Dividing perennials to refresh a crowded clump

Dividing a crowded perennial clump restores vigor, improves flowering and creates new plants at little cost. A clump that has grown for several years often develops a dying center, fewer flowers and stems that flop because they are packed too tightly. Lifting the clump, separating it into smaller sections and replanting the healthiest parts usually fixes all three problems.

Timing and technique both matter. Most perennials divide best in early spring as new growth begins or in early autumn, when temperatures are cooler and the plant has time to recover before winter. Spring-flowering plants are generally divided after they finish blooming, while fall-blooming plants are better divided in spring. Plants that bloom in midsummer can be divided in either season, so choose the period when you have time to replant and water the sections properly. The steps below suit most clump-forming perennials, though a few plants are exceptions.

When a clump needs division

Signs that a clump needs dividing include a hollow or dying center, flowers that become smaller or fewer, and stems that crowd or flop. Many perennials benefit from division every few years, although the interval depends on the species and how fast it grows. Fast spreaders can be divided more often, while slow growers may go many years without it. Avoid dividing during heat or drought, because divisions have less moisture and fewer resources to regrow. Dividing in cool, moist weather gives each section the best chance.

A clump that flowers well only at its outer edge, while the center has gone bare, is usually telling you it has outgrown its space. The same pattern appears in plants left undisturbed so long that their roots have matted into a dense mass with little room for new growth.

Some perennials resist division. Peonies dislike being dug up and can take a season or more to flower again, yet they are usually divided in early autumn when a clump has grown too crowded to bloom well. Plants with deep taproots, such as false indigo and oriental poppies, resent division because their roots are hard to split without damage. Grow more false indigo from seed instead. Oriental poppies are usually propagated from root cuttings taken after the foliage has died back, or from seed.

Digging and separating the clump

Water the clump a day before you start so the soil is moist but not wet. Trim heavy or floppy top growth to make the work easier. Insert a garden fork or spade well outside the outer edge of the clump, then lever the whole root mass out of the ground, working gently rather than pulling on the top growth, which can tear the crown. Shake or rinse off the loose soil so you can see how the crown is arranged. Separate the clump by pulling sections apart with your hands, by pushing two garden forks back to back through the center to pry it open, or by cutting with a sharp spade or knife. For tough, woody clumps, cut through the crown in one firm stroke, and use a saw for very dense clumps with thick roots. Wash your tools afterward, since disease can travel on a dirty blade from one plant to the next. Each division should have healthy roots and several shoots or growth buds. Discard the woody, dead center and any section with soft rot or a damaged crown. If the clump is too large to lift comfortably, you can instead dig out only the vigorous outer sections, replant them and leave the old center in place for a while. This partial approach spreads the disturbance over a season or two.

Keep the divisions out of sun and wind while you work, and replant them as soon as you can. Roots that dry out during the process set back regrowth noticeably, so it helps to work in the shade and keep the sections covered with damp soil or cloth until they go back in the ground.

Replanting and aftercare

Replant divisions at the same depth they grew before, so the crown sits level with the soil surface. Planting too deep can cause crown rot, and planting too shallow exposes roots to drying. Space divisions at roughly the mature spread listed for the plant. Water the new sections deeply enough to settle the soil around the roots, then keep the soil moist but never waterlogged for several weeks while roots reestablish, since soggy soil can rot newly divided roots. Add a light mulch to slow drying, keeping it clear of the crown.

Consider how the replanted sections will fit the space. Smaller divisions can fill gaps or start a new bed, while larger sections are best placed where they will not be crowded for several years. Filling new areas this way is a simple method for extending a border without buying new plants.

A divided clump usually shows the benefit within a season or two, with stronger foliage, more flowers and an open, healthy center. Keep one or two divisions as spares, and share the rest with neighbors who want fresh plants.