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Timing, technique play key role in dividing perennials

  • Martha Proctor
  • Do you know the clues that gardeners note when determining which perennials need to be divided?

    Some gardeners suggest that it's best to divide a plant at the end of the year after the plant has looked its best. Alternatively, these symptoms indicate the plant would benefit by being divided:

    • If growth in the center of the plant is dying out;
    • If flowering is reduced or flowers are smaller;
    • If there's loss of vigor;
    • If the plant needs staking or has outgrown its bounds.

    The growing conditions — the type/pH of the soil and the availability of moisture and nutrients — greatly affect the timing for dividing perennials. Many gardeners attest to the old saying: "the first year they sleep, the second they creep, and the third they leap."

    In general, bearded iris, coreopsis, and Shasta daisies are examples of perennials that need division every one to three years. Some perennials (day lilies, bergenia, evening primrose) can easily go five years or more before becoming overcrowded, while others — peonies, hellebores and large hostas — rarely, if ever, need to be divided.

    When dividing perennials, timing and technique are important. Division can be done at any time of year if the new plants are watered so that the roots receive adequate moisture. However, plants adapt best when the soil is warmer than the air temperature during part of every day. This typically occurs just before peak daffodil season in spring and as the nights become cooler in early fall. Such conditions allow the new roots to grow while the tops stay low and out of the sun and wind. Division often is started when growth resumes in the spring; but if divisions are made in the fall, plants have more time to set new roots before facing hot summer temperatures.

    Make sure that the clumps to be divided aren't drought-stressed; if the soil isn't moist, water the plant thoroughly the day before you divide it. In the fall when the plants have top growth, reduce transpiration plus improve your visibility by cutting the clump's foliage back by half to two-thirds before dividing.

    Tools for the job depend on the size of the plant and the density of the clump. Smaller plants such as geraniums can be split by hand. Plants with a small, fibrous root ball such as asters or lamb's ears can be divided into viable pieces with a spade.

    Start by placing a spade at the center of the crown of the root ball, then with a quick motion, split the crown in half. Repeat the process until you split off the appropriate number of pieces for the size of the plant being divided. Two pitchforks inserted in the center of a plant and drawn apart works best with larger, more fibrous clumps such as day lilies. Perennials that tend to form huge clumps, such as ferns and grasses, may require the use of a handsaw.

    Consider a plant's shape and condition before taking a spade or sharp knife to it. Note the natural dividing points between stems and root sections. When the time is right, break off pieces that would fit into a quart- or gallon-sized container because divisions in these sizes grow more vigorously and tend to produce stronger, longer-lasting blooms. Each new plant must have its own roots and at least one bud or shoot.

    If an overgrown clump is just halved, it will more than double its size in a season and need dividing again next year. Replant only the healthiest pieces — discard the old, dead center and trim off any discolored stems and eroded crowns and roots. Note that blooming plants may not be able to grow as many new roots as nonblooming plants.

    It is best to plant your new divisions soon after dividing them so they don't have a chance to dry out. The divisions should be kept moist and shaded while the new planting site is prepared. Place them in buckets in a cool (50 degrees is ideal), shaded place if they won't be replanted right away. Cover the containers with newspaper to retard moisture loss. Sprinkle the newspapers with water to dampen them if the roots appear to be drying out. If the roots do dry out, soak them in a bucket of water for an hour before replanting.

    When replanting, renew the soil with compost in the area where the divisions are to be planted to provide nutrition, reduce pest problems and maintain fertility. Place a division into a hole that is at least as wide as its roots when spread out. Avoid forcing a new plant to fit into a hole that is too small or planting it upside down because doing so will upset the plant's natural regrowth mechanisms. If dividing plants in the fall, mulch should be placed around the divisions to minimize weeds and help moisture retention in the soil.

    Look around your garden this fall — division is a handy skill to learn as a way to rejuvenate tired plants while improving their bloom production, encouraging stronger stems and decreasing disease problems. As an added bonus, dividing perennials gives you lots of extra plants, which you can use to fill new borders or share with friends.