a rare trick. Plant them once, in autumn, and they’ll come back year after year — quietly multiplying while you’re busy with everything else.
When to plant alliums
Allium bulbs start appearing for sale in late summer, and the window for planting runs from September to November. Get them in during that stretch and you’ll have flowers from May through to July — right at the point when the tulips have bowed out and the summer perennials haven’t quite got going. Alliums hold that gap beautifully.
There are exceptions. A few newer varieties, like ‘Millenium’, can go in during early spring and will flower right through to August. Worth knowing if you miss the autumn window, or if you want to stretch the season at the other end.
I’ll admit autumn bulb planting is one of my favourite jobs of the year. There’s something quietly optimistic about it — burying small brown parcels in cold soil on the promise of what they’ll do in eight months’ time. Alliums repay that faith more reliably than almost anything else I grow.
Getting them in the ground
Alliums want two things: sun and drainage. Give them a spot in full sun with soil that doesn’t sit wet, and they’ll tolerate more or less everything else. Waterlogged ground is the one deal-breaker — these are bulbs, and bulbs in standing water rot.
Depth matters more than precision. The rule I use is two to three times the depth of the bulb itself, pointy end up. For spacing, around 10cm apart for the smaller varieties, and at least 20cm for the big statement ones — they need room for those heads.
For a natural look, don’t plant in rows. Scatter the bulbs over the bed and plant each one where it lands. It feels wasteful of your careful planning instincts, and it works every time.
Two small extras worth the effort. If you’re on clay, drop a pinch of grit in the bottom of each hole to keep water moving. And if you’re growing one of the very tall varieties, choose a sheltered spot — a metre of stem with a sphere on top is a sail in a gale, and there are few sadder sights than a snapped allium in June.
Water them in after planting, then largely leave them alone. Unless there’s a genuinely dry spell, alliums in the ground look after themselves.
Alliums in pots
Alliums make brilliant container plants — that vertical shape is even more striking rising out of a pot. The whole game is drainage.
Choose a deep pot with proper drainage holes, and raise it on pot feet or a couple of bricks so water can actually escape. Half fill it with peat-free compost mixed through with a few handfuls of grit, sit the bulbs on the surface about 10cm apart, then top up with more compost, firm it down and water in.
Pots dry out faster than the ground, so you will need to water — but gently does it. Sodden compost is the fastest way to lose the lot. If in doubt, less.
Afterwards
This is where alliums earn their keep twice.
When the flowers fade, don’t deadhead. The spent heads dry into these extraordinary sparkler-like seed heads — all structure and no colour — and they’ll stand in the border looking sculptural for the rest of the summer. They give food and shelter to insects and birds too. You can cut a few to dry indoors, where they last more or less forever. But leave most of them standing. A border with dried allium heads in August has a kind of glamour that fresh flowers can’t quite manage.
There’s no lifting required either. Unlike some spring bulbs, alliums can stay in the ground over winter, year after year. Pots are the exception — move them somewhere sheltered before hard frost, or lift the bulbs, clean and dry them, and store them somewhere cool and dark for replanting in autumn.
After a few years you may find your alliums have multiplied into a crowd. That’s not a problem; it’s a dividend. Once the flowers have died back, lift a clump with a fork, pull a few bulbs apart, and replant the spares elsewhere in the garden. Or pass them to a neighbour. Bulbs are the easiest generosity there is.
What to put them with
Alliums are threading plants. On their own they can look a little marooned — a lollipop in bare soil — but woven through other planting, those spheres appear to float, and that’s when they really sing.
The classic pairing is grasses and salvias for a loose, prairie feel, and I think it’s classic for good reason: the movement of the grass against the stillness of those globes is the whole effect. For something bolder, clash them with orange geums — purple and orange shouldn’t get on, and somehow they’re brilliant together. They sit happily among cottage roses and lavender too, and underplanting with late tulips gives you a proper relay of colour from April onwards.
If you plant one thing this autumn, you could do far worse than a scatter of alliums. An onion, showing off, asking nothing. Every garden should have one.