Alliums are versatile spring bulbs that are easy to grow, loved by bees and other pollinators, and make great cut flowers. The rounded flower heads complement various garden styles and add architectural interest to beds and pots.
I adore alliums. Their ball-shaped flowers add a playful look to any border, particularly when they bob above a sea of mounding perennials like geraniums or nepeta. They vary in size, from short varieties like Allium 'Millenium' up to the two-metre-tall Allium giganteum. Most bloom in shades of pink, purple and white, but there are more unusual varieties, such as the striking blue Allium caeruleum.
Most alliums flower in May and June, bridging the gap between early spring bulbs and summer perennials. A few varieties, including Allium sphaerocephalon, flower in July and August. The spent flower heads are also attractive, whether you leave them on the plant, use them in a dried flower arrangement or spray them with paint for unique Christmas decorations.
You can buy allium bulbs from July to plant in the autumn or potted plants of smaller varieties in spring and summer.
Where to plant alliums
Plant your allium bulbs in well-drained soil in a sunny bed or border. A sheltered spot is ideal, particularly if you want to grow tall varieties such as Allium giganteum or Allium 'Gladiator', as a strong wind could blow them over. You can plant shorter varieties near the front of borders, but taller alliums look best scattered throughout the middle and back of borders where they can rise above lower-growing plants. The more you plant, the bigger the impact.
Alliums aren't commonly naturalised in grass, mainly because they bloom later than other spring bulbs and therefore need the grass to be left unmown for a good part of the year. If you have an area you can leave unmown, they're a lovely addition to a wild meadow. Species alliums (e.g., Allium hollandicum, Allium nigrum) are more likely to spread than named cultivars, many of which are sterile.
While allium flowers are stunning, the foliage is more lacklustre, particularly when it yellows, which can happen just as the plant starts flowering. To disguise this, plant your alliums with herbaceous perennials or shrubs that will cover the allium leaves. Alliums will happily flower for several years. Since they don’t like being dug up, it’s best to plant them in a bed where they won’t be disturbed.
Alliums look wonderful in a cottage-style border or paired with golden or bronze ornamental grasses in a prairie scheme. You can also plant alliums in pots, either on their own or with other spring bulbs. Consider the size of the flower head and the height of the stalk when deciding which variety to plant and what to plant it with.
When to plant allium bulbs
September and October are the best months to plant alliums. If you miss this window, you can still plant them in November or even December, but the alliums may flower a little later. If your bulbs have been in storage a while, check that they're firm and show no signs of mould before planting.
How to plant allium bulbs
Allium bulbs vary in size and are often short and squat compared to tulip or narcissus bulbs. Some giant allium bulbs can be the size of tennis balls! Dig a hole four times the depth of the diameter of the bulb. You don't need to add compost or manure when planting, but if you're planting in clay soil, it's worth adding some grit to the bottom of the planting hole to help prevent bulbs from rotting.
Like all bulbs, you need to plant alliums with the tip of the bulb pointing up. Allium bulbs don't always have an obvious point, but you can usually see a bulge on the bottom of the bulb where the roots emerge. Plant your bulb with the bulge at the base of the hole and fill in the hole with soil. Space bulbs according to their size: you can plant small alliums around 10 cm apart, large alliums 20–25 cm apart, and giant alliums 30–40 cm apart.
When planting alliums in pots, make sure your container is deep enough for the size of allium you're planting. You can either use a specific potting compost or mix general peat-free compost with horticultural grit. Add a thick layer of compost to the bottom of the pot and plant your allium bulbs at the same depth you would in the ground.
How to care for alliums
Alliums planted in the ground usually survive fine with just rainwater; however, if the spring is very dry, you may need to water them a few times in April or May. Avoid watering later in the summer, as this can cause bulbs to rot. Plants in pots will need more regular watering.
Allium stems are sturdy, and most varieties don't need staking. The exception might be if you're growing giant alliums in a windy spot, as these will be more vulnerable to wind damage. Alliums are hardy and don't require winter protection, other than making sure that bulbs in pots don't get waterlogged.
