Most perennials spread yearly and eventually will need to be divided to control their sizes and revitalize them. Plant division is also a method of plant propagation, so you can have more of the plants you especially like.
How do you know if the perennials in your garden need to be divided? During the growing season, observe the plants for reduced flowering, stem dieback, dead centers in the clumps, and an unhealthy appearance. The roots of overcrowded plants will be growing in a thick, tangled mat. If the plants are healthy in appearance, they do not need to be divided unless new plants are desired. Most perennials, on average, need to be divided every three to five years. Some, like chrysanthemums and asters, should be divided every year while others such as peonies and bleeding hearts, seldom, if ever, need division. Certain perennials, such as Lenten roses, should never be divided since once established they are difficult to transplant. The best time of the year to divide perennials is during the fall after they have finished blooming and have begun going dormant
A day or two before dividing the plants, water them thoroughly. The best method of dividing perennials is to use a shovel and dig down deep on all sides of the plant. Pry underneath the plants to lift them out of the ground, shake off the loose soil, and then remove the dead stems and leaves. Place the clump into a wheelbarrow, and then use a knife or pruners to separate the underground structures into individual plantlets. Perennials have different types of root systems that determine the method of division. Ones with spreading roots, such as Black-eyed Susan and garden phlox, have roots that lack a distinct pattern in their growth. They should be divided into smaller clumps each having a few viable shoots. Tuberous roots, which are the potato-like structures on plants, such as canna lilies and dahlias, should be divided by separating the individual tubers with a sharp knife. Each tuber should have a piece of the original stem and a growth bud attached to it. Those that spread by creeping stems, such as irises, (the stems growing underground are referred to as rhizomes and those above ground as stolons), should be dug and separated by pruners. Each division should have a few inches of the rhizome or stolon and a set of leaves attached to them. Clumping root systems, such as ornamental grasses, are divided using a sharp knife or a shovel to pry apart the roots to separate into smaller clumps.
Keep the divisions moist until planting. Prepare the planting area by tilling the soil, adding organic matter such as compost, peat moss, or top soil, and then applying some low nitrogen fertilizer such as 5-10-15. Plant the divided sections at the same depth they were originally growing, taking care to pack in the soil around the roots to eliminate air pockets. Cut back top growth to 6 to 8 inches from the ground. Apply a couple of inches of pine straw or pine bark mulch and water as needed.
Dividing perennials are beneficial in a multitude of ways and are necessary to have healthy, attractive plants.
Timothy Daly, is an Agricultural and Natural Resource Extension Agent with Gwinnett County. He can be contacted at 678-377-4010 or tdaly@uga.edu
