The many varieties of alliums can give your garden a splash of colour

Alliums create a real wow factor at RHS Garden Harlow Carr, helping to frame the much-photographed vista of the Main Borders which run from the top to the bottom of the garden. These giant ornamental onions provide amazing architectural structure and colour right throughout the year, from the initial flush of their beautiful pom-pom heads blowing in the early summer breeze to the structural elegance of the seed heads.

More than 8,000 alliums are planted within the Main Borders and are growing fast due to the longer daylight hours and mild winter we have experienced this year. If you look closely you can already see the flower heads beginning to open, hot on the heels of the tulips and narcissus which put on an extra-early show this spring. Alliums are a great addition to any herbaceous border, providing early colour amongst different textures of green foliage. It’s a good idea to plant these striking bulbs amongst perennials as they can help to disguise the alliums’ foliage which can begin to look messy and untidy later in the season.

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There are many varieties of allium available to provide a splash of colour, flower shapes and different heights to a border: the taller varieties like ‘Mont Blanc’ can reach up to two metres in height and are great for the middle or back of a border; shorter ones like A. hollandicum ‘Purple Sensation’ are ideal planted towards the front of a display. Colours range from white through various shades of purples to very dark varieties like Allium sphaerocephalon; more unusual offerings are available, such as Allium Schubertii which is particularly dramatic and eye- catching, and looks almost like a firework.

Once flowered, alliums begin to fade into the background as perennials and grasses come to the fore. The seed heads are highly decorative and can be picked and collected to dry for indoor arrangements in the winter or left insitu to add sculptural interest to your planting schemes even after flowering. You may discover little seedlings of alliums if they are left through the season in your border. They are not invasive and look like onion seedlings as they are in the same family – Alliaceae – although they are purely ornamental. They will take a few years to mature to reach flowering, but be patient if you wish to grow them on, either leaving them in your border or potting them on. In time, this will enhance your display through natural propagation, meaning you get new plants for free!

Alliums work well with so many herbaceous perennials such as salvias, eryngiums and geums. One of my favourite combinations on the Main Borders at Harlow Carr is salvia nemorosa ‘Caradonna’ mixed with eryngium ‘Jos Eijking’ and Allium nigrum. This collection provides great textures and colours, as well as a rich source of nectar for bees and insects.

It is best to buy the bulbs of alliums and plant them in the autumn as you would with tulip and narcissus bulbs, planting them at a depth three times the size of the bulb. Alliums are very versatile

and tolerant of many soil types and aspects, liking full sun to partial shade. If, however, you have clay or wet soil and are concerned about them rotting, try planting your bulbs on a bed of sand to give them the best possible chance. Good luck!

Jobs for the week

* Begin hardening off summer plants to be planted early June but keep the fleece handy!

* Empty containers of spring bulbs and allow them to die back if you wish to replant them in the autumn.

* Earth up potatoes as shoots push through

* Keep any new plantings well watered.

With thanks to Tom White, Horticulturist at RHS Garden Harlow Carr


23 & 30 May at 11am: Harlow Carr Garden Tours Find out more about the garden on a tour with the Friends of Harlow Carr every Tuesday throughout May. A donation of £2 per person is suggested.

Until 30 June: The ‘Language of Flowers’ exhibition The Victorians believed that every flower - and some vegetables - carried a symbolic meaning: a hellebore for scandal, a rose for love. Learn about the symbolic meaning of flowers at an exhibition taking place at Harlow Carr’s library. Normal garden admission applies.

1 – 30 June: Outdoor Exhibition - Gardeners’ World at 50 RHS Garden Harlow Carr hosts an outdoor exhibition throughout June charting fifty years since Gardeners’ World first broadcast. See displays from the BBC archives and the RHS Library reflecting how gardening has changed over the years and how Gardeners’ World - now Britain’s longest running gardening show - responded to, and sometimes led, the changes. Normal garden admission applies.