7 Tips For Dividing Chrysanthemums

Chrysanthemums appear with a splash in autumn in jewel tones of the season. Hundreds of varieties with different forms, colors, flower shapes, and bloom times bridge summer and fall, showing color as many other flowers fade. Mums are also one of the easiest perennials to grow. They’re an asset throughout the growing season, with attractive foliage in addition to the fall bloom show.

Hardy chrysanthemums (also called garden mums) offer seasonal decor for porches, patios, and indoor autumnal displays but also provide lasting interest as thriving outdoor selections and showy cut flowers. Mums feature distinct flower forms and petal arrangement classifications that include pom pom, spider, cushion, anemone, and decorative, among others. They glow in shades of bronze, gold, purple, scarlet, pink, and bicolor.

These carefree perennials grow with neglect – you may even plant them and forget until the delightful flowers emerge. However, form, habit, and vigor improve by meeting a few cultural requirements. As vigorous growers, chrysanthemums divide easily and benefit from the practice. Enjoy expanding your mum colony with simple steps that result in healthy, robust new plants.

Chrysanthemum Overview

The genus Chrysanthemum belongs to the aster and daisy family (Asteraceae), and there are about 20 species of garden mums. These vigorous clumping plants have upright, woody stems and soft new growth. They also have underground shoots, or stolons, that increase their ability to survive over the winter. Hardy mums are perennial and overwinter in USDA zones 3-9.

Chrysanthemums have attractive, dense, deep green and lobed foliage. They bloom in sprays, with many small flowers, or as large-flowered selections with showy, single blooms per stem. Depending on plant form and pruning, they softly mound, cascade, or grow upright in the border.

In the fall, we often purchase them at garden centers as “meatball mums” (as a professor of mine used to call them)—perfectly rounded, potted specimens fully budded for seasonal color. When transplanted in the right conditions, these grow year-round in containers or in the ground.

Planting Guidelines

Garden mums grow best in full sun with consistent moisture. Morning sun exposure is ideal. They prefer fertile, loamy, or sandy soils with good drainage. Good air circulation is essential for preventing fungal diseases.

The best time to plant mums is in the fall or spring when roots can establish before freezing or hot temperature extremes. For fall planting, six to eight weeks before heavy frosts help roots develop in advance of winter.

Planting nursery mums late in the fall presents a risk of surviving winter since most of their energy goes into flowering rather than establishing roots. Protect roots with an insulating layer of mulch. Cut back stems after they die back from heavy frost. Use a light layer of mulch or soil on crowns or evergreen boughs for added protection against ice and frost-heaving as soils freeze and thaw.

The perennials overwinter in containers, too; transplant them into larger pots and place them in a cool, sheltered location like a garage or basement until the warming temperatures of spring. Check the soil occasionally for moisture to keep roots from drying out completely.

Why Divide Chrysanthemums?

Chrysanthemums clump heartily, though they don’t spread aggressively or take over a garden bed. Mother plants benefit from division to maintain their health. They have shallow root systems, and they are easily dug and transplanted.

Over time, the central plant becomes less vigorous. Dividing the main clump prevents overcrowding and promotes flowering. It creates new plants for years of recurrent color. In addition to expanding the collection, division also rejuvenates growth.

When To Divide Chrysanthemums

The best time to divide chrysanthemums is in the spring when new growth appears. Clumps are viable for division as shoots reach one to three inches long.

Chrysanthemums benefit from division every few years. A good rule of thumb is to split the main plant every two to three years. Divide when established plants show crowded central crowns with less vigor and flowering.

Tools and Materials

Dividing these flowering perennials is a straightforward process that requires only basic garden tools. You’ll need a digging fork or spade to lift the plants. A sharp knife, hori hori, or spade cuts the divisions. Pruning shears help clean up the new and existing plants.

Prepare your transplant area, whether in the existing growing spot or a different garden location. Check soil quality and add compost or broken-down organic matter to increase richness. Lightly turn the soil to aerate it and promote root development for the transplants.

How to Divide Chrysanthemums

Now to the fun part of getting our hands dirty—digging the mum to improve health and expand the colony. With the spade or digging fork, create a wide berth around the main plant to lift it out of the soil. The shallow roots make it easy to probe beneath and raise them without much intrusion. Dig the whole plant.

Loosen roots and shake off excess soil. Work with the outer perimeter to begin dividing. Separate the clump into sections with your sharp blade or spade. Each segment should have viable stems (growth points) and roots attached. These become the new garden transplants.

Gauge the viability of the existing mother plant. Is the main central crown healthy and productive? If so, divide it into segments for replanting. If it is densely packed and lacking vigor and new growth, compost the old portion and rely on the newly divided plants for fast development.

Remove old, woody stems and debris. Clip off dead or diseased parts and dispose of them away from the bed.

Replant the Divisions

Once divided, replant the segments in good time for viability. With the planting area prepped and amended, tuck the divisions into their loose, well-drained, organically rich soils.

Make sure the new crown and upper growth remain above soil level rather than settling too deep. Plant the segments 18 to 24 inches apart, depending on the variety. Allow for mature growth to ensure plenty of airflow between plants.

Topdress with a layer of mulch to regulate soil temperatures and retain moisture while suppressing weeds. Water the transplants thoroughly.  A few weeks after planting, apply a balanced granular fertilizer.

Young divisions, free of crowded roots and stems, grow quickly. They’ll flower in the fall after a spring division.

Care

Mums are low-maintenance, but newly transplanted divisions need consistently moist soils as roots establish and in the summer heat. 

When they flower in late summer and fall, deadheading spent blooms promotes more budding and flowering to keep them going through frost. Post-flowering in autumn, cut plants down to eight inches tall to prepare for winter dormancy. Add extra mulch for cool-season protection.

Pinching and Disbudding

Pinching new growth on small-flowering (spray) varieties at various phases of development creates healthy, full forms and all-over flowering. Pinching creates compact, sturdy stems to hold buds and blooms.

When stems reach eight inches tall, remove the light green growth tip to encourage side branching. Further pinching removes the tips from the side shoots for dense growth. Depending on the variety’s bloom time, pinching occurs every couple of weeks through late June before buds set.

Disbudding large-flowering varieties promotes energy to single, large buds and blooms rather than a series of less substantial flowers. Pinch off side shoots and remove buds other than the primary central flower bud.

I’m guilty of not pinching or disbudding my garden mums. Fortunately, these reliable growers flower in a blanket of color with a natural mounding form regardless. Enjoy a long-lasting display from your expanding mum colony as we move into fall.