Plant Guide: Iris

There are many species of bulbous iris that, unlike their larger relatives with creeping rhizomes, have true bulbs as storage organs for reserve food. The leaves are not arranged in a fan shape as is typical of the sword-shaped foliage of rhizomatous irises, but are usually narrow and lance-shaped or rolled. The flowers have the characteristic iris form with six petals, three inner and three outer. The outer petals are often distinctly spotted or veined. A large number of species and varieties are blue with yellow markings on the outer petals. Some forms are yellow with brown or green spots on the outer petals, while others are white with yellow markings. The blue forms range from deep purple to delicate porcelain blue.

Many bulbous irises are early-flowering dwarf forms suited to rock gardens or the front of borders; they are also excellent plants for shallow pans in a greenhouse or conservatory. Other bulbous irises are larger and flower in summer; they are ideal for mixed borders and make good cut flowers. There are also a few spring-flowering species that are planted far less often than the other groups.

POPULAR TYPES

Bulbous irises can be divided into three main groups: Reticulata irises, Xiphium irises and Juno irises.

Reticulata These irises have bulbs with a net-like tunic, which gives the group its name. They are dwarf forms reaching about 15 cm tall, with flowers appearing early in the year, usually in February and March. I. danfordiae has faintly scented flowers with brownish-green-spotted, yellow petals. I. reticulata also has scented flowers; its petals are narrower than those of I. danfordiae and are blue or purple with yellow markings. Several cultivars are available. The flowers of I. histrioides and its cultivars are larger with shorter stems; they are dark to light blue with dark blue, yellow and white markings. The flowers open before the leaves are fully grown.

Xiphium This group of summer-flowering irises is popular and easy to grow. It consists of Dutch irises, which flower in early summer with white, yellow or blue flowers with contrasting markings and streaks; English irises, which flower from early to mid-summer with white, purple or blue flowers; and Spanish irises, which flower in mid-summer with flowers in various shades of white, blue, purple and yellow.

Juno Juno irises are less well known than the other bulbous irises, perhaps because they are harder to grow. This group includes I. bucharica, with yellow or white flowers with yellow outer petals, and I. graeberiana, which has lavender-blue flowers with a white ridge on the outer petals. These two are among the easiest Juno irises to grow; other species include I. fosteriana, I. magnifica and I. rosenbachiana, which do particularly well in a cold greenhouse.

GROWING CONDITIONS

Position All bulbous irises like an open, sunny spot.

Location Reticulata irises are suited to rock gardens, raised beds or containers; Xiphium and Juno irises prefer sheltered borders. The soil must be well-draining; Juno irises need soil rich in organic matter.

GROWING

Planting Plant Reticulata irises 7.5 cm deep and 10 cm apart. Xiphiums are planted 10–15 cm deep and 15 cm apart, and Juno irises 5 cm deep and 20 cm apart. Take care not to damage the brittle, fleshy roots. All bulbous irises should be planted in September or October.

Feeding Extra feeding is usually not needed.

Problems In soil that is too wet, the bulbs can rot. In a fairly warm climate, bulbous irises can suffer from "ink disease": black streaks on the bulbs and yellow spots on the leaves. Destroy affected bulbs.

FLOWERING

Season Reticulata irises flower in February and March, Junos in April and May, and Xiphiums in June and July.

Cutting Xiphiums are excellent, sturdy cut flowers that stay fresh for a very long time.

AFTER FLOWERING

Care Remove spent flowers. Most bulbous irises are best left undisturbed for as long as possible; propagation is possible by lifting the bulbs after flowering and dividing them if necessary. Only lift Juno irises once the foliage has died back, and handle the bulbs very carefully. Xiphium irises are best lifted once the foliage has died back and then replanted in September; this allows the bulbs to ripen better.

AT A GLANCE

A varied group of plants with colourful flowers in spring or summer. Suited to many different uses.

Recommended varieties:

• Reticulata group: I. danfordiae, I. reticulata 'Katherine Hodgkin', I. histrioides 'Major'
• Xiphium group: 'Bronze Queen', 'Excelsior', 'Ideal'
• Juno group: I. bucharica, I. graeberiana