It's a blessing that we can refresh and revitalize even the oldest tenants of our gardens by occasionally dividing them.
Perennial plants are healthiest and most productive when they are young and have room to spread; but like many things in gardening, timing and technique are everything. Most perennials tolerate division most any time of the year, but doing it at the wrong time can leave you with tattered-looking plants you'll have to coax along.
A good rule of thumb is to divide spring and summer-blooming plants in fall, and fall-flowering plants in early spring. This gives them time to reestablish before they need to put out energy to bloom. You can also divide many spring-flowering plants in late spring or early summer, after they've finished blooming.
The most important rule to ensure survival when dividing perennials is to make sure they get extra water, especially during the high heat of summer, and extra mulch in winter, at least for the first year. When they're going into dormancy is the best time to divide perennials, followed by coming out of dormancy. If you must split a plant when it's in bloom, cut the flowers back a few days before you start digging. To further aid in the survival of summer divisions, make some shade for the new transplants, like mesh fabric over some bamboo stakes. And always have the planting area you're moving the plant into ready to receive it so it won't have to wait with roots exposed longer than necessary.
There are two ways to divide: dig up the entire root ball or cut out sections still in the ground. Which technique depends on whether the plant is a clumper or a spreader.
With clumpers, like hosta, remove the root ball by digging 4 to 6 inches from the base with a garden fork or sharp spade. I prefer to cut the ball into sections with a pruning saw since I feel it gives me more control. But a sharp spade gets the job done quickly and cleanly.
For spreaders, like Solomon's seal (Polygonatum sp.), slicing with the spade is the best practice. Start at the edge of the mass, dig under and around the piece you want and gently lift.
Here are some popular perennials and their preferred division seasons, growth habit, USDA zones and tips for successful division. Certain plants may be invasive in your area. Check with your local county extension service or state Department of Natural Resources.
• Artemisia (Artemisia spp.); zone 4-9; spring; spreading; roots easily; divide every 3-4 years to prevent open centers.
• Bee balm (Monarda spp.); zone 4-9; spring and fall; spreading; easiest in spring when foliage is emerging.
• Blanket flower (Gaillardia x grandiflora); zone 3-10; spring and fall; clumping; roots pull apart by hand; divide every four years.
• Coreopsis (Coreopsis spp.); zone 4-9; spring and summer; clumping; quick rooting; divide every two years, tend to be short-lived if undisturbed.
• Hosta (Hosta hybrids); zone 3-8; spring, summer and fall; clumping; best done is spring when clumps are small.
• Lily of the valley (Convallaria majalis); zone 3-8; spring; spreading; divide in early spring or after blooming; spreads rapidly, divide often to establish new areas of groundcover.
• New England aster (Symphyotrichum novae-angliae, Aster novae-angliae); zone 4-8; spring; clumping; divide every three years to avoid dead centers.
• Peony (Paeonia spp.); zone 3-8; summer and fall; clumping; make sure each division has at least three growing points; may take several years to bloom after division.
• Sedum, tall (Sedum spp.); zones 4-9; spring and fall; clumping; can be divided in summer; easiest in spring when foliage is small.
• Tall garden phlox (Phlox paniculata); zone 3-8; spring and fall; clumping; regular division makes plant less prone to powdery mildew; discard woody center of clump.
• Yarrow (Achillea spp.); zone 3-9; spring and summer; spreading; easy to divide; most types spread quickly so divide every 2-3 years.
• Alyssum (Alyssum spp.), candytuft (Iberis sempervirens), carnation (Dianthus caryophyllus), delphinium (Delphinium x elatum), foxglove (Digitalis spp.), lavender cotton (Santolina chamaecyparissus), lavender (Lavandula spp.), rose campion (Lychnis coronaria), Russian sage (Perovskia atriplicifolia), sea hollies (Eryngium spp.), silvermound (Artemisia schmidtiana), and trillium (Trillium grandiflorum) are best not divided.garden
Author Joe Lamp'l, host of "Growing a Greener World" on PBS: www.joegardener.com.