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~ Tall Bearded Irises ~

Growing tall bearded irises in Eastern Canada is a challenge.

Blame the iris borer.

The iris borer dooms any and every iris planted in the East. If you plant an iris, the borer WILL find it, and severely cripple it. You're unlikely to see the insect; but it's there, guaranteed. Don't bother trying to grow irises unless you can commit to a chemical control program that involves systemic insecticides, and keep meticulously clean beds.

The adults emerge in September and October from the soil and mate. The tiny creamy-white to lavender eggs are deposited in small clusters on dead leaves; for this reason is it particularly important to remove such dead leaves in the fall. A single moth lays over a thousand eggs. Hatching occurs the following spring, in May in the northernmost range. The minuscule, young larvae feed on tender new leaves, causing notches and the sap to run. They eat their way downward, until they find both food and shelter inside the rhizome, where they gorge themselves on the flesh until early August, and enlarge to 4-5 cm. The injuries caused by the borers greatly increase susceptibility to bacterial rot, and rotting leaves is another symptom of borer infestation. The full-sized larvae leave the rhizomes, crawling through the top 5 cm of the soil for several meters. Still underground, the larvae metamorphose into non-feeding, chestnut brown pupae that twitch when touched. These pupae are found all over gardens where irises are grown when gardeners dig, plant and cultivate the soil. They should be destroyed manually, or placed on a bird feeder as a special treat for insectivorous birds. The 4 cm, dark grey, nocturnal moths emerge from the pupal cases in September, completing the yearly cycle.

Your ability to grow irises almost entirely hinges on your ability control this pest. Read on below for a description of the means available to home gardeners.

As for the rest, their beauty and color variety can speak for themselves. You decide whether they are worth it - below are photographs from my garden, you can guess where I stand on the issue.

Caroline de Monaco

Dusky Challenger

Elegant Impressions

Sea Power

Good Vibrations

Jennifer Rebecca

Jurassic Park

Maid of Orange

Trumpet Call

Poem of Ecstasy

Savannah Sunset

Everything Plus



Rhizomes should be planted flush with the soil surface, with their top face exposed. They like good drainage. If your soil is heavy, work in some peat moss, and grow the irises on a moat.

They can be divided in late July and August. Trim down the fans so that the newly and shallow-planted rhizomes don't fall down.

Irises require full sun, and most importantly, freedom from the borers.

Identifying and Controlling the Iris Borer

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This rhizome features iris borer damage, in particular a tunnel filled with frass, a fancy name for caterpillar feces. This rhizome will not be able to bloom next year.

Click to enlarge

Here, on the lower lefthand corner, there is a borer nestled in its tunnel, chewing the rhizome away, sapping its vigor, ability to flower and increasing its vulnerability to rot.

Click to enlarge

This borer has been removed from the rhizome. It's flesh-pink, and oozes a yellow-green liquid when disturbed.

This is an iris borer pupae. When the larva (worm) has completed its growth, it moves out of the rhizome, migrates several meters away from the iris, and enters a pupal stage before transforming into an adult. They are chestnut colored, can be found quite far from the iris. They "wiggle" when touched.

The iris borer moth is nocturnal, not easily seen and difficult to distinguish from other moths by the average gardener.

An exposed rhizome show an entry hole.
New growth is small and stunted.
Sawtooth edges on leaves dripping with sap.

Borer Prevention

Prevention may or may not foil the borer; the insect is extremely prolific and cannot be controlled by prevention and clean cultivation alone.

Prevention largely consists in the removal and destruction of the previous year's as early as possible in the spring, in the hope of reducing the number of borer eggs. However, the moths are very adept at finding alternate surfaces to lay their eggs in the vicinity of the iris, including bricks, mulch, wood chips and the twigs of nearby shrubs.

Borer Chemical Control

Since 1956 Cygon 2E (dimethoate), a systemic insecticide, has been only recommended pesticide for iris borer control. It chief advantages was that (1) it did not need to be sprayed, but could be simply added to water and poured around the plants and (2) absorbed by the roots, it made its way through the plants' tissues, rendering it poisonous to chewing and sucking insects, including the iris borer. Owing to toxicity issues, Cygon 2E has recently been pulled off the market for use in home gardens in the USA and Canada.

Alternatives exist. Admire and Merit (imidacloprid), manufactured by Bayer, can be applied by drenching (like Cygon 2E) and does not need to be sprayed. However, there is emerging evidence of bird and bee toxicity. A single application in the spring suffices, towards the end of the tulip bloom period. Do not apply near vegetables or plants that may be consumed by humans. This pesticide is also found in granular form to sprinkle around the plants, eliminating hazards associated with spraying. Bayer Advanced Lawn Season Long Grub Control Ready to Spread Granuals also contains imidacloroprid and may be easier to find. This product contains Merit and might be easier to find.

Orthene and Isotox Formula 4 contain acephate, another systemic insecticide. Like imidacloprid, acephate is toxic to birds and bees. Unlike Admire and Merit, 3 applications are required at tulip time in the spring.

Iris Links

Schreiner's Iris Garden - Located in Oregon, the world's premier supplier and hybridizer of bearded irises. Fees for phytosanitary certificate are hefty (U$40 at the time of writing) but their rhizomes are plump and inexpensive when purchasing in sufficient quantities.
McMillen's Iris Garden - Ontario-grown rhizomes. Wide variety of cultivars.
Iris & Plus - Irises and other perennials.
University of Nebraska - Iris borer life cycle and control.
Iris - Up-to-date info on iris borer control.

iris bold fashion
Iris germanica 'Bold Fashion'

iris nordica
Iris germanica 'Nordica'with 'Everything Plus' in background.

iris edith wolford sea power
Iris germanica 'Edith Wolford'(left) and 'Sea Power' (right).

iris elegant impressions
Iris germanica 'Elegant Impressions' (right).

iris trumpet call
Iris germanica 'Trumpet Call' (right).

caroline de monaco
Iris germanica 'Caroline de Monaco' (right).

iris maid of orange
Iris germanica 'Maid of Orange'

Iris germanica (clockwise)
'Alizes' - 'Dusky Challenger' - 'Savannah Sunset' - 'War Chief' - 'Pink Bubbles'.

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