Bearded iris, Iris germanica, is one of the most easy and rewarding perennials you can grow. They can survive in xeric gardens with as little as 10 inches of rainfall a year, and will grow and bloom without care as long as they get plenty of sun. Multiplying perpetually, there is no limit on a bearded iris’ lifespan. They rarely if ever need fertilizer. They tolerate a wide range of soil types. In dry-summer climates they’re highly resistant to disease. They’re able to push back and hold their own against aggressive weeds and groundcovers, without ever becoming invasive themselves, since a clump of irises only creeps outward by about 3-4 inches per year.
They’re also rapid multipliers, which is both a blessing and a curse—a few new plants will become a cluster and a cluster will become a dense, thick mat of roots and leaves that eventually choke each other off and stop blooming. Like many clump-forming perennials, they’re prone to developing the “crowded doughnut” shape, with weak, depleted plants in the center of a cluster and relatively vigorous plants around the edge where there is still access to fresh soil and sun. To avoid this situation (or fix it), it’s best to dig and divide bearded iris every 3-4 years.
Timing
In truth, bearded iris is tough enough to be divided any time of year. But since it takes some time for roots to re-establish, the most often-recommended time to transplant is right after they bloom: in June for Northern Hemisphere gardens. The reason for that is not so much about the health of the plant (the least-stressful time to dig them, from the plant’s perspective, would probably be early fall) but so that the plants are completely recovered by the following spring when you want them to bloom.
It’s notable that June and July are very stressful months for plants to be transplanted, forcing them to re-establish roots in the midst of long, hot days when demands for water are at their peak. June is also the most productive time of year for photosynthesis, an opportunity missed if plants are dealing with their re-establishment phase when they’ll have fewer leaves. But this is the best time of year to do it if you want to maximize blooming the year after. Since bearded iris is such an easy and vigorous plant, one that can survive being unrooted for a long time and multiplies fast anyway, you can go ahead and dig it in summer without much harm. Why not? Spend the spring and fall transplanting seasons attending to your more temperamental plants.
How bearded iris grow
Irises grow from a rhizome, a form of modified stem that creeps along the ground just beneath the surface. Irises have a fat, fleshy rhizome for storing water and starch, which contributes to their durability. (Other rhizomatous species have simple, spindly rhizomes that help them spread rapidly but won’t enable them to live long outside soil). The top of the rhizome is usually exposed, revealing ribs where now-dead leaves used to be attached. The underside of the rhizome is covered with stringy, strong, fibrous roots that eventually die and slough off, leave small superficial holes in the rhizome.
A keen gardener who looks at the structure of different plants eventually comes the realization that all vascular land plants, from 350-foot-tall redwoods to dainty daffodils, are composed of the same three basic parts: leaves, stems and roots. Leaves and roots always attach to the stem, which, for most plants, is usually the only type of tissue that generates the other two types. Stems always grow new tissue from specialized cells at the tip, and produce new branches at nodes that lie dormant at the base of a leaf or leaf scar until it’s time to grow. Cells at the growing tip of a stem divide rapidly to form the basic structure, then stop dividing and go through an elongation phase to reach the full size, then harden and stop growing as they mature.
Whether plants grow from bulbs, tubers, crowns, corms or woody trunks, the dramatic differences in the way plants grow are simply a matter of the way these three main parts are arranged. In an onion, for example, the stem is a small mass in the bottom of the bulb called the basal plate, where the onions layer’s (which are modified leaves) are attached. The roots emerge from the bottom of the basal plate. Only this basal plate tissue can perpetuate the life of the onion; you can replant the base of an onion bulb to generate a new cluster of leaves, but you can’t plant leaves or chopped onions to grow new roots, nor can you generate onion plants from the spindly roots. In a potato, the entire edible tuber is made of stem tissue and each of the eyes is able to sprout new potato plants. If you look closely at a carrot, which is a tuberous root, the stem is a small area at the top of the root where a cluster of leaves emerges. Neither the root, nor the leaves, are viable on their own.
So the bearded iris’s rhizome is the plant’s main stem. The new cells are created inside a small, actively-growing region inside the fan, which is located at the young growing tip of the rhizome. New leaves are always emerging from the center of the fan, and old leaves are always sloughing off the outer edges, exposing new sections of rhizome. New roots emerge from just behind the spent leaves. Iris leaves are constantly being renewed, and roots are not long-lived either; they live about a year, and after they die, the older portion of the rhizome simply sits on top of the ground, anchored by the actively-growing regions at the tips where new roots are always forming.
Irises multiply in two phases. The most prolific is related to blooming. When an iris prepares itself to bloom, the actively-growing tip of the rhizome—in other words the growing tip of the plant’s stem—curves itself upwards and hoists itself into the air as a flower scape. There, the stem terminates, and the fan that it originated from will die.
Like so many other plants, pinching or terminating the growing tip of a stem forces it to branch. This is also true when a rhizome terminates in a flower scape. In fact, in early spring, before the flower buds even appear, the emergence of offsets that pop up just ahead of the main fan is a strong indication that this particular iris will bloom. Weak or shaded irises will produce 1 or 2 offsets per flower scape. Moderately-vigorous plants will produce 2 to 3 offsets, and highly vigorous plants will produce 4-5 new offsets to replace the portion that bloomed. That gives you a sense of how quickly irises multiply.
The second source of new offsets on a bearded iris comes from older nodes in the aging portion of the rhizome. These auxiliary shoots are usually small and will require multiple growing seasons to reach blooming size. Any old chunk of iris rhizome, whether it has leaves or not, will produce one or two of these types of offsets if it is separated from the rest of the plant. If you want rapid propagation of your irises there’s no reason you can’t keep every bit of living rhizome to create lots of new plants, but many gardeners will discard older portions of the rhizome when they divide their iris since the offsets from old portions take 2 years to bloom.