I prefer to leave allium flowers to develop into seedheads, but if you want to avoid self-seeding or you want to let other plants shine, you can cut spent flower stalks at the base to remove them. It's important to leave the leaves to die back naturally, as this helps the plant build up energy reserves in its bulb for the following year.
How to propagate alliums
Alliums naturally spread by producing bulb offsets and by self-seeding. The easiest way to propagate alliums is by separating allium bulblets from the parent plant and replanting them. It's worth doing this every three or four years, so clumps of alliums don't get too congested.
Wait until the plant has finished flowering and the foliage has died back before carefully lifting the bulb. You'll see a number of small bulblets attached to the main bulb. Remove these offsets and replant them back into the soil.
Some alliums, including Allium Sphaerocephalon, produce tiny bulbs in their flowerheads. These bulbils are trickier to remove, but you can separate them and plant them as you would a bulb offset. As they're much smaller than offsets taken from the bulb, they will take longer to mature.
Growing alliums from seed
Growing alliums from seed is straightforward, but it takes several years for allium seedlings to grow large enough to flower. The plants will also be hybrid varieties, so may not look exactly like the allium you took the seeds from. Collect ripe seeds and sow them in trays of moist, gritty compost. Top with a layer of grit and leave them in the shade outside. Allium seeds take around 12 weeks to germinate.
Common problems
The best way to keep your ornamental alliums healthy is to avoid planting them near edible onions and garlic. This helps prevent onion fly, allium leaf miner, onion downy mildew and onion white rot from spreading. These problems are more common with edible alliums, which is why it makes more sense to separate the two.
Onion fly and white rot both affect the bulbs of the plants. An early symptom is yellowing leaves, though it's important to bear in mind that allium leaves can naturally turn yellow before the plant flowers. If you suspect an issue, dig up a bulb. If you spot onion fly larvae or a white, furry growth, dig up and dispose of bulbs, and avoid planting alliums in the same location.
To help prevent downy mildew, make sure there's good air circulation around the plants and don't overwater them. Remove any affected leaves.
Allium leaf miners gnaw holes in plants, which then rot from fungus and bacteria entering through the wounds. Leeks are most commonly affected.
Our favourite allium varieties
Allium 'Purple Sensation' is a popular mid-height allium with vibrant purple flowers. It doesn't have the largest flowers, but the densely packed, perfectly spherical flower heads make this allium particularly versatile.
Bigger may not always be better, but Allium 'Globemaster' seeks to prove otherwise. This giant allium adds a wow factor to any border with large violet flower heads the size of footballs!
Allium cristophii, also known as the star of Persia, has a very unique appearance. Its star-shaped flowers have very narrow petals that give the enormous flower head a delicate, lacelike appearance. The purple flowers have a silver shimmer, and the overall effect is rather like a giant floral sparkler.
Allium sphaerocephalon has egg-shaped flower heads that start off green before maturing to a rich claret, giving a lovely two-toned appearance. This drumstick allium flowers later than other varieties, making it a good choice for a mid or late summer border.
FAQ
When do alliums flower in the UK?
Most alliums flower in late spring and early summer, usually in May and June. Allium 'Purple Sensation' is one of the first alliums to bloom, followed by larger varieties such as Allium 'Globemaster' and Allium giganteum. Allium atropurpureum flowers a little later, in June and July, and the late-blooming Allium sphaerocephalon flowers throughout July and August. Plant different varieties of allium bulbs to enjoy flowers from late spring all the way through the summer.
Are alliums perennials?
Yes, alliums are perennial plants that die back after flowering and shoot up again the following year. Individual plants are relatively short-lived, lasting around four years, but you can lift and divide older plants to give you a continual supply of alliums for your beds.
Do allium bulbs multiply?
Alliums produce "bulblets" – small offsets attached to the parent bulb. Over time, these grow into large clumps that can cause groups of alliums to become congested. It's worth lifting allium bulbs every three or four years and removing these offsets, which you can then plant in a more appropriate location.
What should you do with alliums after flowering?
You can either remove allium flower heads or leave them on the plant. I prefer to leave them, as the seedheads add structural interest throughout the summer and autumn, but if you want to prevent your alliums from self-seeding, remove the flowers by snipping the stalks at the base. Allium cristophii and Allium 'Globemaster' have particularly attractive seedheads.
