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When, why and how to dig and divide bearded iris

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.

Bearded iris are one of the most carefree and reliable garden perennials

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.

These rhizomes are still connected because they grew from the same original plants. The red lines are reasonable places to cut or break off portions for re-planting. If iris are divided and replanted in early summer, these divisions would be able to bloom next spring.

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.

Dividing iris will produce a lot of older portions of still-living rhizome. Some gardeners will discard them, but if you plant them they’ll produce shoots that bloom in 2 years.

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.

How to naturalize tulips for a carefree spring garden that returns and multiplies

Few plants evoke the joy and nostalgia associated with tulips, the quintessential spring flower with big showy blooms and a color palette broader than almost any other category of bulbs.

For many gardeners and designers, tulips are treated as a short-lived perennial or even an annual—one that requires extra planning since the bulbs must be purchased and planted in fall, leading to a show of spring color that is all too short compared to box-store annuals that bloom from May until frost.

That’s because many tulip varieties don’t perennialize well in common garden conditions. Though rugged wild populations of Tulipa gesneriana, the first tulip species dug up from the mountains of Iran and Turkey to be cultivated, are vigorous even in dry rocky soil, centuries of breeding and coddling in northwestern Europe have made them fussy. Many of these popular commercial varieties like carefully-controlled temperature treatments and soil components, resent summer rain, and in the garden they are prone to “shattering” (dividing excessively) in their second year into countless small bulbs that are each too small to bloom. The next year’s growth is mostly small spikes of foliage, and things continue to devolve from there. If that’s what growing tulips really entailed, I’d say why bother and put my focus on a different selection of plants.

But that scenario doesn’t have to be the case. The high-maintenance tulip cultivars, albeit popular, don’t need to be representative of a truly rugged and resilient genus of plants. With a little bit of discrimination and planning, tulips can be an excellent and carefree long-term investment in the garden in any region that has a cold winter and mild spring. Particularly in the semi-arid American West, tulips can be some of your most reliable and consistent low-maintenance perennials.

Tulips that return year after year: Getting your bulbs to naturalize

First, a definition. When it comes to bulbs, the word “perennialize” generally means flowers return for at least 3 to 5 years or, at best, they can return indefinitely but typically don’t increase. Since we can’t control nature, a bad year in which uncommonly destructive weather or an invasion of hungry rabbits and deer lops off the tops of the plants could permanently knock the population of perennializers down. Because it can’t return quickly, a simple perennializer will eventually have to be replenished. Meanwhile, the word “naturalize” means the plants multiply and spread like wildflowers, rebounding from almost any temporary setback.

The genus Tulipa is composed of dozens of species native to central Asia, cool mountainous parts of the Middle East, and southeastern Europe, in various climate regions that range from dry alpine to Mediterranean. In other words, tulips are native to climates that are a lot like western North America. These different climates share the presence of a cold winter, then a brief but mild spring with at least some precipitation or residual moisture from melting snow, followed by a dry summer. They often grow at high altitudes where temperatures can swing unexpectedly and snowstorms can occur well into spring. It’s why the ephemeral tulip evolved to be exceptionally tolerant of growing and blooming amidst occasional hard frosts and cold, and can emerge from the soil, grow quickly, bloom and return to dormancy within a span of two to three months before summer’s heat. Wild tulips tend to be tolerant of a broad range of climatic and cultural conditions, while domesticated tulips can be more particular, often resenting a wet summer or soil that is too rich or dense.

With that in mind, the most important step in achieving a tulip garden that naturalizes is choosing the right kinds of tulip to begin with.

For a perennial tulip garden that doesn’t need to be constantly restocked, right off the bat we have to give up on some of the most popular classes of commercialized tulips. That means to avoid building your garden out of things like towering single late tulips, showstopping double tulips, exotic fringed, viridiflora and rembrandt tulips, trumpet tulips, and the formal cup-shaped single early and triumph tulips. Sure, there are a few of these varieties contain cultivars that perennialize decently, and some might multiply well if you are fortunate enough to have the perfect soil and microclimate in some corner of your garden. But, in general, I recommend saving these short-lived varieties for an accent, planted here and there in a contrasting color for a striking standout effect that only requires a few bulbs. Meanwhile, I recommend sticking to tried and true multipliers en masse for the tulip garden’s backbone.

Fortunately, there are so many kinds of tulips in so many shapes and colors that the remaining classes—the naturalizers—give us a lot to work with.

