Dividing Perennials

Perennials are plants that come back from the roots each year. They provide the landscape with an abundance of beauty through blooms and foliage. However, over time because of new growth through the root system and new shoots, perennials periodically need to be divided. It may seem like a daunting task, but understanding how, when, and why to divide will make it manageable and even enjoyable.

When perennials begin to crowd out other plants or expand beyond their designated space, when the blooms get smaller and smaller, when the centers begin to die, or when they get so large they begin to droop and fall over, it is time to think about dividing them. Alternatively, don’t wait until they begin to decline and simply divide them every three years. However, some plants prefer to be divided more often and some simply should not be divided at all. Those with single woody bases like lavender (Lavendula) and Russian sage (Perovskia) should not be divided. Those with single taproots should also be left alone. That includes, but is not limited to false indigo (Baptisia), butterfly weed (Asclepias), lupine (Lupinus), clematis, and columbine (Aquilegia). To find specific information about 125 common perennials, when and how to divide them, see this site: https://extension.umn.edu/planting-and-growing-guides/dividing-perennials

Ideally divide perennials on a cloudy, overcast day. Sunny days dry out the plants rapidly. If it is dry, water the area a day or two in advance to make the ground easier to work. Prepare the new location in advance so that everything is ready on the day of division. Dig the hole wide enough to accommodate the roots and deep enough that the crown will be even with the soil level. Add some organic matter or compost to the hole.

Dig the entire plant out of the ground before trying to divide it. Using a garden fork and digging 4-6 inches beyond the drip line will minimize damage to the root system. 

There are three types of root systems and each is handled a bit differently. Spreading root systems such as asters, tickseed (coreopsis), beebalm (monarda), and coneflower (echinacea) are intertwined and likely matted. Pull the smallest sections apart by hand, cut larger sections with sharp knife. Exceptionally large sections may require two forks back-to-back and prying them apart. Divide the plant into clumps with 3-5 shoots per section. Discard the center if it is dying.

Clumping root systems can be quite challenging if they have gotten large. Astilbe, daylily (Hemerocallis), hosta, and most ornamental grasses take some muscle to divide. They originate from a central clump with multiple growing points. Divide with a sharp knife, back-to-back forks, or a machete. Each piece should have a few buds with roots attached.

Rhizomes spread out from the plant like spreading root systems, however they are easier to divide. Bearded iris are typical in this category and the rhizomes grow horizontally at or just below ground level. Simply dig them up, cut away any diseased or damaged parts, leaving a few inches of foliage.

Plant the new divisions immediately, making sure to fill in all the air pockets, firming the soil well.  Water immediately and keep them watered for the next few weeks. Do not use a high nitrogen fertilizer until next year as it promotes top growth at the expense of root growth.

Enjoy the rejuvenated landscape, start a new flower bed, or share with a friend now that the work is done.

Carol Shirk

Certified Master Gardener