Allium bulbs

Allium bulbs: ornamental onions for a border in bloom from May to July

Allium bulbs give a garden a silhouette like no other plant. Tall, bare stems carry perfectly round flower heads in purple, white, pink or blue, some as big as a grapefruit. Alliums flower just as the tulips are finishing and most perennials are still getting going, so they bridge that quiet gap between spring and summer beautifully.

In this collection you will find giant alliums for the back of the border, mid-height varieties that pop up between perennials, and small, unusual species for the front of a bed, a pot or a gravel garden. All of them are planted in autumn, alongside the rest of your flower bulbs, and they come to you fresh from the grower.

Why grow alliums?

Ornamental onion bulbs are among the most rewarding bulbs you can plant in autumn. Here is why so many gardeners buy allium bulbs every year:

• A magnet for pollinators: the flowers are rich in nectar and draw in bumblebees, honeybees and butterflies at a time when little else is in bloom.
• Structure in the border: crisp spheres on tall stems add rhythm to planting schemes and look wonderful with grasses and airy perennials.
• Interest that lasts: once flowering is over, the seed heads stand on like silvery sculptures, often well into summer.
• Left alone by rodents: thanks to their oniony scent, mice and voles usually leave the bulbs in peace.
• Back year after year: most varieties are fully hardy and return reliably in a sunny, free-draining spot.

When to plant allium bulbs

The short answer: in autumn, from September to the end of November, as long as the ground is not frozen. That puts alliums in the same planting session as your tulips and daffodils, even though they flower a good deal later.

The best planting window

In soil that is still warm from summer, the bulbs quickly put down roots, ready to grow away in spring. Large bulbs such as Gladiator and Mount Everest benefit most from an early start in September or October. Smaller species like Allium sphaerocephalon can happily go in as late as November. Bulbs planted late may flower a little later or slightly smaller in their first year.

How deep to plant allium bulbs

The rule of thumb is to plant at a depth of three times the height of the bulb. Large bulbs go in roughly 15 to 20 cm deep, medium bulbs around 10 to 15 cm and small species such as caeruleum or sphaerocephalon 8 to 10 cm. Set each bulb pointed end up and firm the soil gently afterwards.

The right spot and soil

Alliums love sun and soil that drains well. Waterlogged ground in winter is the main risk, as the bulbs can rot. On heavy clay, a layer of sand or fine grit beneath each bulb makes a real difference. Light shade works too, but stems tend to stretch and lean towards the light.

When do alliums flower?

Depending on the variety, alliums flower from May into July. By combining early and late types, you can enjoy more than two months of alliums in bloom:

• Mid May: Allium Purple Sensation is one of the first to open, with deep purple heads on stems of around 70 cm.
• Late May and June: the big heads of Gladiator, Mount Everest, Ping Pong and Pinball Wizard follow, together with Allium christophii and its metallic, star-shaped florets.
• June: Allium caeruleum turns sky blue, while Allium siculum shows off elegant, nodding bells.
• July: Allium sphaerocephalon, the drumstick allium with small wine-red heads, closes the season and mingles beautifully with ornamental grasses.

Which alliums should you choose?

The range runs from towering statement plants to little alliums you can tuck between perennials. A few groups to help you decide:

Giant allium bulbs for the back of the border

Gladiator carries large lilac heads on sturdy stems and is one of the tallest alliums you can grow. Mount Everest is its white counterpart, with tight, snow-white spheres that almost glow at dusk. Ping Pong forms remarkably round white balls, while Pinball Wizard has large purple heads with a slightly looser shape. These look best in groups of three to five behind lower planting.

Mid-height alliums for every border

Allium Purple Sensation is the classic: dependable, early and an intense purple that goes with almost anything. Purple Rain flowers a little later in a warm purple, and Miami brings cheerful colour to the middle of the border. Scatter these more loosely through your planting so they look as though they have simply appeared.

Unusual and smaller species

Allium christophii stays short but produces huge, open heads packed with starry flowers, and it remains a striking ornament long after flowering. Allium siculum stands out with hanging bells in cream, green and pink. Caeruleum is one of the few truly blue summer bulbs, and sphaerocephalon keeps the garden moving in July. Perfect for the front of a border, a gravel garden or among grasses.

Ready-made mixes

Can't decide between colours or varieties? Go for a mix. The Purple-White mix pairs purple and white for a calm, elegant look, and Buzzing Beauties has been put together with bees and butterflies in mind. You will find more combinations, including other bulbs, in our bulb mixes.

Alliums in the garden: borders, pots and grasses

Alliums look their best in groups that repeat through the garden. It creates rhythm, even in a small space.

In the border

On most alliums the foliage starts to yellow while the plant is still in flower. Plant the bulbs among perennials such as catmint, hardy geraniums or lady's mantle, which hide the leaves neatly. Combine them with outdoor plants and ornamental grasses for a natural, pollinator-friendly border that stays interesting all season.

In pots and containers

Mid-height and shorter varieties such as Purple Sensation, christophii and caeruleum do well in a roomy pot. Make sure there is a drainage hole, add a layer of grit and use good-quality compost. Plant the bulbs a little closer together than in the ground, but just as deep. A sturdy container from our range of outdoor pots and baskets gives the tall stems the stability they need.

How many bulbs per square metre?

Allow 5 to 10 bulbs per m² for large varieties, 15 to 25 for medium varieties and 25 to 50 for small species. For a natural effect, avoid straight lines: toss a handful of bulbs onto the soil and plant them where they land.

Caring for alliums after flowering

Alliums need very little attention. Follow these simple tips and they will flower again year after year:

• Leave the foliage: only remove the leaves once they have turned completely yellow and dry. The bulb uses them to build up energy for next year.
• Keep the seed heads: leave the faded flower heads standing for extra summer interest. They also look lovely in dried arrangements.
• Control self-seeding: some varieties seed around. If you would rather they didn't, cut off the heads before the seeds ripen.
• Feed lightly: a little organic fertiliser or compost in spring is plenty. Too much nitrogen gives lots of leaf and floppy stems.
• Keep them dry in winter: on wet ground, lift the bulbs after a few years and replant them somewhere with better drainage.
• Safe at home and in the garden: bulbs are not for eating. Keep them out of reach of pets and young children.

Combining alliums with other bulbs

Because alliums flower late, they are the perfect follow-on from your spring bulbs. Plant them among tulip bulbs: as the tulips fade, the purple spheres take over and the gaps in the border barely show. That is why alliums are often grouped with summer-flowering bulbs, even though you plant them in autumn. For a natural garden that gets richer every year, pair small species with naturalising bulbs such as grape hyacinths and bluebells. Not sure what goes together? Our Personal Plantshopper is happy to help.

Order your allium bulbs from Green Bubble

✔ Fresh from the grower
✔ From Purple Sensation to giant alliums like Gladiator
✔ Clear planting and care tips for every variety
✔ Personal advice from our Personal Plantshopper

Plant your alliums this autumn and enjoy a border full of colour, bees and butterflies next spring and summer.