Plants - Perennials
Irresistible irises: Secrets to eight weeks of blooms
With a bit of planning, you can enjoy a colourful array of bearded irises throughout spring and early summer
Dividing: Split and replant irises every three to four years to promote flowering; this is also the easiest way to propagate them. Gently loosen the iris clump with a garden fork about 15 centimetres from rhizomes; ease plant out of ground. Wash soil off clumped root mass; with sharp knife, divide into single plants with one leaf fan each (cut down to 15 centimetres). Discard old, soft or diseased rhizomes. Dry rhizomes in an open cardboard box, single layer, in a warm, dry location for a day to allow cuts to seal; if replanting in same spot, amend soil with compost or composted manure and a couple of handfuls of alfalfa pellets. Work on one clump at a time to avoid mixing up colours.
Fertilizing: Don’t feed irises in their first season after planting. In subsequent years, apply fertilizer in spring as soon as growth starts and right after flowering. Avoid high-nitrogen types, which promote soft, succulent growth, poor flowering and bacterial soft rot; look for mixes such as 5-10-10 or 10-20-20. Alfalfa pellets are a good organic choice; sprinkle a handful around each plant and water in. Do not get fertilizer on the rhizomes.
Grooming: After flowering, cut or snap off each flower stalk at its base. Remove brown or withered leaves (new ones will form). In fall, rake up all dead leaves, which can harbour pests and disease; do not compost if you suspect pests.
Mulching: If your area doesn’t get reliable snow cover, evergreen boughs make an effective, airy mulch; apply after freeze-up and remove in early spring.
Watering: Bearded irises are drought-tolerant, but need 2.5 centimetres of water throughout the first growing season; in following years, natural rainfall is generally sufficient, but you should water in periods of drought.
What went wrong?
Iris borer: Attacks irises in Ontario and eastward; not found in Western Canada. (Borers are the larvae stage of moths that lay their eggs in fall; they hatch when daffodils bloom, then tunnel down from the top of the iris to the point where the leaves separate.) You won’t see borers on the foliage because they chew from the inside, but there are telltale signs, such as notches or moist “sawdust.” Three weeks after irises bloom, borers can eat their way into the rhizomes, eventually hollowing out and destroying them. Control As a preventive measure, Chuck Chapman, vice-president of the Canadian Iris Society, recommends an application of dormant oil spray early in the season, just as the leaves begin to grow. Clean up all iris leaves in early spring before the daffodils bloom, or in fall; dispose of them in the garbage or by burning, not in the compost pile (where eggs may overwinter). Both Chapman and Ann and Bob Granatier of Trails End Iris Gardens endorse a manual control. In mid-June, look for notched leaves or damage where foliage separates. Trace chewing (discoloration shows where tunnels are located) until you find borers; kill by squeezing, or remove pest and toss into soapy water. Nematodes (organisms that live in the soil and feed off other soil life, including insect pests) are another option—the most promising appears to be Steinernema carpocapsae (Sc). Apply just before irises start to flower; nematodes work best in moist environments, so first water your patch or wait for rain.
Leaf spot fungus: Small brown spots on leaves; harmless but unsightly. Control Remove and dispose of unsightly leaves, and remove all old iris foliage in fall; do not compost.
Soft rot: Soft, rotted spots on rhizomes, usually in spring. Control Dig out rotted tissue (Chapman uses an old tablespoon with a sharpened edge). Spray treated area with a solution of 10 per cent chlorine bleach and water.