Divide and conquer: Prevailing over perennials


— Although many gardening books tout dividing herbaceous perennials every two to five years, don't rely strictly on the calendar. Instead, consider a plant's shape and condition before taking a spade to it.

If a plant looks crowded and is performing poorly, it probably needs to be divided. If a perennial is thriving and continuing to flower year after year, leave it be.

The root balls of some perennials can be simply pulled apart with your hands; others need to be sliced with a spade or even cut with a saw. Here's a list of dozens of perennials and the best way to divide them. Divide these plants by hand: Blanket flowers (Gaillardia spp.) Bleeding hearts (Dicentra spp.) Columbines (Aquilegia spp.) Coral bells (Heuchera spp.) Forget-me-not (Myosotis sylvatica) Hellebores (Helleborus spp.) Jacob's ladder (Polemonium caeruleum) Lady's mantle (Alchemilla mollis) Lamb's ears (Stachys byzantina) Moss pink (Phlox subulata) Primroses (Primula spp.) Pussytoes (Antennaria dioica) Speedwell (Veronica spicata) Stonecrop (Sedum spectabile) Violets, pansies (Viola spp.) Wormwood (Artemisia ludoviciana) Yarrow (Achillea millefolium) Divide these plants with a spade or pitchfork: African lilies (Agapanthus cvs.) Anemone (Anemone X hybrida) Asters (Aster spp.) Bee balm (Monarda didyma) Bellflowers (Campanula spp.) Big bluestem grass (Andropogon gerardii) Black-eyed Susans (Rudbeckia spp.) Cardinal flower (Lobelia cardinalis) Catmint (Nepeta X faassenii) Coneflower (Echinacea purpurea) Daisy (Leucanthemum X superbum) Daylilies (Hemerocallis spp.) Garden phlox (Phlox paniculata) Hostas (Hosta spp.) Lemon balm (Melissa officinalis) Monkshood (Aconitum napellus) Penstemons (Penstemon spp.) Perennial sage (Salvia X superba) Pinks (Dianthus plumarius) Poppies (Papaver spp.) Red hot pokers (Knifophia spp.) Sedge (Carex morrowii) Siberian iris (Iris sibirica) Yarrow (Achillea filipendulina) Slice apart woody crowns with a handsaw: Amsonias (Amsonia spp.) Astilbes (Astilbe spp.) Bear's breeches (Acanthus spinosus) Doll's eyes (Actaea pachypoda) Foxtail lilies (Eremurus spp.) Gayfeather (Liatris spicata) Goatsbeard (Aruncus dioicus) Joe Pye weed (Eupatorium maculatum) Lilyturf (Liriope spicata) Male fern (Dryopteris filix-mas) Meadowsweets (Filipendula spp.) Peonies (Paeonia cvs.) Solomon's seal (Polygonatum odoratum) Wild indigo (Baptisia australis) Cut up rhizomes and tubers with a knife: Arum (Arum italicum) Bergenia (Bergenia cordifolia) Caladiums (Caladium spp.) Calla lily (Zantedeschia aethiopica) Cannas (Canna spp.) Corydalis (Corydalis lutea) Dahlias (Dahlia cvs.) Elephant ears (Alocasia spp.) Irises (Iris spp.) Lady fern (Athyrium filix-femina) Lily-of-the-valley (Convallaria majalis) Rhubarb (Rheum palmatum) Rodgersia (Rodgersia pinnata) Wild ginger (Asarum europaeum) These perennials are best not divided: Alyssums (Alyssum spp.) Candytuft (Iberis sempervirens) Carnation (Dianthus caryophyllus) Delphinium (Delphinium X elatum) Foxgloves (Digitalis spp.) Garden sage (Salvia officinalis) Geraniums (Pelargonium spp.) Lavenders (Lavandula spp.) Rose campion (Lychnis coronaria) Rosemary (Rosmarinus officinalis) Russian sage (Perovskia atriplicifolia) Silvermound (Artemisia schmidtiana) Sweet pea (Lathyrus latifolius) Trillium (Trillium grandiflorum)

All spring blooming plants as well as irises and oriental poppies can be divided as soon as the blooms fade or in the fall. All others should be divided in very early spring for our zone 3-4 climate. Allow at least four to six weeks before the ground freezes for newly transplanted plants to become established.

If the weather has been very dry, water the plant thoroughly the day before you divide. If possible, work on a cool, cloudy day or late in the evening to reduce moisture loss.

Prepare the area that you plan to put your new divisions in before you lift the parent plant. Use a sharp, pointed shovel or spading fork to dig down deep on all four sides of the plant, about 4 to 6 inches away from the plant. Pry underneath with your tool and lift the whole clump to be divided.

If the plant is very large and heavy, you may need to cut it into several pieces in place with your shovel before lifting it.

Try to avoid root damage. Gently shake the soil from the roots or rinse the soil off with a gentle stream of water from a hose. Prune the stems and foliage to 6 inches from the ground to ease division and to cut down on moisture loss. Remove damaged or diseased sections. Divisions should be taken from the outer edges of the plant because this younger growth will produce more vigorous plants.

Make sure each division has at least three vigorous shoots. Small shoots will take longer to flower. Replant the divisions as quickly as possible, setting them at the same depth as the parent plant. Water thoroughly and provide mulch to prevent the soil from drying out. A layer of mulch will also protect the developing roots from frost.

Kathy Conlon is a Master Gardener through the Colorado State University Cooperative Extension office in Routt County. Questions? Call the CSU Cooperative Extension office at 879-0825 or e-mail:


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