There are some 300 species in the genus Iris. The most familiar irises are the tall bearded irises (Iris germanica).
The distinctive flowers have three large outer petals called "falls" and three inner upright petals called "standards." The falls may have beards or crests. Bearded iris are so-called because they have soft hairs along the center of the falls. In crested iris, the hairs form a comb or ridge.
How to Care
Plant iris in mid- to late summer. They prefer fertile, neutral to slightly acidic soil. If your soil is very acidic, sweeten it with a bit of lime, and limit summer watering, which can lead to rot. Irises need at least half a day of sun and well-drained soil. Without enough sun, they won't bloom.
Dig a shallow hole 10 inches in diameter and 4 inches deep. Make a ridge of soil down the middle and place the rhizome (fleshy roots) on the ridge, spreading roots down both sides. Fill the hole with soil and firm it gently.
When planting, top-dress with a low-nitrogen fertiliser, and again in early spring. After planting, water thoroughly.
Feeding and Watering
Avoid applying high-nitrogen fertilisers to the surface or carelessly mulching with organic matter, which may encourage rhizome rot (root rot).
Keep rhizomes (fleshy roots) exposed. Unlike bulbs, which thrive deep underground, iris rhizomes need a bit of sun and air to dry them out. If they're covered with soil or crowded by other plants, they'll rot. Irises may benefit from shallow mulching in the spring.
Don't trim iris leaves. Leaves carry on photosynthesis for next year's growth. Cut off brown tips and cut the flowering stalk down to the root to discourage rot.
After 2 to 5 years, when clumps become congested or lose vitality, divide and replant sound rhizomes (fleshy roots) in fresh soil. The best time to replant irises is soon after bloom. Transplant them to places where they will have "wet feet but dry knees."
Pests and Diseases
Slugs and snails, thrips and aphids are troublesome to iris.