Dividing Perennials by Donna Duffy

If your garden looks like mine, you probably have lots of overgrown perennials. The abundance of rain during the past couple of months has encouraged lots of plant growth. Take a walk around your garden and make note of plants that are ready to be divided.

Perennials are ready to divide when the side shoots or runners become crowded by other plants. Some perennials are aggressive and may need to be divided every two to three years. Examples include: Achillea (Yarrow);
 Aster; Centaurea montana, (Mountain bluet);

 Centranthus ruber, (Red valerian);
 Coreopsis;
 Dianthus; Geum; Lychnis coronaria, (Rose campion);

 and Veronica.

Some species of Veronica will benefit from dividing every two to three years.

The bloom time of your perennials will determine when they should be divided. Not every plant is ready to be divided at this point in the summer. Plants that flower in mid- to late summer and fall, such as chrysanthemums and asters, should be divided in the spring before growth begins. Ornamental grasses can also be divided in the spring. Iris and daylilies usually are divided immediately after flowering. Spring and early-summer blooming perennials, such as peonies and poppies, are usually divided in the fall or when foliage dies (mid-September through mid-October). Dividing them in the spring will sacrifice a season of bloom.

Daylilies are easy to divide and transplant

I've been dividing Daylilies and Veronica like crazy the past few weeks. I've also been digging up volunteers of several perennials (Penstemon cardinalis, Cape Blue Forget-Me-Not, Red Leaf Rose) and potting them for planting later or gifting to fellow gardeners. I keep them in a semi-shady spot  while they adjust to transplanting.

Penstemon cardinalis reseeds readily, providing "volunteers" for transplanting

To divide perennials, use a spade, shovel or fork to dig around and under the entire plant and lift it out of the soil. This is easier if the soil is not saturated. Remove most of the soil from the roots by hand or by washing with a hose. Use pruning shears or a sharp knife to cut apart the healthiest part of the plant, often on the outside. Most clumps can be divided into four or five smaller clumps, after dead and discolored parts are removed. Replant divisions as soon as possible and protect with mulch. Don’t forget to water the new transplants and keep them moist until they are re-established.

Check out CSU Extension’s Fact Sheet#7.402 for more information on growing perennials.