“We are planning to sell our house later this year and I am trying to spruce up the garden without spending much money. I would like some advice on dividing perennials to fill in some open spaces in my borders and then to apply it to my new garden in future years.”
— Robert Appleton, Morton Grove
Most perennials are best divided in spring when they are showing a couple inches of growth, so April is a good time to do it. A general rule of thumb is to divide perennials that flower from early spring until the middle of June in early fall.
I have divided these in early spring and shortly after they finish flowering with good results. The plants then have an entire season to recover before flowering again.
Perennials that flower after the middle of June should be divided in early spring and will often flower the same year after being divided. With your goal in mind, you should be able to successfully divide perennials now as needed to fill in your empty beds.
Making the divisions early in spring gives the plants more time to recover and look their best when your home is put on the market. It is best to avoid dividing plants when they are flowering.
There are some plants that are best divided in a more specific time frame. For example, peonies are best divided in late fall and true lilies (Lilium) in mid to late fall. Hostas and daylilies can be divided at nearly any time. Once, I left hosta divisions above ground over winter and then planted them in spring when they started growing.
I do not divide the perennials in my garden on any kind of routine schedule but do monitor the plants for needs. If a perennial such as an ornamental grass or Siberian iris has developed a dead center, it is time to divide and replant the healthy portions from the outer edge.
Some perennials such as Zagreb threadleaf tickseed (Coreopsis verticillata) will continue to expand and take over sections of your bed without occasional dividing.
I also divide certain plants when I need to fill in other parts of the bed. You may have a clump of perennials like a bearded iris that has stopped or significantly reduced flowering. These are good candidates for dividing. Bearded iris is generally divided shortly after flowering in July or early August, but if they have not been flowering well, go ahead and divide them in spring.
A lack of flowering, especially when we’re talking about sun-loving plants, can be caused by increasing shade from nearby trees.
A sharp, flat spade works well for dividing perennials. New spades generally do not come with a sharp-enough edge, and I find most gardener’s spades to be very dull.
It can be difficult to break up a clump of perennials if your spade is dull. Use the spade to cut around the perennial’s root ball, about 2 to 4 inches around where the shoots are coming up and at an angle that goes under the plant.
Once you have cut the roots, gently pry the clump out of the ground. It is easier to split the clump if you cut off the bottom “point” of the root ball to make it flat. At this point, use the spade to cut off any excess soil that is on the clump. Some perennials have dense roots and take some work to get out, while others are more delicate and will require some care not to break them apart as you lift them.
Use the spade to cut up the clump into smaller sections. When dividing a plant, choose the strongest-looking parts of the plant to replant and discard the rest.
Use a tarp to put the divisions on to make cleanup easier.
For more plant advice, contact the Plant Information Service at the Chicago Botanic Garden at plantinfo@chicagobotanic.org. Tim Johnson is senior director of horticulture at the Chicago Botanic Garden.
