The roots of perennial plants live for several years. Herbaceous perennials such as daylilies die back to the ground every year. Deciduous, woody, perennials have branches and stems all winter but lose all their leaves. Evergreen perennials such as boxwood keep their stems and leaves during the winter.
Perennials not only return year after year, giving our gardens reliable beauty, but they increase in size providing more backbone to our gardens with more stems and flowers. Perennials grow taller, larger and wider, taking up more room as the years go by.
Spring is the ideal time to divide many of them.
There are four reasons to go to the work of digging, dividing and replanting them, including: Some perennial clumps die out in the middle with age, others stop blooming and need to be rejuvenated, some get invasive weeds and grass in their roots, and others have grown too large for their location.
In general, the perennials to divide in spring are those that bloom in the summer or fall. Spring and early summer blooming perennials are divided in the fall.
Divide spring blooming perennials after they finish blooming, including: Primrose, Rock Cress (Aubrietia), Basket-of-Gold (Aurania), Moss or Creeping Phlox (Phlox sublata).
Perennials to divide in the spring include: Lady’s Mantle (alchemilla), Artemisia, Autumn Joy Sedum and other stonecrop, Blanket Flower (Gaillardia), Black-eyed Susan, Bugleweed (Ajuga), Catmint (Nepeta), Coreopsis, Cornflower, Daylilies, Dianthus/Pinks, Phlox, Joe Pye Weed, Hardy Geraniums, Hosta, Hellebore - Lenten Rose, Obedient Plant, ornamental grasses (Maiden Grass (Miscanthus) and Fountain Grass (Pennisetum), Mums, Pachysandra, Purple Coneflower, Salvia, Shasta Daisy and Yarrow.
They will recover after being divided and most will bloom as usual.
Wait until fall to divide Peonies, Oriental Poppies and true Lilies unless you are willing to sacrifice some flowers.
Whether you take on the task to improve the health of the plant or divide it to make more plants, plan to dig up the entire clump as soon as you see signs of growth. Dig up as much of the root as possible by putting the shovel in the ground about 4-inches beyond the shoots. Dig all the way around the clump and then pry it out of the ground and move it to a tarp (a retired shower curtain works well).
Remove extra soil by dropping the clump or rinsing the roots with water from a hose. With the soil removed you will be able to make cuts into the mass of roots.
Use a knife to cut into the clump of roots. You can cut it into dozens of small pieces but most of the time decent size pieces will be more likely to thrive than tiny ones. Look at the roots and try to identify logical places to cut. If that is not obvious, begin by cutting the clump in half, then in half again. Each piece that you plan to replant should have healthy roots and shoots that were above ground.
While it is difficult to throw away pieces of plants we love, scraggly bits probably will not become plants. Replant the healthiest looking cuttings from the outer edges of the parent plant. Those roots will be younger and will recover more quickly. Put the older roots in the compost pile.
Plant the divisions in prepared soil at the same depth they were growing as part of the original plant. Water them in and add more soil as needed. Expect to give them extra care this year just as you would any new plant.
If there are more divisions than you can use, plant them in pots. They can be back up plants for your garden or gifts for other gardeners.