By Mary-Jane Pilgrim, Master Gardener
Happily, perennial plants increase in size over the years, and at some point they will benefit from being divided. In general, the best time for this task is in the spring when they have just emerged from their winter hibernation.
Perennials should be divided when:
- They have started to die out in the middle
- They don’t flower as well as they used to due to congestion or the roots grew to be old and woody
- They have used up all of the nutrients in the soil near them, resulting in stunted growth, yellowish leaves or lack of bloom
- They are infested with weeds
Rule of Thumb
One rule of thumb for division is this: perennials that flower between early spring and mid June are best divided in early fall. Perennials that flower after mid-June are best divided in the spring.
Summer and fall-flowering perennials have the whole spring and early summer to recover from being divided, and most will give you an excellent flower display the same year.
Three plants that prefer to be divided at other times are Peonies (fall only) and true Lilies (mid to late fall). Daylilies (Hemerocallis) can be divided at nearly any time.
The first time or two that you divide perennials will be a bit nerve-wracking and anxious. This is normal!
The basic steps of dividing are simple. Once your plant shows signs of growth in the spring (an inch or two of new shoots is fine), dig up the entire clump with a deep shovel. Try to be generous and get as many thick roots as possible. Dig all the way around the clump, then pry it out of the ground. Put down a tarp somewhere handy, and transport your clump there.
Try and knock off any loose soil. Find a knife — a Hori-Hori knife, root knife or even an old bread kniofe. Look closely at your clump in an attempt to find a natural point where it can be easily separated. Cut directly down the center with your knife, from top to bottom. Once it’s split in two, then look at each half to see if there is a sensible spot to cut yet again, then split these each into two. Try and keep the sections generally of a good size, say the diameter of your fist or larger.
Discard old and woody roots from the middle (add them to the compost pile).
Once your dividing task is complete it’s time to replant the pieces. Try to plant them at approximately the same depth they were growing. Water them in well at planting time, then maybe once a week for the first month unless spring rains are generous.