Irises

Growing Irises

Iris was the Greek goddess of the rainbow and her name is therefore an appropriate one for a genus containing more than 300 species of multicoloured flowering plants which have been cherished for their beauty since antiquity. The genus Iris is native to almost every country in the northern hemisphere.

Flower structure

Iris flowers vary greatly in colour, shape and size but in general structure all are the same:

Each flower has six segments in two sets of three.
The three upper segments are petals and called standards.
The three lower segments are called falls.
Between standards and falls are three flat-coloured bands which arch over the stamens. These are style-arms.
The size and shape of the parts vary depending on the type of Iris.

Iris Terminology

Bicolour: a flower with two colours
Bitone: a flower with two shades of the same colour
Blend: a flower with three or more colours
Plicata: a flower featuring a light ground with the edges stitched, stippled or banded with a darker colour
Self: a flower with standards and falls the same colour

Bearded Iris

Bearded Iris are identified by the 'beards' or tufts of pale hairs on the lower petals. They prefer a sunny dry location and will adapt to almost any type of well-drained soil.

Miniature Dwarf Iris are the smallest bearded Iris and are earliest to bloom. They are up to 20 cm unbranched with one or two terminal buds. Flowers are 5-8 cm across.

Standard Dwarf Iris are 21-40 cm tall with flower sizes of 5-10 cm. Most have one branch.

Intermediate Iris are 41-70 cm tall. They bloom after Dwarfs but before Tall Bearded. Flower size is 7-12 cm.

Border Bearded Iris are 41-70 cm tall with flower size of 7-12 cm.

Miniature Tall Bearded Iris are 41-70 cm tall with slim, wiry flower stems. They are well branched and carry 7 - 12 blooms. Excellent for flower arrangements.

Tall Bearded Iris are the best known of the Bearded Iris. They are over 71 cm tall and have stiffly erect stems. They are the last of the Bearded Iris to bloom. Stalks are well branched and carry 7 - 12 blooms.

Planting and Cultivation

When planting, dig two slanting holes with a ridge in the middle. Place the rhizome on the ridge, spreading the roots on either side. Make sure the top of the rhizome is slightly above the ground level.
Ideal planting time is mid-July to early September. Earlier plantings will reduce the following season's bloom. If planted later, they will not be fully established before winter, increasing the chances of winter kill.
Iris must be replanted every three or four years to ensure vigorous growth and large healthy flower stalks.
Any fertilizer used must be low in nitrogen and must not come in contact with the rhizome.
Weed carefully as Iris roots are just below the surface.

Blooming Season

The blooming season in southern Ontario begins about the first week in May. The Miniature Dwarf Bearded bloom first, followed by the Intermediates, Border Bearded and Tall Bearded which bloom until about the first of July in a normal season.

Species Iris

There are at least 230 distinct iris species found in the wild, along with many varieties of each. (The garden hybrids made by man are cultivars; natural hybrids or variants within a specific range, are varieties.) All the iris species originated in the northern hemisphere; none have ever been found in the tropics or south of the equator. Their native habitats range from deserts such as are found in Israel to arctic-like Alaska but most are found in a Mediterranean-style climates.

Canada has only a few native species. The last ice age wiped out most plant life and irises have only come back slowly from the south. They have, however, also come from the north as parts of Alaska were not glaciated. There are several varieties of I. setosa in Alaska, indicating it has been there a long time in isolated colonies each of which developed to meet local conditions. I. setosa has spread from Alaska to naturally cross with I. virginica, resulting in our wild blue flags, I. versicolor, of swamp lands. I. virginica has been found as far north as London, Ontario. There is a variety of I. setosa, found along the shores of the St. Lawrence in Quebec and in Newfoundland, known as var. canadensis, a smaller version of I. setosa. I. versicolor is most hardy, perhaps a result of hybrid vigour, and has spread widely into northern Ontario.

Non-native species Iris which can be grown in Ontario are those from areas that have a climate similar to ours. This includes I. missouriensis from Montana and the western plains, I. pseudacorus from England and central Europe, I. sibrica, I. apylla, I. variegata, I. croatica, I. gramina and I. pumila from central Europe, I. balkana and I. lactea from Russia, I. attica from Greece, I. tectorum and I. chrysographes from China and I. italic from Italy. Mountain or alpine species are usually hardy here at our lower altitudes.

Spring and Fall Planting

Siberian Iris are very hardy and once established will grow well for many years They like a slightly acid soil and lots of water but resent transplanting. Peatmoss added to the soil and as a mulch aids acidity and helps to retain moisture. They grow well in partial shade. Their blooming period falls in early July.

Spuria Iris bloom after the Siberians. The plants are tall and bloom stalks often reach 1.5 m. They need good drainage, are heavy feeders and prefer neutral to slightly alkaline soil. A drying off period in July through August is beneficial. Do not replant too often as they resent moving.

Japanese Iris is a large, flat, beardless Iris, usually with only 3 or 6 petals, blooming in early July. Plant in mid-September in a rich, well-prepared spot, making sure roots don't dry out. They should be planted in damp areas or near water. They are heavy feeders: make yearly applications of garden sulphur and iron sulphate and feed twice yearly with a Rhododendron and Azalea fertilizer. Plants will die back completely after planting but will resume growth in spring.

Pests and Diseases

Cleanliness is a must for disease and insect control. All dead leaves must be cleaned out and destroyed in the spring.
Iris borer is the main insect enemy. Its eggs are laid on the leaves in late fall. Cleaning out the leaves in the spring will therefore destroy the eggs. The borer hatches in early to mid May, crawls to the top of the fan and slowly bores its way through the heart and rhizome, emerging in late July as a 2.5 cm long cream-coloured larva with a brown head.
Bacterial soft rot is a soft mushy rot with a very strong, offensive odour. It is caused by too much moisture, too rich and lush a growth (due to too much Nitrogen) or untidiness. Cleaning out the leaves in the spring allows the sun and air to dry out the rhizome reducing the chances of soft rot. The rot can be cut off and the rhizome dusted with captan or, if severe, the whole rhizome may be dug, cleaned and left in a dry location for one to two weeks.
Botrytis fungus rot is a dry, spongy rot with no odour. It has a grey spore mass on the rhizome at ground level, occurring in late winter and early spring. Dig up and destroy.
Leaf spot does no particular harm unless it becomes serious. Cleaning out the leaves in the spring destroys spores that cause leaf rot. Remove the diseased leaves and spray the healthy leaves with a nontoxic fungicide such as sulphur.

Toronto Master Gardener Factsheet