Beautiful Iris are Healthy Iris… Keep Them That Way

Iris, particularly the rhizomatous kinds, have fewer pests and diseases than any other important group of ornamental plants. This fact, coupled with the ease with which they may be grown under widely varying conditions, largely accounts for their great popularity among home gardeners. Occasionally a few insects and microorganisms do attack them, particularly in old or neglected plantings, and control or preventive measures must then be practiced.

Public enemy number one of iris is the borer, a pinkish caterpillar with a brown head. t grows to be nearly 2 inches in length before ending its feeding cycle. Inexperienced gardeners rarely detect it before it has hollowed out the rhizome and caused the leaves to wilt. Actually borers get started in very early spring (April) after hatching from eggs deposited the previous fall by moths on dead iris leaves and other nearby debris. The tiny caterpillars move up the green leaves and chew on the inner edges of the iris fans. They soon bury themselves in the folds of the leaves and eat their way down inside the leaves, Finally reaching the rhizome. Leaves infested with young borers have tear stained or water-soaked markings, due to the exuding drops of leaf sap.

Once in the rhizomes (early July) the caterpillars feed voraciously and increase in size rapidly. They then leave the hollowed-out rhizomes and bury themselves in the soil a few inches below the plant, where they change to brown pupae. After about a month, the pupae wiggle their way to the soil surface and split open to release the adult (the moth stage).

Moth Attack

Moths fly at night in late August and September. At twilight they may be seen crawling around the base of iris plants. They are brown with black markings, and have a wingspread of 2 inches.

Water-soaked areas resulting from the mines of the young caterpillars in the leaves are good indicators of the presence of borers. Such areas should be squeezed between the thumb and forefinger, commencing at the base of the leaf and pulling upward in order to crush the young borers.

A second practice is to remove and destroy old iris leaves and other debris in the fall. This will eliminate many of the overwintering eggs.

Pest Control

Where large plantings are involved, pinching the tunneled areas in leaves is impractical. In such situations it is best to protect the new growth with weekly applications of an insecticide like Malathion or natural Neem Oil. The first application should be made after the first warm spell in April and repeated once a week until blooming time. Because some of the tiny borers may hatch from eggs deposited on nearby debris other than iris leaves, it is wise to spray the areas immediately adjacent to the iris plants also.

Gardeners should bear in mind that if the insecticide application is delayed until after the young borers have entered the leaves, the treatment will be ineffective.

In old, heavily infested plantings the iris should be lifted in July or August, and all unsound portions, as well as any larvae contained therein, should be cut away and then destroyed (burning is a preferred method). Such material should never be thrown onto the compost pile.

The common stalk borer is the second pest which occasionally infests iris, in addition to many other kinds of plants.

This borer has alternating lines of white and chocolate brown down the length of its body with a median girdle of solid brown. Unlike the iris borer, it is agile and moves readily from plant to plant. In May it leaves other host plants such as ragweed and moves over to the succulent iris fiower stalks. It climbs up the stalk for a short distance, bores a hole and then enters. The frass this borer throws out from the entrance hole is a dead giveaway of its presence. The Malathion treatment is recommended for the iris borer will also control this borer.

The rose chafer, a long-legged, yellowish-brown beetle about one-third inch long. occasionally feeds on iris flowers. It frequently appears in swarms rather suddenly in June or early July and continues to feed for several weeks. Where only a few iris are involved, the beetles should be hand picked in early morning. when they are least active, and smashed between a couple of 2×4′s although a shoe heel works great too! Where a large number of plants are concerned. spraying with Malathion is sometimes effective.