Dividing Perennials: Only When Needed

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Many gardening books and Web sites insist on the importance of dividing perennials according to a specific schedule. For each variety described, they inevitably recommend dividing it every three years, four years or five years. There never seem to be any exceptions to the rule. 

I’ve always wondered about the reason for this obsession with division. After all, in the wild, nobody divides these plants and they manage to thrive anyway. Why then should I have to divide them just when they’re starting to look their best? (Because it takes about three or four years for most perennials to really reach their peak appearance.)

Personally, I don’t divide my perennials according to somebody else’s schedule: I only divide on an “as needed” basis. If a perennial has expanded so much that it encroaches on its neighbors, I divide it. If it starts to bloom less because it has become too dense over time, I divide it. If it dies out in the middle leaving a ring of growth around a dead center, I divide it. If I need more plant material to start a new flower bed, I divide it. But if it is beautiful, productive, floriferous and not interfering with its neighbors, I just let it do its thing. As a result, I’ve never divided most of my perennials: they simply don’t need it.

Some Do, Some Don’t

I find it very hard to generalize about dividing perennials, even among related plants. For example, some astilbes and daylilies start to decline after four or five years and bloom less abundantly. I’ll divide these … or replace them with something more durable. But I have other astilbes and daylilies that are still beautiful 20 years after I planted them, so I just let them be.

This Hosta ‘June’ is almost 25 years old, has never been divided and is more and more beautiful every year. Photo: laidbackgardener.blog

Dividing according to need makes even more sense with slow-growing perennials. When a book or Web site recommends I divide my hostas and peonies every five years (I’m not exaggerating: you really do find that kind of advice!), I can scarcely believe it. After all, these plants grow with all the speed of an anemic snail: it takes at least ten years before the plants look their best … and even after ten years, the less often you divide them, the more beautiful they become. There are spectacular hostas and peonies that have not been divided in 50 years or even longer: they are much more beautiful than the scrawny specimens divided every five years.

I suspect that the authors who encourage unneeded plant maintenance have a bit of a puritanical element to their nature: sort of an “only through hard work can you reach heaven” attitude … and dividing perennials can be very hard work (as you know if you’ve ever tried dividing some of those big perennial grasses)! Well, I get to enjoy my piece of heaven right here on earth thanks to my flower beds and they have very little need of my attention!

The Exceptions

Gaillardia or blanket (Gaillardiagrandiflora): this short-lived perennial practically blooms itself to death. Divide or take cuttings every 2 years to keep it going. Photo: sunlightgardens.com

Obviously, there are exceptions to every rule. Thus there are some short-lived perennials, such as gaillardia, perennial flax, and certain coreopsis that really should be divided frequently, in fact, every two or three years; otherwise they tend to disappear for no apparent reason. But rather than chopping the plant into pieces and thus reducing its bloom for the summer, I find it much easier to take stem cuttings. Just cut a stem, remove any flowers or flower buds and root it in moist soil. It couldn’t be easier … and it doesn’t stress the mother plant which will then continue to bloom.

Columbines are charming but short-lived perennials that don’t like to be divided, so just let them self-sow. Photo: http://www.whiteflowerfarm.com

And there are other perennials that are short-lived; yet that hate being divided. Nor do they grow from stem cuttings either (or at least, not very vigorously): lupines, columbines, mauves, etc. These are the easiest short-lived perennials of all: simply let them self-sow … and they do so abundantly!

So here in a nutshell is the laidback gardener’s rule to dividing perennials: only divide perennials when they really need it. ’Nuff said!

Fall is for Division

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20150903AYep, while it’s true you can divide a perennial in almost any season, summer division is more stressful for the plant… and often, because of the heat, for the gardener as well. So usually gardeners divide their perennials in the early spring, just as they are starting to sprout, or in the fall, in September or October, as they prepare for their winter rest.

Also, it isn’t wise to divide a plant while it’s blooming or about to bloom, so it’s usually best to divide fall-blooming perennials in the spring. That also means the ideal season for dividing spring bloomers and early summer bloomers is right now, in the early fall.

As for plants that bloom in July or early August, you can divide them either in spring or fall.

Why Divide?

Actually, in spite of the fact that many books insist on the importance of dividing perennials every 3, 4, or 5 years, most really don’t need division. The vast majority of perennials will continue happily on their way, blooming up a storm, as years go by… as long as you keep neighboring plants from invading them. But there are many good reasons why you should consider dividing perennials. Here are the main ones:

  • The plant starts to flower less over time (yes, some perennials do start blooming less if you don’t divide them regularly, including some, but not all, daylilies and astilbes). Division rejuvenates these types and they start to bloom abundantly again.
  • The plant dies out in the center;
  • The plant has become too big and you want to reduce its volume;
  • It’s starting to invade its neighbors and you want to put it back in its place;
  • It’s a short-lived perennial (like a gaillardia or certain coreopsis) that needs to be rejuvenated every 2 or 3 years to survive;
  • You have to move the plant anyway (for various reasons) and it would simply be easier to move a small section rather than the entire plant.

Complete Division or Just a Section?

Division doesn’t have to mean digging up the entire plant.

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For most perennials, division simply means digging up a clump from the outer rim of the mother plant.

