Growing Healthy Iris: Facts To Know For Strong Iris Plants

Modern bearded iris is probably the most spectacular and desirable of all late-blooming spring perennials in eastern Massachusetts.

The flowers are large, of beautiful form, and in magnificent colors ranging from black purple through the spectrum to pure white, including the newest color break approaching true pink.

Growing Iris PlantsPin

These are a far cry from the rather dull, small-flowered old types one commonly sees growing en masse in borders and along roadsides.

Major Pests And Diseases Of Iris

Iris are susceptible to certain diseases and pests, which must be controlled if they are to be well grown so the full beauty of flowers and plants may be realized.

There is considerable variation in the prevalence of diseases in different parts of the country.

Here in Massachusetts, there are two major diseases of bearded iris and one serious pest.

Two types of rhizome rot are found, and fungus leaf spot is prevalent.

The iris borer is widespread and destructive. Other diseases, severe in some parts of the country, are of minor importance here.

In our garden at Waltham, we have adopted certain methods of growing bearded iris that seem beneficial in controlling rot and leaf spot, and we have eliminated borers.

Proper Growing Methods

Almost everyone who grows any quantity of iris has his methods, and we are no exception.

We believe that the modern bearded iris needs as near-perfect drainage as possible, both surface and under drainage.

They enjoy porous soil rich in humus and plenty of food, and they respond to measures that keep them in active growth from spring to freezing weather.

The beds in which our iris grow are elevated 3” to 6” inches above surrounding levels, except on slopes where elevated planting is unnecessary.

Ideal Winter Mulching Materials 

Here in New England, some winter mulching is essential.

Newly planted and unestablished clumps are protected throughout the winter—the rhizomes and surrounding soil being covered, the leaves exposed.

Many good mulching materials are available. We have used several of them, but we now use only two for iris.

Pine needles have proven to be one of mulch’s cleanest and most satisfactory.

The material is easily obtained here and costs only the time spent gathering it.

Vermiculite, house fill grade, applied half an inch deep, is a perfect mulch. It is an excellent insulator, preventing rapid soil temperature changes and markedly reducing heaving.

It is very quickly and easily applied, sterile, and not removed in spring. It is an excellent soil conditioner.

Spraying Program

Our spraying program starts in late March or early April with the advent of the first warm period.

Following this period, the iris borer hatches from eggs deposited the previous fall on dead leaves, debris, and shrubs near the iris.

The tiny larvae make their way to nearby iris plants, crawl up the leaves and enter the fans.

They hatch throughout the spring, and it is thought some may hatch during the summer.

They must be intercepted and killed before entering the leaf fans, for after this has occurred, they are invulnerable to sprays and must be hunted out and killed by hand.

We spray weekly from the first application to the blooming season. Not only is it necessary to thoroughly cover the iris with spray, but also the contiguous ground, surrounding plants, and shrubs.

Regular spraying is imperative. The material we use in the ordinary 3-gallon pressure sprayer is:

  • 3 gallons water
  • 6 heaping tablespoonfuls 50% percent wettable Malathion
  • 3 teaspoonfuls Phyton and 18 drops of Liquid Sticker Spreader

The malathion and Phyton are made into a paste, added to the water, and thoroughly mixed.

The sticker spreader is diluted with a tablespoonful of water, added to the finished product, and thoroughly mixed.

This is a suspension, not a solution, and frequent agitation during the application is necessary.

When much spray material must be used in large plantings, larger equipment such as a wheelbarrow type or a small power sprayer is desirable.

We use such a sprayer of 20 gallons capacity. The formula for this sprayer is:

  • 20 gallons of water, 8 oz. by weight of 50% percent wettable malathion
  • 5 tablespoonfuls Phyton
  • 1 ½ teaspoonful (in 2 oz. water) Sticker Spreader

With this equipment, there is no work to spray even large plantings. This program has entirely eliminated iris borers in our garden.

It must, however, be used in sufficient concentration, early enough, often, regularly, and thoroughly.

About mid-April, mulch material, except vermiculite, is removed. All dead iris leaves and detritus are also removed and burned.

Rhizomes are inspected, and if rot is found, the rhizome is cut out and burned. The remaining hole is filled with sand.

As soon as growth starts, a surface application of commercial fertilizer and wood ashes is cultivated into the soil, together with vermiculite, where it has been used as a mulch.

Any dead or diseased leaves are promptly removed and hurried to help control leaf spots throughout the season.

The fungicide Phygon is used in the spray material to control leaf spots.

Division And Replanting Methods

Following the June flowering, any plants requiring it are divided and replanted.

New beds are planted immediately with our own stock and with new stock as soon as it can be obtained from the dealers.

Chimps are planted 3’ feet apart, and individual rhizomes 1’ foot apart. This provides good air drainage and helps to prevent fungus diseases.

Proper Fertilizer Application

When large clumps are lifted, the area is built up with a mixture of the following:

  • Soil
  • Sand
  • Humus
  • A small amount of organic and inorganic fertilizer

New beds are built of new soil, with liberal quantities of old cow manure, compost, vermiculite, and the addition of complete commercial fertilizer and Milorganite.

Liberal quantities of superphosphate have previously been added to the cow manure compost. Thus a good textured, porous, rich soil is provided.

This method of preparing beds promotes the health of the plants, and we believe it reduces rhizome rot.

Plants are kept thriving as long as possible, with ample watering during dry weather.

The second application of complete fertilizer and wood ashes is given to all the beds in August.

Culture And Spraying Program For Healthy Growth

During the summer and fall, weekly spray applications detailed above are used, omitting malathion and adding Neem oil to kill aphids that occur throughout this period.

We do not allow a rainy spell to interrupt the spraying program, taking advantage of any temporary break to apply the mixture.

This is particularly important during hot, muggy weather when leaf spot fungus is active. Mulches are applied before hard, freezing weather.

Such a program of culture and spraying also gives complete coverage of all perennials which may be growing with the iris in mixed plantings.

It promotes healthy growth and helps produce plants with the stamina to withstand disease.

It helps to make gardens in which one may take pride and satisfaction of work well done and results in favorable comments upon the health of the plants and the freedom from disease and pests by all who see the garden.

Preventive hygiene pays abundantly in garden attractiveness.

42737 – by Dr. Erving M. Fraim