Let the roots be your guide when it’s time to divide perennials

By Bonnie Orr
WSU Chelan/Douglas County Master Gardener

Yellow and red flowers in the garden.
Perennials are great garden fillers until they aren’t. When they become overgrown, the flowers are smaller, the plant’s center is empty, and the plants are less healthy. It is then time to divide perennial plants. – Provided photo/Bonnie Orr
Bonnie Orr
Bonnie Orr – WSU Extension Chelan/Douglas County Master Gardener – photo by Don Seabrook, Wenatchee World

Many perennial plants need to be separated every few years to maintain healthy growth. Many extra plants can be created by separating, and these can be used to fill in a border, to create new beds or to give to gardening friends.

Perennials grow different types of roots, and the roots are the guide to dividing the plants.

The spreading roots create a large family of closely spaced off-shoot plants that are separate. Divide these plants in early spring before they develop leaves. Common plants in our area: Astors, Astilbe, Campanula, Phlox, Echinacea/Rudbeckia.

Clumping roots create a very wide plant that has a strong crown. These are divided by cutting apart the crown after the plant has bloomed. Common plants in our area: ferns, Hyssop, Penstemon, autumn joy Sedum, daylily, Heuchera.

Rhizome or tuber or thickened roots are dug in the fall after the plants have gone dormant. Often, it is best to dig the entire plant and just replant the root parts that are most vigorous. Common plants in our area: iris, hosta.

The most effective tool to use to divide plants is a spading fork because you can dig into the ground with less chance of breaking the roots. The fork allows you to lift the entire plant and then separate out the parts into new plants. Moist, not soggy, soil makes the lifting easier. Divide plants early in the day before it is hot and cover the bare roots with a damp burlap bag until you are ready to plant them.

Water the plants in their new location. Do not amend the soil nor apply nitrogen fertilizer. The nitrogen promotes foliage growth, but the plant needs to put its energy into growing new roots. If you only amend the area around the transplanted plant, the roots will not expand into the native soil in the flower bed. Worse, water does not travel from one soil composition to another, so you end up making a swimming pool around the transplanted plant.

When selecting a site to plant perennials, keep in mind that many perennials do not want their roots disturbed and cannot be transplanted. If a plant’s roots are disturbed, the plant may die or take several years to re-bloom. As a perennial gets bigger over the years, it needs to be in an appropriate place. University of Minnesota Extension has an extensive list of perennials that lists those plants that do not want to be disturbed. You can find the list online at bit.ly/umndividing.

Grasses are hardy perennials as well and often become overgrown. Divide them in early spring, before they have started new green growth. Because the fibrous roots are matted and very dense, often these plants are divided by sawing or axing the plants apart.

A WSU Chelan-Douglas County Master Gardener column appears weekly in The Wenatchee World. To learn more, visit bit.ly/MGchelandouglas or call (509) 667-6540.