Many 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.
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!
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!