There are many species of Iris and most of them originated in the Northern temperate regions. They are monocotyledons and some of them spread by rhizome while others come from bulbs. Like other monocots, their flower parts are usually in groups of 3. They have 3 upright petals called standards and 3 horizontal or recurved petals called falls. Illustration of flower parts.

In milder climates, there are enough different species of Iris to keep them in bloom practically all year round. Each plant may only flower for 1 to 3 weeks but the sequence of blooming species can keep the flowers coming.

Since this is a fairly large genus of plants, the required site conditions can vary greatly among different types.

  1. Typical Site - Most of the types of Iris need a well drained soil to avoid root rots. This includes the tall bearded iris.
  2. Wet Site - Some iris such as the Siberian and yellow flag iris can actually grow in a pond or bog garden.

Check on the species of iris you wish to grow.

Other site conditions would include full sun.

The best way to learn how to plant something is to look at growing on other locations. Iris that grow from rhizomes always grow with the rhizomes just at the surface of the soil. Therefore, avoid the most common mistake which is to bury the rhizome as if it were a root. Plant these types of iris so that the rhizome is just beneath or at the soil surface.

For the bulb type iris, follow the old rule of thumb for all bulbs. The depth of the planting hole should be 2 to 3 times the height of the bulb. Most bulbous iris are rather small so they don't need to go very deep into the soil.

Plant the bulbs in the fall and the rhizomateous ones may be planted in either spring or fall. If planted in the spring, you won't generally get any flowers that summer. Also regardless of the season, sometimes iris will not bloom the first year after being planted.

In the early spring before growth begins, clear the beds and borders of any debris from the previous growing season. It is especially important to remove an old foliage from iris that were infected by leaf spot diseases the previous year.

Once the growing season begins, you can side dress the iris with the same fertilizer that you use for other perennials. It is always good to have your soil tested every 3 or 4 years to be sure the pH and nutrient levels are adequate for plant growth.

When the blooms fade, cut off the spent flowers (deadhead) since there is no sense letting the iris use its energy to produce unwanted seeds.

Take care while weeding around the iris so that the rhizomes don't get damaged. This could open them up to root rots and encourage iris borers to attack.

  1. Bacterial Soft Rot - This is by far the most damaging problem of iris. It is caused by a bacteria in the Erwinia genus similar to the one that causes fireblight on pears, apples and other trees.

    For the bacteria to enter the plant, it needs some sort of opening in the top layer of the rhizome. This could happen during weeding, winter injury, by stepping on the rhizome or by iris borer feeding. Once it is infected, the rhizome will turn to mush and have a peculiar odor.

    This type of rot is more prevalent in irises that are under stress. This may be from being planted too deep, poor drainage in the soil, too much fertilizer or when clumps become crowded.

  2. Iris Borer - The larvae of a certain moth emerges from the egg in the spring and crawls to the top of the iris foliage. They bore an opening and begin to feed as they move downward between the layers of the leaf. Eventually, they reach the rhizome where they finish feeding and bore out to pupate in the soil nearby and turn into the moth which lays her eggs in debris near the iris and it all starts again.

    First indications of an infestation are small, ragged notches on the edge of the leaves and the accumulation of sawdust looking frass (droppings) at the base of the leaf. As they work their way downward inside the leaves, the foliage may turn yellow and then brown. When they have finished feeding later in the summer, the drill their way out of the rhizome leaving pencil lead sized holes. The larvae then drop to the ground, pupate in the soil and turn into the moth.

    So, be sure to clean up the dead foliage and other debris from the iris patch in the fall if borers have been a problem. Sometimes the borers can be felt inside the leaf and may be killed by squeezing. Certain insecticides may also be used to help control this pest.

  3. Leaf Spot Disease - This fungal disease is more of an aesthetic concern rather than a serious enemy of the plant. If there are large numbers of spots, it may cause a weakening of the plant due to loss of leaf surface for photosynthesis. Again, removing debris from the previous year's foliage will help to minimize this problem. Also, planting iris in full sun will help to keep the foliage dry and, thus, discourage fungal diseases from starting.

If your plants are selected properly and are rated for your specific USDA Hardiness Zone, there should be no need for extra winter care.

When the clumps of iris get very crowded with newer rhizomes, they may begin to flower less and less each year. When this becomes a problem, it is time to divide the plant and reset the divisions so that they allow proper spacing between plants.

Note: We have provided some general information and observations on this topic aimed at the home gardener. Before you take any serious action in your landscape, check with your state's land grant university's Cooperative Extension Service for the most current, appropriate, localized recommendations.


Copyright 2000-