Three Myths About Bearded Irises

by Tom Waters

Every area of human knowledge has its myths: ideas firmly believed by large numbers of people that are not actually true. The rise of the internet and social media has further complicated the process of separating myth from reality. In earlier times, a curious person might seek out a book or an expert to resolve a question, and stand a fair chance of getting accurate information in return for their effort. But today, when a curious person does an internet search instead, the information they find is just as likely to be wrong as it is to be right.

On the subject of growing bearded irises, I have found three myths that seem to be ubiquitous, and inevitably resurface during any internet discussion of the subject. This article addresses each of these three myths, in the hope that a clear exposition of each will provide a little island of solid information that is often missing from untethered internet exchanges.

Myth #1: Bearded irises will not bloom unless the tops of their rhizomes are exposed to sunlight

Although, as I shall explain in a bit, there are some good reasons for planting irises with the tops of the rhizomes exposed, it is not necessary to do so to ensure bloom. Irises bloom just fine if planted with an inch or more of soil over the rhizome. Nothing magical happens when sunlight strikes the surface of an exposed rhizome.

Where did this myth come from? I think it has three sources. The first is a very basic piece of advice: iris rhizomes are not bulbs. Most spring-flowering bulbs (daffodils, tulips, crocuses, etc.) need to be planted fairly deeply (three times their height is a common recommendation). If you plant an iris as deep as a tulip, it may indeed not bloom. In fact it may not survive at all. The second is a general remark about growing conditions irises prefer: they like full sun, or at least half a day of sun, and will not bloom well in too much shade. Finally, there is a bit of traditional advice that irises should be planted "like a duck in the water", with the top half of the rhizome above the soil surface. I think these last two points (a recommendation for planting with exposed rhizomes and the fact that irises bloom best in sunny locations) led people to blend these two ideas together and conclude that it is sunlight striking the tops of the rhizomes that causes irises to bloom. The advice not to plant them deep like tulips or daffodils then reinforces this notion.

Okay, if it is not necessary to expose the rhizomes to direct sunlight to ensure bloom, should I plant them exposed or covered? What is the best planting depth?

The short answer is that it just doesn't matter very much. In most gardens, irises with the rhizomes exposed and irises covered with a half inch or inch of soil will both do equally well. If you look at an established clump, you will see that the rhizomes themselves sometimes grow down into the soil and sometimes grow up onto the surface. It's all good.

In some locales, particular climate conditions can favor either shallow or deep planting. In a climate that is often rainy and humid, exposed rhizomes are less likely to rot from wet soil. Much of our traditional gardening advice comes from places with such climates: the UK and the eastern seaboard of the US. I believe the advice to plant with rhizomes exposed originated in these areas, and then was simply repeated.

In areas with very cold winters, Irises may benefit from being planted more deeply, making them less susceptible to heaving during freeze-thaw cycles.

In dry, hot regions (such as much of the western US), planting with the rhizomes covered offers some protection against sun-scalding and desiccation from heat and wind. The rhizomes appreciate being below the soil surface, where conditions are a little cooler and moister.

Bottom line: Plant covered or uncovered, according to your preference, experience, and local advice. Irises will bloom just fine either way.

Myth #2: Irises can "revert" to some other color

It seems like everyone has heard a story of a beautiful clump of irises, say nice ruffled pinks, "reverting" to white or purple after a few years. In fact, this does not happen. Irises do not spontaneously change color. (There is one minor qualification to this statement, which I will address below.)

No, this iris will never "revert to purple"

Where does this myth come from? One source, I think, is that some plants do appear to behave this way, particularly annuals that reseed each year. If one plants a hybrid zinnia or morning glory, for example, the plants that come up from their seed in future years will not look like the original, and in fact may show simple "wild type" colors common in the original species from which the hybrid was developed. A second source of this myth comes from the fact that if different irises are planted together, one of them may multiply faster and eventually take over the planting, making it seem to the casual observer that the irises in the planting have "changed" from the color that was originally common in the planting to the one that eventually took over. But note carefully that this is competition between two different plants, not a single plant changing color.

In almost all cases where people say their irises have "changed color" or "reverted", this is the explanation: there was more than one variety in the planting to begin with, and one that had not bloomed the first year or two grew well and came to dominate the planting in later years.

It is possible for the coloring of an iris to appear somewhat different from one year to the next, because of weather differences or chemical exposure. The blue and violet pigments, in particular, are somewhat sensitive to unusual weather. These changes are changes in the darkness or saturation of color, though, and cannot result in a whole new color or pattern. A pale blue iris may appear to be cool white in one year or sky blue in another year, for example, but will never become yellow or pink. Some herbicides cause deformed blooms with color strongly depleted in some parts of the petals, but the deformity is obvious.

