Time to Divide Many Perennials

By Marilyn Loser
2014 May 28

Over the long Memorial Day weekend I spent time dividing perennials. Especially in our cold climate, spring is an ideal time to carry out this garden activity.

As I walked around my garden I noticed a number of large clumps of green and some were crowded together. I could tell there were dense root systems underneath with little room to expand. I also figured that the healthy plants had used a lot of the soil nutrients so I would need to add new soil, compost and/or soil conditioner.

I soaked the clumps a few hours before I began working.  Digging up plants is stressful to the plant (and perhaps to the gardener). Saturated roots help the plant stay hydrated and make the soil easier to dig.

In one garden I dug up a one and a half foot round clump of Blue Fleabane (Erigeron ‘Azure Fairy’) and two clumps of Dianthus that were a bit smaller. To dig them up, I used a spading shovel which is narrower than the typical shovel. Usually I dig straight down six-eight inches all around and about three inches away from the clump if there is room. I then switch to a spading fork, which is sturdier than a typical pitch fork, to reach under the plant and lift it up.  More roots stay attached when I use this method instead of sticking with the shovel.

I then laid the clumps on the grass (some people use a tarp) and inspected them. All three were very dense and I couldn’t break pieces away by hand.  I couldn’t even divide them with sharp shovel thrusts so I cut them into pieces with a large knife. You could also try using an axe for tough clumps. It’s important to make sure each division has leaves and roots and that any dead parts are discarded. I divided the Fleabane into four parts and the Dianthus into six parts.  I added a mixture of top soil and compost and replanted one of each.

I’d already chosen places for the other divisions and dug them in trying to make sure they were planted evenly with the surrounding soil. I watered all spots thoroughly and will watch them over the next few days to see if they need more water than the watering systems provides.

Other plants such as Jacob’s Ladder (Polemonium caeruleum) and Meadow Anemone (Anemone candensis) are quick spreaders and their root systems are easy to dig and separate. You don’t need to dig an entire clump; just use a trowel to remove a portion and leave the rest alone.

Some species, such as Stella D’oro lilies (Hemerocallis stella d'oro) bloom less vigorously as clumps become large.  I usually divide them when they get about 10 inches wide.

Some perennials tend to die out in the center as they spread, leaving an unattractive doughnut shape. In my garden, this happens with Snow in Summer (Cerastium tomentosum), creeping Veronica (Veronica oltensis), and low Sedums. These three are easy to deal with as the clumps pull apart with little effort. 

I usually have some winter die-off, so can replace the dead plants with new divisions. If you have a small yard you may need to share the left overs with a friend. Some cities or gardening clubs have plant exchanges. I think it’s a great idea, but I seem to be too busy tending my garden in the spring and don’t have time organizing a formal exchange.

A general rule of thumb is to divide plants when they are not blooming. I wait for early bloomers to finish blooming before dividing them.  This includes Grape Hyacinths (Muscari), Creeping Phlox (Phlox stolonifera), and Golden Banner (Thermopsis montana).

Traditionally, the time for dividing Bearded Iris is shortly after flowering. But if you have stubborn clumps that refuse to flower, then you might as well go ahead and divide them in the spring, since they likely won’t bloom this year anyway. Several gardeners I know cut about a third off the tops of the Iris leaves when transplanting. The idea is that the roots will recover sooner as they have less leaf mass to support. ).   Peonies are usually divided in the fall and Oriental Poppies (Papaver oriental) after they’ve bloomed and gone dormant in July or August.

Some plants don’t divide and transplant well.  These include Bleeding Heart (Cicentra spectablilis) and Butterfly Weed (Asclepias tuberosa).

"And when your back stops aching and your hands begin to harden...You will find yourself a partner in the Glory of the Garden." Rudyard Kipling