Tulips varieties that naturalize

Greigii tulips

Greigii tulip ‘Calypso’ is a vigorous multiplier even in adverse conditions, with a tropical-hued flower that is hard to put into words. It blooms in early to mid spring before most other tulips, standing less than a foot tall. A cluster of 2-3 bulbs will eventually expand into a dome-shaped bunch. This image shows bulbs I planted with great success in a neglected corner of the yard.

My favorite class of tulips are hardy Greigii tulips, with big midseason blooms, sturdy stems and interesting patterned foliage that is attractive enough to appeal even when it’s not in bloom. Some varieties of Greigii tulip are vigorous enough that a small offset bulb can grow in early spring to produce a standard-sized plant and flower that is nearly as big as one from a commercially-bought bulb. Additionally, Greigii tulips tolerate wear and tear and grazing from herbivores at a level that would be fatal to other tulip classes.

Darwin hybrids

Darwin tulips are the quintessential tulip, coming in the familiar reds, yellows and oranges many of us remember from childhood. Newer Darwins are available in many other colors. They return and multiply well. This cultivar is ‘Golden Apeldoorn.’

Darwin tulips are the most popular and familiar of the classes of tulips that can multiply, and are most likely to be seen blooming in an old garden that hasn’t been restocked in a very long time. You might even see one or two of these tulips peeking out in an abandoned lot. Created by crossing vigorous fosteriana tulips with showy single late tulips, Darwin hybrids have a very “classic” and recognizable tulip shape, and bloom mid-spring. They exist somewhere along the threshold between perennializers and naturalizers—although they can multiply in many conditions and are easy to bring back year after year, they won’t usually form expansive clumps and could eventually lose the battle in a garden that becomes crowded out by more vigorous perennials or weeds. Still, in a garden that is even moderately tended, you can get them to maintain their numbers and even gradually increase your stock.

Emperor tulips

Fosteriana tulips have elongated flowers and bloom earlier than most other varieties. This unique cultivar, fosteriana ‘Purissima blonde,’ has variegated leaves.

Fosteriana tulips, also known as emperor tulips, are half of the Darwin hybrids’ genetic heritage and the side that gives them their perennializing ability. Fosterianas are big and showy early-spring bloomers that flower with a peculiar hourglass-shaped bloom, and spread their petals wide on a sunny day to resemble an oriental poppy as much as a tulip. They are solid naturalizers that will multiply in most circumstances.

Kaufmaniana tulips

Kaufmanniana tulips are the earliest-blooming tulips, and have distinct star-shaped flowers with striking patterns in the center. When the flowers close in the evening, they are reddish-pink.

Kaufmaniana tulips have an un-tulip-like “waterlily” shape, spreading their petals wide to reveal striking bullseye patterns in the center of a starlike flower. On a cold or cloudy day, their tightly-closed flowers appear slender and often show different colors on the outsides. The plants are short—five to eight inches tall—exceptionally tolerant of snow and cold, and bloom the earliest of any type of tulip, sometimes even before the daffodils do.

Species tulips

Finally, there are species tulips, a class that is really a catch-all for anything left over from all other tulip categories. While the other classes are, generally, different blends of a few closely-related species within the Tulip genus, each type of species tulip is a unique species, many of which are too diverse to hybridize—so there’s more genetic diversity in this class of tulips than in all the others put together. They are sometimes referred to as “wild tulips” since most varieties were only recently brought into cultivation. This category tends to contain lots of smaller plants—some as dainty as two inches tall—and holds some of the most aggressive and resilient multipliers (although not all of them will multiply as quickly). Many of them produce multiple flowers per stem or send up successive flowers from the same bulb. They can spread through your garden not only by offsets but by seed, and are diverse and unusual enough that visitors might be surprised to learn that some of your species tulips are tulips at all.

The best sites for naturalizing tulips

With the right varieties in tow, you have a wide range of options for planting tulips to return year after year. Although tulips will generally suffer in waterlogged soil, and tend to prefer a sandy soil, naturalizers will still naturalize even in poor clay, and don’t really require fertilizer as long as mulch or compost is periodically used as a top-dressing to recycle nutrients in your garden.

A basic rule will be that tulips prefer full sun to part shade—I’d say at least 4 hours of direct sunlight per day will work if you live in a mostly cloud-free climate like New Mexico or Colorado, but 6-10 hours of direct sun with a bit of afternoon shade is ideal. In the Pacific Northwest or Eastern U.S., you should aim for at least 6 hours of direct sun.

