Plant profile: Siberian iris

Plant profile: Siberian iris More refined than their bearded cousins, Troy Scott Smith selects his favourites for a brief but glorious flowering

Parham House & Gardens Most of the images for this month’s plant profile were taken at Parham House & Gardens in West Sussex, which is renowned for its colourful herbaceous borders. parhaminsussex.co.uk

Ihave known Siberian irises ever since I was a child in Yorkshire and we moved to a house in the countryside. It was May and the garden was surrounded by fields bordered with ancient ash pollards and woolly hedges of hawthorn frothing in the breeze with great billowing clouds of f lowers. The garden was full of cottage garden f lowers, and standing with their heads and shoulders above the crowd were several large groups of an unidentified form of steely blue-f lowered Iris sibirica. It was a beautiful sight as the sunlight caught f lashes of gold in the flowers’ throats above a darker undercurrent of Geranium phaeum teamed with just the right amount of discord from the acid-yellow flowers of Alchemilla mollis. The overriding impression on that day, 38 years ago, was one of great beauty, and since then I have experimented with growing many different forms of this most elegant and easy-going iris.

I love the verticality of their simple stems, the structured lines they provide in the fecundity of the spring border, and the fact that despite bulking up quickly they tend to stay where they are put. Flowering is brief but glorious, but they are grown as much for their narrow, grass-like, architectural foliage as for their flowers.

Siberian irises have been grown in British gardens since the late 16th century, and have naturalised here and in other temperate regions. Most garden selections are from I. sibirica and I. sanguinea and their hybrids. I. sibirica is found mainly in central Europe, although it is now vulnerable in Czechia, Hungary and Ukraine; extinct in Slovakia, and rare and endangered in Poland. I. sanguinea is found from Russia to Japan and Korea. But it is botany more than geography that links these two closely related species. Both have many similarities but some differences. I. sibirica has branched flowering stems with five to seven flowers per stem, held clear of the leaves, while I. sanguinea has unbranched stems with one or occasionally two flowers per stem; hybrids exhibit traits from both.

Although natural breeding and selection have inf luenced the Siberian iris, the real changes to f lower size and habit occurred when the American breeder Currier McEwen created tetraploid plants in the 1960s. This technique, first used on hemerocallis, doubles the chromosome count, creating more robust plants with stronger stems and larger, sturdier f lowers. It also enriched and intensified the colour range of f lowers available, as with I. ‘Lavendelwein’ and ‘Silver Edge’. Pre-tetraploid forms, such as the classic I. ‘White Swirl’, typically have narrower foliage and thinner stems holding several delicate f lowers that constantly move and flutter. While I was head gardener at Sissinghurst, I took advantage of this quality in the White Garden where I grew the pure, unadulterated ‘White Swirl’ by the bucket-load in tandem with the foaming flowers of Anthriscus sylvestris. Illuminated by the fullness of a May moon, the effect was magical.

All Siberian iris are easy-going and perform well in most average garden soils, but for best results provide a rich soil containing liberal amounts of organic material, with a mildly acidic pH of between 6 and 7, but they will often succeed elsewhere. They prefer a sunny situation but will tolerate partial shade if they get a few hours of full sun each day. Like most irises, Siberians are hungry plants and so a balanced fertiliser, applied at a ratio of 35 grams for every square metre, should be applied in March and then again after flowering will help.

Thinking about successional planting, I like to add narcissus bulbs in and around clumps of Siberian irises. Light-limbed types, such as N. pseudonarcissus, or the wild-looking ones, such as N. ‘Actaea’ (a beautiful, red-eyed narcissus, similar to pheasant’s eye) work best, as their foliage is discrete and quickly hidden by the expanding leaves of the irises. You could also try adding something that flowers earlier, such as snowdrops; again, their fading foliage will be conveniently hidden first by the narcissi and then the irises. For later interest, and contrast of leaf form, try planting the fern Polystichum setiferum nearby. n

• Troy Scott Smith is head gardener at Iford Manor in Wiltshire. His recommendations for the best Siberian irises can be found over the next five pages.