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Culture Notes

Details on the Bearded Iris and their care.

The Bearded iris is a hardy, long-lived perennial that requires a minimum of maintenance. The flowers have six petals; three upright petals (called standards) and three hanging petals (called falls). A fuzzy line or beard runs down the middle of each fall. Flowers come in many colours including blue, pink, purple, reddish, white, yellow, and bi-colours. Most bearded iris flower in the spring (October to December depending on cultivar and region grown)

Bearded irises are classified into several types: miniature dwarf (less than 20cm high), standard dwarf bearded iris (20-30cm high), Intermediate Bearded Iris (30-40cm high) (a cross between a dwarf and tall), border (30-40cm high) (a genetically smaller tall iris) and TALL BEARDED IRIS (flower mid November, 80 cm+ high)

The shorter iris flower first, followed by the intermediate, and then the taller irises.

Some of the new cultivars re-flower in the summer and autumn. The second flower display is not as showy as the spring display but last into autumn. Many re-blooming iris are fragrant.

Cultivars

There are thousands of bearded iris cultivars to choose from, with new introductions each year.

Growth Habit

Iris have thick, fleshy, underground stems (called rhizomes ) that store food produced by the sword-shaped, semi-evergreen leaves. The rhizomes grow best when planted at or slightly below the soil surface with feeder roots penetrating the soil below. Each year underground offsets develop from the original rhizome. Buds produce a large fan of leaves and several flower stalks. Success with iris depends on keeping the rhizomes firm and healthy. In general, this is done by providing the rhizome good drainage while the feeder roots below remain moist but not wet.

Site Selection and Preparation

A full sun exposure is preferred; however, some of the delicate pink and blue iris hold their colour better in partial shade. Excessive shade will reduce or prevent flowering. Good soil drainage is essential to prevent rhizomes from rotting. It may be necessary to plant the rhizomes in raised beds (at least 15 cm high) to obtain proper drainage.

Iris will grow in many soil types but a light, loamy soil with a pH of 6 to 7 that has been amended with organic matter is preferred. A tight clay soil may keep the rhizome too wet and should have organic matter (compost) incorporated to improve drainage. Manure is not usually recommended for iris but can be used if well-rotted and incorporated at least 15 cm deep into the bed (should not come in contact with rhizomes).

Fertilisation of iris is important to obtain best results, but must be done in moderation. Nitrogen, potash, and phosphorus are essential for iris, but excessive nitrogen promotes lush growth that is more susceptible to rot diseases. At planting, incorporate a low-nitrogen fertilizer such as 5-10-10 . Taking and following the results of a soil test is the preferred method to determine fertiliser amounts.

Planting

The best time to plant bearded iris is late January through March. This will allow them to become well established before winter. Container-grown iris can be planted in the spring. In a well-prepared bed, dig a shallow hole large enough to accommodate the rhizome or clump of rhizomes. Form a mound of soil in the centre for the planting base. Make the mound high enough so the top of the rhizome is slightly above soil level. Spread the roots around the mound, fill with soil, and water. Keep the newly planted rhizome watered for the first month until new roots establish. For a mass of colour, plant at least three rhizomes (spaced 20 to 25 cm apart) or plant undivided clumps; point each fan of leaves away from the centre of the group. Clumps should be spaced 45 cm apart. Mulch can be applied to autumn-planted iris to reduce heaving during a heavy winter in colder regions.

Care and Maintenance

Before flowering, water plants often enough to keep the soil moist but not wet. Reblooming iris should be watered during the summer, while spring-flowering iris will tolerate drought. After flowers fade, bloom stalks should be cut off close to the ground after blooming, or grab the old bloom stalk and bend it sharply towards the ground, snapping the stalk off close to the soil line. Plants that are growing well (good green foliage) may not need fertilising. In Central Otago , autumn application with super phosphate and a small amount of blood and bone, and in early spring with a low nitrogen fertiliser such as ½ cup of 5-10-10 fertiliser per iris clump Fertiliser can burn the rhizomes; it should be applied around but not directly on them.

Reblooming iris should be fertilised in the spring as new growth begins and after spring flowering ends. Iris respond to shallow (2 to 5 cm) cultivation and should not be mulched.

In late autumn, as Iris go into winter dormancy, their leaves naturally die back and turn brown. To help your Iris get ready for winter, remove limp outside foliage (not green healthy leaves), and cut off and remove brown or any diseased leaves. Also cut off old bloom stalks that may remain blooming. After the first hard freeze, cut remaining leaves 6 to 8 cm from the ground.

Keep the iris weed free. Weeds can also be prevented by using pre-emergence herbicide before weeds begin growing in spring. Remove leaves that have died back in autumn to prevent excess moisture and disease lying around.

Aphids can be a problem with Iris. Use a rose spray if necessary to control aphid and any fungal disease. An organic method of dealing with these sucking pests is using insecticidal soap (1/2 cup of dishwashing liquid mixed with a gallon of water and sprayed onto the plants). With heavy aphid infestation, you may need to spray once with insecticidal soap, wait a few days, and spray a second time after new aphid babies have hatched.

After 3 to 5 years, iris generally become crowded and should be divided. Iris can be divided any time, but many growers prefer to divide 4 to 6weeks after the flowering period. Cut the leaves to one-third their length. Dig the clump and wash soil off with a hose. Cut rhizomes apart so that each section has at least one healthy fan of leaves and firm, white roots. Older rhizomes may seem firm but should be discarded since they have limited flowering capacity.

Problems that can occur with Bearded Iris

Bearded Iris is very hardy against disease, especially in the colder and drier areas of New Zealand. Problems can occur:

Poor flowering -- is normally due to planting in excessive shade, using excessive nitrogen fertilizer, or planting the rhizomes too deep. Limited flowering may also occur if plants become too crowded and need dividing.

Bacterial soft rot -- is the most serious iris disease. Bacteria enter through injuries or cuts to the rhizome. Soft rot causes the rhizomes to become mushy and have a disagreeable odour. Use of fresh manure or excess nitrogen, coupled with poor drainage, contribute to soft rot development. Dig up and destroy diseased rhizomes. If the rot is not extensive, cut off and destroy diseased plant parts.

Crown rot fungus -- causes a rot at the base of leaves where they join the rhizome and causes them to fall over. It is identified by reddish-brown "mustard seeds" which are produced by the fungus. Trim leaves to admit more sunlight and air movement to the rhizomes; carefully remove and destroy all diseased leaves.

Leaf spots -- After flowering, leaves may become dotted with small, brown spots. Bacterial leaf spot has a watery, streaked appearance. Water-soaked margins around the spot turn yellow. Fungal leaf spots are rust-coloured, drier, and more confined. Since disease organisms overwinter on old foliage, cut and destroy leaves of infected plants in the fall. Spray with a registered fungicide during extended periods of high humidity or rainy seasons.

Mosaic -- is a viral disease that causes a mottling of leaves and flowers. It is transmitted by aphids. Remove and destroy infected plants and control aphids.

Iris borer -- The first symptoms of iris borers are small notches on the leaf edge or small accumulation of sawdust frass in early spring. Iris later develop loose, rotted bases and holes in rhizomes. Bacterial soft rot readily attacks borer-infested plants. Carefully remove and destroy old leaves, stems, and plant debris in the fall. A registered insecticide can be applied to the rhizomes in the spring as new growth occurs. 

Adapted from the American Iris Society website