With few exceptions, most perennials slowly decline in vigor unless they are rejuvenated by being divided every few years. A general rule of thumb is to divide spring-blooming plants immediately after they flower and summer and fall bloomers in early spring when they have about three inches of top growth. In the South, however, you may want to divide spring and summer bloomers in the fall so they can readjust during cool weather. It is good to time your division of plants so that their root systems can re-grow before stressful weather arrives. It is best to divide perennials at least four weeks before hot or freezing temperatures arrive.
You can turn one mature perennial into many by dividing the roots. Plants with woody root systems such as astilbe can be divided by using a spade to split the root ball into several sections containing crowns. The sections can then be spaced out and replanted to increase the size of the plantings, used elsewhere, or shared with friends.
Fast growing plants need more frequent division than slow growers. The appearance of the plants will be an indicator of that need. Clumps resembling donuts, with active growth on the outer edges and dead centers, should be divided.
Techniques for dividing plants vary according to the growth habits of the plant. Some plants, such as chrysanthemum and shasta daisy, have shallow, fibrous root systems. Once the clump is dug up, you can easily pull it apart into many sections with your hands. Study the top growth to locate individual crowns, then drive the blade of a sharpshooter through the space between the crowns, dividing the clump into several sections. For fleshy-rooted plants such as daylily and hosta, you will cause the least harm to the roots if you divide them with two garden forks. Plunge the forks back to back into the middle of the clump, then press the handles outward, prying the clump into two parts. With your hands, pull apart individual crowns with their roots attached. Large ornamental grasses, such as maiden grass and zebra grass, have such tough roots that you might have to use an ax or a chainsaw to divide them. Fortunately, these need dividing only once every ten years.
You may want to use the occasion of dividing to improve or replenish the soil before replanting the divisions. Turn the soil over with a fork and add organic matter and super phosphate. Plant divisions as soon as possible, taking care to protect them from drying air and sun. If they cannot be replanted immediately, heel in the divisions in a temporary location or in a pot of soil until you can plant them. Water them and mulch the soil well.
(Source: “Better Homes and Gardens Complete Guide to Flower Gardening”)
Barbara Thornhill, former Smith County Master Gardener
Texas A&M AgriLife Extension Service