Dividing Perennials

Most gardeners have a spring ritual that includes walking their garden in search of emerging plants. We begin to get an idea of which plants have expanded well beyond their borders. The quick use of a sharp spade now is better than attempting the eradication of a full-blown invasion later. Thus, it is time for the great perennial divide!

April is the time to divide perennials for many reasons: plants have outgrown their space; plants aren’t doing well in that site because of sun, shade or moisture requirements; plants got way too big; flowers were not the color you anticipated; or maybe you have decided it would look so much better in another spot. Or maybe you just feel like digging. Remember the adage: “Every good garden has been in a wheelbarrow at least three times.”

Dividing can be an invigorating process for plants in which the center tends to die out. Some such as yarrow, aster, perennial sunflower, obedient plant and black-eyed Susan perform better if they are divided every few years to keep them in bounds.

April is an ideal time to move/divide most perennials. However, peonies should be divided only in September. Bearded irises are divided in July and August. Plants that form underground rhizomes or multiple crowns are easy to divide.

Everyone has their favorite method of dividing perennials and their favorite implement of destruction. I prefer a small sharp spade to divide large clumps. Shove the spade into the soil on the outside of the plant and continue all the way around the outside. Cut the plant into the size of sections you want. I often have to jump on the spade to get through thick stems.  

Some gardeners prefer digging around the clump and using two garden forks to pull the clump apart. Divisions can be as large as you want, but four-inch diameter sections work well for most plants. Smaller divisions may not bloom as well for a couple of years.

The whole clump does not have to be lifted. Sections from the outside of the planting can be removed to reduce the size of the planting or to leave the mother plant intact. For many perennial plants the most vigorous shoots are on the outside of the clump. This method works well for space invaders such as beebalm, mint and anything that spreads by runners to form a colony (or in the case of mint, its own country).

Some plants such as daylilies, catmint, and astilbe have more of a central crown. Dig out the whole plant and make divisions using a spade or garden knife or in the case of ornamental grasses an axe works well.

Replant divisions immediately, plant into pots, heel into a pile of moist mulch for planting later or put on your neighbor’s doorstep. Be sure to water plants thoroughly after replanting. Before replanting, amend soil with compost if needed. Most perennials do not flower as well the year they are divided, so don’t be discouraged.

Some plants do not like to be moved or divided. These include baby’s breath, old fashioned bleeding heart, balloon flower, lavender, monkshood, blue indigo, gas plant, sea holly, lupine and butterfly weed.

Other April gardening activities include removing last year’s perennial stems, trim butterfly bush, caryopteris and Russian sage back to 6-8 inches and remove any winter mulch from perennials and roses. Ornamental grasses should be cut down before new growth emerges.