Dividing Your Perennials Will Multiply Yard’s Options

One attraction of perennial flowers is their perennial nature--a single planting may last for years.

But “perennial” does not necessarily mean that the plants will last forever. With age, some perennials spread to form clumps whose centers die out as new growth pushes out the edges. The crowns of other perennials inch upward out of the ground each year, eventually dying from exposure to the elements.

The result, in either case, is fewer flowers. When this happens, it’s time to divide the clump.

Wait until you see new, green growth in spring before dividing a perennial clump. The more vigorous, young growth, which is what you are going to save, will be obvious. Don’t wait too long, though, or the plant will be shocked by this rough treatment.

To divide a clump, work around the edge with either a shovel or a spading fork, thrusting the tool into the ground at an angle to get under the clump. Push down the handle to lever the clump up and shake dirt loose from the roots. Eventually, you will have the clump laying on top of the ground.

Before pulling apart the clump, cut just the crown--not the roots--into pieces with either pruning shears or a sharp knife. Next, separate the crown pieces, teasing apart their attached roots.

Use your hands--or two garden forks held back-to-back in the center of the clump, then pull apart at their handles. Save vigorous young crown pieces from the outside edge of the clump for replanting, keeping them covered with moist burlap or soil as long as they are out of the ground.

Before replanting, improve the soil. A bucketful of peat moss or compost in the planting hole will lighten clay soils and help sandy soils hold more water. Phosphorus is a plant nutrient that moves very slowly in the soil, so mix a handful of bone meal with the soil in the planting hole to ensure a good supply of phosphorus right near the roots.

Next, build a mound of soil in the planting hole on which to set the crown. Adjust the mound height so that, when the soil is firmed, the crown will be at ground level. Then fill the hole, sifting soil in around the roots. Put a layer of sawdust, straw or compost over the ground as mulch, and slowly water the plant with a gallon of water.

To look their best, perennials such as asters and hardy chrysanthemums need to be dug up, cut apart and replanted every spring.

Invasive perennials such as bee balm, tansy, goldenrod and artemisia need division not to spruce them up, but rather to keep them from wandering.

Division only every three or four years is needed for the following perennials: armeria, phlox, coral bells, Canterbury bells, cerastium, Siberian and Japanese irises, veronica, yarrow and Shasta daisy.

Not all perennials want to be divided now. Oriental poppies, bleeding heart, bearded iris and Virginia cowslip go dormant in midsummer, and that is when they should be divided.

And think twice before dividing perennials such as Christmas rose, peony, monkshood, butterfly weed, lupine and baby’s breath. These flowers need division perhaps once a decade, and they often show their resentment to the treatment by not blooming for a year or more thereafter.