Each year perennial flower gardeners must decide whether or not to divide their plants. The University of Minnesota (U of M) offers advice to help make that decision. Diagnose your plant If the center is woody or dying, or it does not have many blooms and has smaller flowers, then it is time to divide. Why divide? o Division will rejuvenate your plant and stimulate new growth and more vigorous blooming. Overcrowded plants or a plant that is too large compete for nutrients, air circulation and water. Division helps control this and keeps plants looking healthy. o To control the size of the plant. Since plants grow at different rates, division helps to keep plants that spread rapidly under control. o Division is an easy way to increase the number of plants in your garden. What time of year? Many perennials should be divided in the spring because the plants are full of stored energy and there is more moisture in the soil. This gives the plants all summer to grow and develop a strong root system before winter begins. General guidelines o Early spring every 1-3 years: Aster, beardtongue, beebalm, carnation (dianthus), common sundrops, coralbells, centaurea, delphinium, fernleaf, bleeding heart, foamflower, garden mums, obedient plant, painted daisy, Shasta daisy, spiderwort, tickseed (coreopsis) and yarrow. o Early spring every 3-5 years: Astilbe, bellflower, blanket flower, catmint, rudbeckia, gay feather, gooseneck loosestrife, mallow, sea thrift, speedwell and yellow loosestrife o Infrequently, early spring every 5-10 years: Cranesbill, goatsbeard, hosta, Japanese anemone, lady mantle, lungwort, meadow rue, meadowsweet and heliopsis o Anytime in late summer or early fall: Daylily, Jacob’s ladder, peony, tall phlox and Siberian Iris. o Do not divide: Baby’s breath, balloon flower, bugbane, butterfly weed, clematis, evening primrose, false indigo, flax, gentian, lupine, monkshood and Russian sage. o Divide only to propagate: Bugbane, globeflower and yucca. How to divide perennials Dig up the whole plant, determine the type of root system it has. A tough, dense root should be cut apart with a sharp knife, sod knife or spade. A root that is tangled and fibrous can be separated by hand or by inserting two garden forks back-to-back pulling them apart. Discard any soft roots or woody, old centers. Use only good, healthy growth with at least two or three new shoots for each division. When placing back in the ground, put them at the same depth and water them well. Keep the soil moist (but not so moist that it promotes root rot) for several weeks while new roots are forming.