There are a couple other ways an iris of a different color can appear in a planting, even if only one variety was planted to begin with.

The first is hybridization. Just as your morning glories may reseed themselves, so a bearded iris may occasionally form a seed pod and drop its seeds into the soil around the plant. If these seeds sprout, the seedlings may well be a different color than the parent, and when they bloom (perhaps three years after the seeds are first produced), the gardener may be in for a surprise! To prevent this from happening, you can remove the bloom stalks after the flowers fade, so that seed pods do not develop.

Although possible, seedlings appearing in a bearded iris clump this way seldom happens. Most bearded irises do not produce seed on their own. (In my garden, I see maybe two spontaneous seed pods for every thousand blooms.) And bearded iris seeds don't germinate well in many climates without special attention. If seedlings do sprout in an established clump, they will likely be crowded out by the parent. Hybridizers go to a great deal of trouble to get bearded irises to cross-pollinate and to grow the seeds to maturity. The process can and does happen without human intervention, but only seldom. (If you grow beardless irises like Siberians, the appearance of unexpected seedlings is much more likely.)

Finally, an iris may experience a mutation that causes the flower color to change. Such mutations, called "sports", are extremely rare events. Except for a few historic varieties that are prone to such mutations, most irises will never produce a sport. You can grow a thousand different varieties for a decade and never see one. I started growing irises in the 1970s, and have never seen a sport in my garden, or in the gardens of any of my iris-growing friends.

Bottom line: Bearded irises do not spontaneously change color. Each iris is a unique individual, and will retain its original color and pattern forever. If you see a different colored iris in a planting, it must be a different variety that was already there and just had not bloomed, or had not been noticed, before.

Myth #3: Iris foliage should be trimmed back in the fall

It's a ritual that some gardeners swear by: attacking their iris beds in August or September with shears, resulting in a defoliated war zone that looks as though someone had come through the garden with a lawn mower set at 8 inches. Sadly, those irises are now deprived of much of their food source: photosynthesis in green leaves.

Why do people do this? What makes them think that cutting leaves in half is good for their plants? I think there are two sources for this myth. The first is that many perennials do benefit from being cut back at certain times of year, to stimulate new growth, and a new flush of bloom in some cases. But if you are an observant gardener, you will notice that the anatomy of these plants is different from that of irises. These plants have buds along their stems. Removing the tops of the stems encourages the lower buds to grow, resulting in bushier, more vigorous plants. But irises do not grow this way. All the leaves of a fan emerge from a single bud at the tip of the rhizome. When you trim a fan back, you are just chopping leaves in half, not removing any upper buds to stimulate lower buds into new growth.

The second source of this myth is that when irises are dug and divided, the fan is traditionally trimmed back. This is how irises are generally sold: bare-root, with roots and fan trimmed back to about 6 or 8 inches. This trimming is a good idea for an iris that has been dug and divided. Its growth has been interrupted, and it will take some time for new growth to emerge from the rhizome. During that time, a big fan of leaves can weaken the plant by drawing too much water and energy from the rhizome. The leaves lose water by transpiration, which the old damaged roots are not able to replenish. A big fan also makes it easy for the newly planted iris to topple over or become uprooted. But these reasons only apply to plants that have been dug and divided; they are not applicable at all to plants left growing in the soil, undisturbed.

Some have said that trimming back in the fall helps discourage iris borers, which lay their eggs in the leaves at this time of year. The eggs, however, overwinter in dead, dry leaves, not growing green leaves. Removing dead foliage is helpful; cutting green leaves in half is not. The recommended procedure for borer control is to remove all dead foliage and burn it in late winter or early spring.

Some just think the trimmed fans look tidier. This is understandable. By the end of summer, iris foliage often looks pretty tired and unattractive. Many leaves are drying at the tips, getting a little pale and floppy, and perhaps suffering from damage from insects or other ailments. Ironically, if you trim the leaves back, then the tops where you cut them will just turn brown and dry up, so instead of tall leaves with dry ends, you have short leaves with dry ends. Was it really worth it?

Bottom line: Cutting through the green fans of an iris in the fall does not help the plant, and may weaken it slightly, as you are reducing its capacity for food production through photosynthesis. Irises are rugged, and this slight weakening is something most of them can cope with without suffering much, but why put them through it at all? It does "tidy" your garden, but that only benefits the aesthetic sensibilities of the gardener. It does not help the irises in any way. If you want to tidy up at this time of year, restrict your activity to removing dead foliage and dry leaf ends. Don't cut green leaves!

The foliage on the undisturbed clump on the left should not be trimmed. If you want to tidy up, remove just the dead leaves (1 and 2) and the dry end of leaf 3.