Bearded irises have one of the biggest ranges of flower hues of all plants - aptly enough, as they are named after Iris, the Greek goddess of the rainbow! Their spectacular, bold blooms in late spring offer wonderful potential for creating colour schemes in the garden. The epithet 'bearded' comes from the hairy tufts on their lower petals (called 'falls'). They derive from several Iris species, including Iris germanica (the old-fashioned purple or white 'flag iris' seen growing wild in abandoned country gardens, which starts flowering in winter in Sydney and continues on into spring), Iris pallida, Iris variegata and perhaps other southern European species. They are hardy rhizomatous types with sturdy swordlike leaves and tall stems (to 90 cm) of three to many flowers.
They are grouped into categories according to height: dwarf, intermediate and tall. It is the tall ones (growing 70 cm to 1m tall) which are best suited to Sydney (as the others need frost in winter to do well) and even so, attention must be paid to their cultivation, because as they are Mediterranean in origin, our climate is on the edge of their adaptability. They really don't like a humid summer. The best spot for them is in a sunny, dry-ish garden bed with very well-drained, slightly alkaline soil. Rhizome rot is one of the biggest worries associated with these plants. Avoid the use of animal manure nearby to the irises, as this can cause rot, as can overwatering in spring and summer. The rhizome should not be buried deeply: its top should be at ground level with just a very light covering of soil (to prevent sunburn in summer) but should not be covered with mulch, and the plants should not be shaded by nearby foliage or weeds. Plant them about 30 to 50 cm apart.
To keep the irises vigorous, dig them up and divide them every three or four years in mid- to late summer, as this is when the plant goes into a state of semi-dormancy after active growth that produces new rhizomes, following its flowering in mid-spring. Once it has flowered, a rhizome will not bloom again. Discard the old withered central rhizomes and replant the healthy ones from the edge of the clump, preferably in a different position in the garden. Trim away two-thirds of the foliage, cutting about 20 cm above the rhizome - this will reduce transpiration and help the rhizome remain stable until new roots establish. The roots can also be trimmed, leaving about 5 cm. Fertilise with a slow-release or organic plant food (or rose food) in late winter and when replanting.
Water irises well in late summer and autumn but reduce watering in winter, increasing the supply of water in spring. Remove any diseased or brown leaves and dispose of in the green waste bin, not in the compost heap. Remove the flowering stems close to the ground after blooming is over to help protect the rhizome from rot. Fungal leaf spot and rust can afflict the foliage: remove any affected leaves and dispose of in the green bin, and spray the plant with an organic fungicide. Slugs and snails can ruin the flowers; use a pet-safe bait or control them by collecting them by hand and disposing of them.
Tall bearded irises are natural companions for other Mediterranean plants, such as perennial wallflower (Erysimum), Euphorbia, lavenders, Artemisia, statice (Limonium) and Dianthus, which all enjoy the same growing conditions and are in bloom at the same time. Irises can look effective planted beneath a crab apple tree (Malus ioensis), which grow quite well in Sydney and will be in bloom at the same time. Irises are also often planted amongst roses and they can be grown in pots - in which case they should be divided every two years.
Whilst their flowers are stunning, the blooming season - usually in October - is relatively short (apart from Iris germanica mentioned above). There are some varieties (called the remontants) which rebloom in late summer or autumn, which have an added appeal. Many of these were raised in the USA in the 1920s and 1940s, and in recent times breeders have become interested in this characteristic again and have developed more of them. Some suitable ones for Sydney include 'Victoria Falls' (light blue), 'Cascade Pass' (white), 'Harvest of Memories' (clear yellow), 'Perfect Couple' (mauve, white) and 'Total Recall' (pale yellow). The flowering stems of irises can be picked for vases; some varieties are even perfumed!
There are a number of iris catalogues online, which can whet your appetite for adding a few new ones to your garden.