If you just want a single clump to fill in a blank spot or give as a gift, or if the plant has sent up offshoots, you can leave the mother plant in the ground and simply dig up a section from its outer rim. It’s like slicing a piece of pie: take a sharp shovel or spade, push it into the plant  once, twice, then a third time (I always “harvest” from the back, so it won’t be as noticeable) and cut out a chunk of plant with attached roots. Now just replant the section somewhere else and fill in the hole left behind with soil. The plant will soon regrow and you won’t notice the difference. Hostas, for example, are typically divided this way.

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Sometimes you’ll want to dig up the entire plant so you can divide it.

Sometimes you do want to dig up the entire plant. Maybe you need multiple divisions or you have to move the plant anyway. Or its one of those perennials that are awkward to divide unless you can clearly see where one plant ends and the other begins (peonies are the best example of this).

If so, using a spade or shovel, dig up the entire plant, with as complete a rootball as possible. Now slice through the plant with the spade, cutting it into 2, 3 or 4 sections (or more). If it’s a smaller perennial, you can often just pull the plant into sections with your hands.

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With peonies, you need to rinse off the roots to see what you are doing.

In the case of peonies, I suggest rinsing the soil off the rootball and carefully cutting between the different plants, making sure there are 3 or 4 buds on each new division.

Some perennials have a very tough or woody rootball and a spade won’t be enough. If so, use an ax, a sharp knife, or a saw to cut the plant into sections. Many ornamental grasses fall into this category, as well as goat’s beard and even some astilbes.

Followup

No matter how you choose to divide your plant, you’ll have to replant each division. Do so without burying the collar (the point where the roots joint the stem)… in other words, plant it at the same depth as was the mother plant.

And after you replant, water well and apply mulch. The young plant will require additional watering during periods of drought until it is well established, usually the following season.

Shrubs Too

Although in my explanations so far have only covered perennials, the same methods of division also apply to those shrubs that lend themselves to division. Many don’t, producing a dense woody base you can’t divide. But those that produce multiple offsets at the base, or that spread over time, such as the smooth hydrangea (Hydrangea arborescens), including the very popular cultivar ‘Annabelle’, are easy to divide, although an ax is often necessary to separate plants.

The same rule applies: divide spring-bloomers in the fall and fall-bloomers in the spring.

Do You Really Have to Divide Perennials?

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20150511AMany gardening books emphasize the importance of dividing perennials according to a specific schedule. For each variety described, they inevitably recommend dividing it every three years, four years or five years. There never seem to be any exceptions to the rule.

I’ve always wondered about the reason for this obsession with division. After all, in nature, nobody divides these plants and they manage to thrive anyway. Why then should I have to divide just when they’re starting to look their best? (Because it takes about three or four years for most perennials to really reach their peak appearance.)

Personally, I don’t divide my perennials according to somebody else’s schedule: I only divide on an “as needed” basis. If a perennial has expanded so much that it encroaches on others, I divide it. If it starts to bloom less because it has become too dense over time, I divide it. If it dies out in the middle leaving a ring of growth around a dead center, I may divide it (or not, depending on how it looks). If I need more plant material to start a new flower bed, I divide it. But if it is beautiful, productive, floriferous and not interfering with its neighbors, I just let it do its thing. As a result, I actually rarely divide most of my perennials: they simply don’t need it.

I find it very hard to generalize about dividing perennials, even among related plants. For example, some astilbes and daylilies start to decline after four or five years and bloom less abundantly. I’ll divide these… or replace them with something more durable. But I have other astilbes and daylilies that are still beautiful 20 years after I planted them, so I just let them be.

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This Hosta ‘June’ is almost 20 years old, has never been divided and is more and more beautiful every year.

Dividing according to need makes even more  sense with slow-growing perennials. When I find a book that recommends I divide my hostas and peonies every five years (I’m not exaggerating: you really do find books that recommend this!), I can scarcely believe it. After all, these plants grow with all the speed of an anemic snail: it takes at least ten years before the plants look their best… and even after ten years, the less often you divide them, the more beautiful they are. There are spectacular hostas and peonies that have not been divided in 50 years or even longer: they are much more beautiful than the scrawny specimens divided every five years.

I suspect that the authors who encourage excessive plant maintenance have a bit of a Calvanistic element to their nature: sort of an “only through hard work can you reach heaven” attitude… and dividing perennials can be very hard work (as you’ll know if you’ve ever tried dividing some of those big perennial grasses)! Well, I get to enjoy a piece of heaven on earth thanks to my flower beds and they have very little need of my attention!

The Exceptions

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Gaillardia x grandiflora: it practically blooms itself to death. Divide or take cuttings every 2 years to keep it going.

Obviously, there are exceptions to every rule. Thus there are some short-lived perennials, such as gaillardia, perennial flax, and certain coreopsis that really should be divided frequently, in fact ever two or three years, otherwise they tend to disappear for no apparent reason. But rather than chopping the plant into pieces and thus reducing its bloom for the summer, I find it much easier to take stem cuttings. Just cut a stem, remove any flowers or flower buds and root it in moist soil. It couldn’t be easier… and it doesn’t stress the mother plant which will then continue to bloom.

And there are other perennials that are short-lived, yet that hate being divided. Nor do they grow from stem cuttings either (or at least, not very vigorously): lupines, columbines, rose campions, etc. These are easiest short-lived perennials of all: simply let them self-sow… and they do it abundantly!

So here in a nutshell is the laidback gardener’s rule to dividing perennials: only divide perennials when they really need it. ‘Nuff said!