Stunning displays of alliums mark the start of summer at RHS Harlow Carr – these beautiful ornamental onions are easy to grow at home
Come to RHS Garden Harlow Carr in early summer and be greeted by the glorious sight of thousands of alliums in flower – around 8,000 give or take a few – standing tall in the garden's Main Borders.
The exquisite Main Borders, first laid out by the Northern Horticultural Society in the late 1940s, feature spectacular drifts of herbaceous perennials, prairie-style grasses, bulbs and shrubs. Alliums are the early summer stars of the show, repeated throughout the beds to create a white and purple haze of pom-poms that float above the borders.
There are more than six different allium species at Harlow Carr. First to flower are the deep purple globes of Allium hollandicum ‘Purple Sensation’, quickly followed by the white half pom-poms of A. nigrum and giant white heads of A. stipitatum ‘Mount Everest’ – which can reach dizzying heights of 1.7m (5.5ft). Both Allium cristophii and A. schubertii are magnificent varieties with starburst-effect flowerheads up to 30cm across. The smaller flowering Allium sphaerocephalon, with blackcurrant-coloured drumsticks, and maroon flowering A. atropurpureum, punctuate the borders with height and colour.
Curator Paul Cook said: ‘We strongly suspect we now have our own Harlow Carr hybrids, based on the subtle colour variations of alliums that pop-up near the longer-established plantings. These have self-seeded and cross-pollinated without any interference from us.’
Most alliums are easy to grow in fertile, well-drained soil, and with the addition of grit to improve drainage, can be grown in heavier clay soils. In full bloom, alliums are a magnet for bees and butterflies, and – depending on the weather – will last for several weeks from bud to seedhead. Their leaves, which can look untidy, are easily hidden by herbaceous perennials, the perfect companions to grow alongside alliums.
After flowering, the alliums' structural seedheads continue to delight and provide a haven for wildlife well into winter if left intact. Alternatively, the seedheads can be harvested and used as decorations – they look particularly striking on Christmas trees.
Paul advises: ‘If you are planning to leave the seedheads intact, alliums can smother other perennials once they germinate – usually a few years after being planted – so it may be worth deadheading some, which will help direct the energy back into bulb’.
Curator's choice: Fabulous alliums for home gardens
One of the tallest alliums, reaching 1.2m in height, with large, pale purple flowerheads up to 15cm across, on sturdy, upright stems.
Smaller, deep violet flower domes (up to 8cm diameter) that contrast beautifully with lighter coloured alliums.
Semi-spherical flowerheads comprised of large, open creamy-white flowers, with a distinctive greenish-black ovary. Up to 75cm in height.
Impressive, dense clusters (up to 15cm across) of star-shaped lilac flowers, on stems reaching 80cm in height.