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Gardening: Dividing irises

The bearded iris is an early summer treat whose rhizomes should be lifted and divided now if you want more for free.

Irises have been cultivated in gardens for millennia and were named by the Greeks after the goddess who personified the rainbow. Pharaohs and ancient Cretan kings even used them as regal symbols. Perhaps the flowers owe their spread west to the early followers of Mohammed. During their speedy conquest of the coast of north Africa and southern Spain, they carried irises to plant by their graves.

Thanks to a massive breeding programme a little over a century ago, there is now an almost limitless range of flower colours - far more than the original blues, yellows and whites. It's thus impossible to positively identify which of today's specimens can be traced back to ancient times.

Bearded irises are divided into six groups: miniature dwarf bearded; standard dwarf bearded; intermediate bearded; border bearded; miniature tall bearded; and tall bearded. Their flowers all comprise three upright petals, known as standards, and three flat or pendant "falls". The fine hairs on standards are sometimes the same colour as the falls. The rhizomes of all these variations should be treated in the same way.

The low-growing dwarf bearded hybrids grow to 30-45cm and are well suited for the front of a border. They also fit easily into random corners, but be sure to plant them in groups of three or five for best effect. An excellent cultivar is Iris dwarf bearded Tinkerbell with its clear, pale blue flowers, and I can't resist the subtle, deep velvety-red flowers of Iris dwarf bearded Cherry Garden.

As a general rule, dwarf bearded cultivars flower earlier than their taller cousins, whose flower spikes can reach 75cm. Among my favourites is the slightly scented Iris germanica Deep Black, which has striking rich, indigo flowers. And the clean, lemon flowers of Iris germanica St Crispin take some beating. A third iris to look out for is Dusky Challenger, a sturdy, vigorous plant with huge, ruffled purple-blue blooms, and up to 13 buds to a stem.

Bearded irises need poorish, very well-drained, alkaline soil. Critically, the rhizomes must poke above the ground in a south-facing bed so they can soak up maximum sun, which is essential for good flowering. Overcrowding will also affect flowering.

To maintain a good show and get some new rhizomes for free, lift clumps of bearded iris every three years. As irises only grow roots for a short period after flowering, the rhizomes need to be lifted and replanted this month to develop a good root system. If you delay into August or September, the plants won't establish a proper anchor and may fall over the following year.

After lifting, separate the rhizomes. Discard the older rhizomes from the centre of the clump and prepare the rest for replanting immediately. Remove the bifurcate or side shoots from the main stems, and cut leaf growth to the shape of an inverted V, 5-6cm above the rhizomes. Plant 30cm apart with the tops of the rhizomes at, or slightly above, soil level. You can apply a blood, fish and bone dressing before planting, but this is not essential.

Although iris plantings look superb for a brief spell, they leave bare spaces for the rest of the year. But there are ways to get round this. Plant small early spring bulbs, such as scylla, chionodoxa, Iris reticulata and Anemone blanda. The foliage will have largely died back before the irises start into growth. At other times, arrange plantings to draw the eye away from the dull rhizomes. Plant taller, upright specimens such as hardy salvias, Verbena bonariensis or varieties of Sedum telephium. These also tolerate dry, sunny conditions. Be sure to keep these plants well staked to prevent them from flopping over the irises.

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