Remember that tulips emerge and bloom in early spring before deciduous trees have leafed out, meaning that most areas in your garden, even under trees, will count as full sun until the tulip season is on the wane. Planting around trees can actually be a great benefit to your tulips because the dappled shade that develops as the trees leaf out later in the spring can protect fading tulip foliage from heat and keep it green longer. That means the plant will have a chance to grow more and bigger bulbs for next year’s blooms.

On the other hand, the South-facing side of a home, fence or building may not be a great site for tulips in sunnier regions. The problems with this setting are twofold: the heat of a southerly exposure tends to break tulips’ dormancy too early in spring, leading them to bloom when hard frosts can still nip the petals. Meanwhile, midday heat dehydrates the petals, leaving them tattered and faded and shortening the lifespan of the flower. If unseasonably warm spring weather raises the temperature past 80, full sun at midday will zap the blooms of all but the species tulips.

One place I love using for tulips is a neglected area near a sidewalk or driveway where shoveled snow gets piled over the winter. Provided that it’s not loaded with salt, tulips appreciate the winter moisture and the insulation the additional snowpack offers. When the snow melts, the soil’s water storage capacity is often great enough to get tulips through the growing season with no supplemental irrigation at all. Then the garden bed dries out in summer when the plants are dormant—conditions that tulips love. In fact, any dry, unirrigated area could be ideal for tulips, which do not lose much moisture to evaporation when temperatures are cool in spring.

Spacing

One important aspect in getting tulips to naturalize well is ensuring that the planting bed is not too dense. While drifts or clusters are always attractive ways to arrange bulbs, the clusters should be loose—the bulbs of large tulip varieties should be started out 8 to 10 inches apart (or clusters of 3-4 bulbs spaced at least a foot from the next cluster), and smaller varieties should be planted 6 inches apart (or in clusters surrounded by a wider space).

The issue is that, after a couple years, each bulb will itself have developed into a small cluster, and too much competition can stunt them. Meanwhile, you’ll find that tulips are somewhat difficult to dig and divide without unearthing the entire garden; right after they die back into dormancy the bulbs are easy to locate, but their skins are still soft and digging can easily puncture them. Later in the summer or fall they’re firm and come out easily, but it’s more likely that the dead foliage showing you where the bulb is buried has already rotted away. Although digging and dividing bulbs is a good way to keep and propagate your collection, it can be difficult to be thorough enough to get all the bulbs out if your goal is to relocate an entire planting or radically thin your stock.

Care and cultivation

Beyond the proper plant selection and planting, tulips grow best with little to no interference. That means, most importantly, leaving the foliage intact after the blooms have faded. Planting tulips in and around other flowers and perennials helps distract from the boring green foliage and reduce the temptation to clip, but this is merely aesthetic. In truth, you don’t have to do anything except occasionally replenish the mulch.

After tulip flowers fade, it’s important to leave the foliage undisturbed so the plants can photosynthesize feed bulbs for next year’s blooms. The foliage sticks around for 1-2 months.

Some varieties of tulips are sterile or reluctant to go to seed, but sometimes you may see the seedpod begin to swell. Since most tulips take several years to grow from seed to flower and that’s under careful conditions, it’s appropriate to snap those off to promote energy flows down to the developing bulb, rather than up to developing seeds. The only exception I would make is for species tulips, which can sometimes go from seed to flower in 2-3 years and don’t seem to loose much momentum by making seed. Still, I like to remove all but a few pods if I have the time, just to give those bulbs and their offsets a little more oomph.

And that’s it. With a little investment upfront, you can create a dramatic and colorful spring garden of tulips, rewarding you, your neighbors and populations of pollinators for years or decades to come.

Summary:

Perennializing tulips bloom for years to come, but may eventually dwindle. Naturalizing tulips multiply and spread.

You can create a permanent, naturalizing tulip garden by choosing varieties that return and multiply. Species tulips, greigii tulips, fosteriana tulips, Darwin tulips and kaufmaniana tulips are most likely to naturalize.

To naturalize well, tulips prefer well-drained soil in dryish areas that receive part sun or full sun in early spring. A spot under a deciduous tree that doesn’t leaf out until tulips are done blooming might be an ideal garden.

To naturalize tulips, plant them in drifts spaced farther apart than the label usually calls for, so that the plants don’t compete with each other and have room to multiply.

Don’t trim or mow foliage after tulips are done blooming. Let bulbs recharge for next year’s blooms by leaving the leaves intact until they die on their own.

On larger tulips, cut off the seed pods after the blooms fade to direct the plant’s energy to next year’s bulbs. Small species tulips may be aggressive enough to spread and multiply by seed and by offsets at the same time.