Quick Guide: summer, hardy, full sun, height: 45cm/18in, width: 23cm/9in

The iris family includes a great number of species and hybrids, with very beautiful flowers of blue or yellow in a distinctive arrangement of petals but varying greatly in size and use. Most irises grow from rhizomes, a few from true bulbs. Several are excellent rock garden plants; those described here are ideal for waterside planting or moist conditions.

GROWING Plant rhizomes in spring or late summer at the water's edge or in water up to 45cm/18in deep, depending on species. The clumps should be lifted every 3 years for division.

PROPAGATION Just after flowering, lift the rhizomes and divide by cutting off pieces from the outside of the clump, discarding the centre. Each piece should have one or two good fans of leaves. Replant immediately.

SPECIES I. laevigata (above), deep blue flowers up to 15cm/6in across, water depth 15cm/6in; named varieties include `Alba', pure white and `Monstrosa', white blotched purple. I. pseudocorus, reaches up to 1.2m/4ft in 45cm/18in of water, half that height in a damp border; vivid yellow blooms; named varieties need only 15cm/6in water. I. versicolor, violet flowers, ideal lakeside plant in 7.5cm/3in water; `Kermesina' is a wine-red variety.

POSSIBLE PROBLEMS Cucumber mosaic virus; leaf spot; iris sawfly.