Divide Your Perennials and Conquer Your Garden’s Blank Spaces

Who has that kind of time to plan for that kind of time?”

I came across a video on Instagram earlier this summer where a gardener was showing how easy it is to divide herbaceous perennials that are taking over areas in the garden and utilize those divisions elsewhere in their landscapes — a somewhat overlooked money-saving fact about many perennials, as she pointed out.

Her approach was bold. There was no hesitation to shove a spade straight through the crown of a flowering daylily, separating a sizable part from the whole, on a mission to get the job done all while the summer sun soars high above. So, pretty much doing the opposite of what any garden book would suggest, which is to wait until autumn for when the weather becomes overcast and damp and the plants begin heading into dormancy.

I felt conflicted. On one hand, I appreciated the reminder that gardening is often about having the confidence to do the task and not always about waiting for “the right time” to get that task done. On the other hand, the idea of tackling such a strenuous project during the heat of summer sounded dreadful. From experience, dividing perennials requires the type of muscle and stamina that I find impossible to summon on 80-degree days. And so, if anyone were to ask me, I’d give them the textbook answer: Just wait for fall.

Now, as we inch toward the equinox, I’m spending the remaining summer days taking inventory of all the perennials in my life that need some attention. Groups of hostas here, a patch of poorly performing irises there. Mounds of Shasta daisy that were collapsing under their own glory by mid-summer can definitely be parceled up. And lest we forget the herb garden with chives, marjoram, and mint encroaching on the territory of walkways and their less vigorous neighbors.

Aside from the fall offering a more comfortable climate, for both you and the plants, considering it’s the time of year when we’re also planting out our spring bulbs for next season, it’s the perfect time to consolidate digging. But even if for some (unsatisfactory) reason spring bulbs aren’t on the garden to-do list, you may find you need to do an excessive amount of digging anyway, so might as well toss some tulips into the mix.

Upon realizing I would need to give some undivided dividing attention to the perennials, I decided to consult one of the massive gardening books in my collection, “America’s Garden Book” by Louise and James Bush-Brown. I figured since it’s nearly two inches thick, it would most likely have the information I was looking for, and a little more information that I could do without.

Where I intended to find some confirmation on proper timing and technique in their chapter about perennials, I instead found them saying things like “at some point, perhaps every five to six years, it is advisable to lift plants and rearrange the perennial garden entirely.” The reason being that “groups of plants become too crowded … and the fertility of deep soil layers requires restoration by incorporation of compost or well-rotted manure.”

Five to six years? Rearranging the entire perennial garden? They talk about it as if it’s as simple as moving the living room furniture around. Do we not curate our garden beds according to what fits where we force it to fit over the course of many years? The approach seems a bit excessive, and who has that kind of time to plan for that kind of time anyway?

A very good point is made, however, that these plants indeed do become overcrowded and deplete soil fertility. A good example is bearded iris, where, over a few seasons, it becomes apparent that without attention they quickly lose vigor and struggle to flower if the rhizomes aren’t divided on a regular basis. Same goes for summer phlox and hardy mums.

A majority of perennials seem to be able to remain vigorous for many years without division. Peonies fall into this category; however, there are plenty of good reasons for wanting to divide them, including your desire for more peonies in your garden.

Depending on where you get your information, some texts like “America’s Garden Book” will say something like “peonies do not readily transplant,” and I regard this as nothing more than industry fear-mongering. Tree peonies, perhaps, will be a tough sell in a new location, but the tuberous sort can be transplanted just fine so long as a healthy number of roots are taken up with them. The drawback with transplanting and dividing peonies, as with iris, is that you’ll more than likely be losing the next season of blooms as they reestablish themselves.

While fall is a great time to go about division projects, some perennials like sedum and rudbeckia could still be going strong and may be of visual interest well past the first frost. It wouldn’t hurt to wait until spring to split them up. Then again, you may feel like now, as we near the end of summer, is as good a time as any, and you would be correct.