Digging and dividing
It’s fairly easy to dig, lift or yank bearded irises out of the ground. There’s not a lot you can do that harms the plant much, even if you lose some of the leaves in the process. When the cluster is out, you can shake or hose off the dirt and separate the cluster into individual plants.
Rhizomes will initially be connected together into large branching structures, tracing back to the original parent portion, which may or may not still be living tissue. To divide iris, you’ll need to break the branches apart into manageable bits. Some guides suggest using a sharp, clean knife but tearing is also fine. The exact point of breakage is not that important; separate the offsets into 2-6 inch sections each containing 1-3 fans. Try to maintain at least two inches of rhizome and at least one highly-vigorous fan on each segment, since it leaves you more likely to get blooms next year. But even smaller sections will grow stronger and produce blooms in the second year.
You’ll be left with a lot of nice-looking healthy green vegetative fans connected to rhizome, and a lot of leafless chunks of older rhizome or rhizomes with only small, weak vegetative shoots. You can feel free to plant these other chunks in a separate part of your garden to propagate more iris, or mix them in with the cluster you intend to keep, or throw them out. They will bloom in 2 years if grown in good conditions. They’ll also root in your compost pile.
Should you trim the leaves off of bearded iris?
Some gardeners consider it good practice to trim the leaves back when transplanting iris. There are good reasons for it, but they might not apply to every circumstance. First of all, if the plants are left out of the ground for a while to be shipped or stored, the leaves will dry up and die back. Irises never go fully dormant, but it is a quasi-dormancy that allows the plants to live a long time without water. Trimming the leaves keeps them looking cleaner and saves a lot of space.
Second, in some climates, particularly those with rainy or humid summers, bearded iris are more prone to disease. The plants will be weakened temporarily by transplanting, giving diseases like iris leaf spot fungus an opportunity to spread. So if you see a significant number of streaks or brown spots on the leaves even before the plants are transplanted, cut them back halfway or more to remove some of that diseased tissue. In climates with dry, warm summers, this is not likely to be a concern.
In all other circumstances, I would make this your standard: after you transplant iris, you want the fans to stand up. Iris are adapted to hold their leaves perpendicular to the sun at midday to keep cool and avoid water loss, and fans that tip or flop over will scorch and brown. If they’re tipping over, cut them halfway down or low enough that they stand up. If they’re standing up already, it’s not necessary.
Planting
To plant your iris, dig a wide, shallow basin of the desired with and 2-4 inches deep. Pile the soil to the side. Place your prepped iris divisions in the basin about 6 inches apart at the tips, with the fans pointed outward—this is the direction they’ll creep along the ground as they resume growth, and this way they’ll spread away from each other rather than getting too dense. After you have them placed, you can invert a couple plants with the fans pointed towards the center of the circle so there won’t be a blank space there.
Use the soil you removed and cover the rhizomes. Iris like to creep along the surface of the ground, but if they end up being buried 1-2 inches deep, they’ll be fine. You might find it’s easier to keep them stable with a little soil covering, and subsequent growth will stair-step up to the surface so the rhizomes are growing at the depth that they want. Gently pack the soil with your hands enough to keep the plants from tipping over. Then, you can replace mulch on top.
Should you fertilize or amend the soil?
Bearded iris are tough plants that do well even in poor soil. That’s one of the things we like about them! Additionally, their roots leave channels in the soil when they die, allowing oxygen and nutrients to circulate deep into the garden. I prefer not to collapse those channels by tiling the soil unnecessarily.
However, if it looks like the clump of iris really depleted the soil (indicated by very weak plants at the center of the clump), you can consider sprinkling a little compost on top of the soil to gradually break down and fertilize the iris over time. You should do this after the iris are planted and covered by native soil. It really doesn’t take much compost to replenish soil nutrients and may not be necessary at all. Then spread mulch over the top, and water them thoroughly to settle the soil around the rhizomes and initiate new root growth.
Summary
- Bearded iris multiply quickly and the number of fans can triple every year. Clumps that become too dense can start to decline in the middle, and should be dug and divided every 3-4 years.
- Bearded iris is hardy enough to be dug or and divided throughout the year, but dividing clumps in June—just after they bloom—gives plants the longest time to re-establish before the next blooming season.
- Rhizomes creep along the ground at or just below the surface and form branching shapes that can be split into 2 to 6 inch segments, each containing 1 to 3 fans.
- Divisions with large green fans can bloom the next year. Older portions of rhizome that are still firm but don’t have much foliage may take two years to grow established enough to bloom.
- Growers in humid-summer climates may want to trim the foliage during transplanting to limit the growth of fungal diseases. Foliage that flops over will scorch in the sun and can be trimmed halfway. Otherwise, if the foliage looks healthy and can stand up straight after the plants are transplanted, it can be left intact, or trimmed part way down to clean it up.
- Bearded irises should be planted in clusters in a wide, shallow hole just 2 to 4 inches deep (not including mulch). Arrange rhizomes around the hole, several inches apart, with the rhizomes laying horizontally and the fans pointing upwards. Point the rhizomes in all directions with the majority pointed outward, then cover with soil.
- Irises typically do not need fertilizer and can grow in poor, compacted soil. Optionally, you can sprinkle a thin layer of compost on the soil to supply a nutritional boost.
- After transplanting, reapply mulch and water the plants in to get them established.