Our garden editor's pick of the best allium bulbs for summer

Alliums are the heroes of early summer, standing out in perfect spherical splendour against the froth of a late May or June border. These sun-loving ornamental onions are rewarding plants that push up their flowers confidently from bulbs planted in autumn, many returning reliably year after year if they are given the right conditions.

Most people are familiar with the architectural outlines of these globe-like flowers, which are often the stars of the RHS Chelsea Flower Show. But the family also includes a host of lesser-known species that are equally easy to grow and which flower at different times from spring to late summer. Many of these are clump-forming alliums, also known as herbaceous alliums, which are more petite than the early-summer globes.

However, let us start with those familiar favourites such as ‘Purple Sensation’. A cultivar of A. hollandicum, this is an amazing plant growing 60-80cm tall, with tightly clustered, impossibly round flower heads about 10cm across in vibrant violet-purple. For some it is just too purple, especially in a naturalistic scheme; in that case, an alternative is A. hollandicum, which is a degree or two less colour saturated. This species will naturalise if it is happy in its situation and it will helpfully tolerate a little dappled shade.

A.‘Globemaster’ is similarly uniform in shape, but with showy spheres up to 20cm across on stems 80cm tall, while A. stipitatum‘Mars’ is taller at 1.2 metres with slightly smaller flowers at 15cm across. A. stipitatum ‘Mount Everest’ is similar in height and one of an increasing number of white forms, with beautiful greenish-white flowers 12-15cm across. This and other white alliums, such as A. amplectens ‘Graceful Beauty’, can look very effective mixed with purple alliums, or on their own against a dark yew hedge, perhaps with an astrantia such as ‘Shaggy’.

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All these tall, globe-like alliums have strappy leaves that are already starting to die back when the flowers come into their own, so it makes sense to plant
them among mid-height perennials, such as nepeta, geraniums, astrantias and alchemilla, which will mask the fading leaves.

Now for what I call the artistic alliums – slightly mad statement pieces that are required only in small quantities for impact. Allium cristophii has enormous, airy flower heads about 20cm across. Its clusters of starry flowers have a metallic sheen and a structural shape, which remains interesting even when the flowers fade. It grows up to 50cm tall, which seems slightly out of proportion to the size of the flower heads. A. schubertii is even more outlandish, with a central cluster surrounded by purple-green filaments that can eventually produce a flower head of about 30cm across. Allium siculum has intriguing flowers on tall stems up to 1.2 metres in height, forming loose, nodding umbels with individual blooms striped cream and maroon.

The flowers emerge gradually, creating unbalanced and interesting shapes, and they are real eye-catchers. Some of the smaller, more delicate alliums can be more suited to naturalistic planting schemes. A. sphaerocephalon has more irregularly shaped flowers than the perfect, well-defined spheres of others, growing to about 60cm. Flowering slightly later in July, its egg-shaped flowers are green and purple when they first emerge, becoming purple all over as they mature. I am growing this drumstick allium in drifts in my gravel garden, set off by the soft grass Stipa tenuissima.

Whereas A. sphaerocephalon grows singly from a bulb like the others mentioned above, the clump-forming alliums have mounds of narrow, chive-like leaves from which multiple flower heads rise. They grow from rhizomes rather than true bulbs and are sometimes offered for sale in pots in spring and summer, or as clumps ‘in the green’ with their leaves still intact in early autumn.

A. tuberosum, the Chinese chive, flowers in late summer and into autumn, with small greeny-white flowers bobbing about on narrow stems about 50cm tall. A. senescens subsp. glaucum is only 25-30cm tall with tiny, pale mauve flowers, while A. angulosum, the mouse garlic, is a little taller at 40cm. Others – such as the mauve-pink A. cernuum (50cm), the nodding onion, and the white and pale pink A. hyalinum (25cm), the glassy onion – have looser, more relaxed flower heads. Both flower in July or August. You can even expand the colour spectrum with blue A. cyaneum and yellow A. moly. Grow these herbaceous types in a gravel garden or in containers, where they will charm you with their pretty clusters of flowers.