What can be better than free plants?
At some point, most perennials benefit from being divided. The three main reasons for dividing perennials are to keep them in bounds, to rejuvenate them and to get more plants for free.
Some perennials grow more vigorously than others and may start to bully their way across the garden, claiming space meant for other more mannerly plants. Dividing them keeps them under control. Sections can be planted in other areas in the landscape or tossed in the compost pile. If they are shared with other gardeners, a word of warning should accompany them, so monsters aren't unknowingly unleashed in their gardens.
As clumping perennials get older, they may form a “doughnut” of foliage that surrounds a dead center. The entire clump is dug, the ring of foliage cut into sections to be replanted and the dead center tossed in the compost bin. Older perennials may not bloom like they once did, either by producing fewer or smaller flowers. Diving them is their proverbial fountain of youth.
I don't know of any gardeners who don't love getting plants for free. Before heading to the garden center when creating a new border, check out the perennials in your own gardens. Some may be large enough to split.
Spring and early fall, with their cooler temperatures and increased rainfall, are the seasons most kind to newly divided perennials. But to enjoy this season's blooms, divide perennials that bloom between early spring and the end of June in early fall. Those that flower after June can be divided in spring. Ornamental grasses are best divided in spring.
There are a small number of perennials that resent ever being divided, like bleeding hearts (Dicentra), blue false indigo (Baptisia), butterfly weed (Asclepias), gas plant (Dictamnus), hellebores (Helleborus), lavender (Lavendula), Russian sage (Perovskia) and sea holly (Eryngium).
A little preparation and dividing and replanting on a cool, cloudy day make dividing easier and more successful. First, check the soil. If it is dry, thoroughly soak it and wait at least several hours — a day is better — to ensure the plant is well hydrated and to make digging easier.
There are two main methods when dividing perennials. Dig the entire clump and carefully split it into sections. This method causes less damage to roots. Or simply use a shovel to slice down at the edge of the plant and remove the outer section.
To divide the whole clump, dig around the perimeter of the plant with a shovel or garden fork and gently lift it from the garden. Remove the soil by either washing or brushing it off so the roots can be examined. The root structure dictates how the plant will be divided.
Some perennials, like hostas, sedum and yarrow, are easy to separate. Others, with tangled or thick roots, like astilbes and Joe Pye weed, must be pried or cut apart. Those with fleshy roots, like pigsqueak (Bergenia) require a knife to slice them apart.
Be sure new divisions have ample roots and leaves to grow on their own and replant them as soon as possible before their roots dry. Plant them at the same depth they were growing before and water generously. Apply a layer of mulch to help keep the soil moist but be sure to keep it pulled back an inch or so from the foliage. If Mother Nature doesn't supply it, give newly planted divisions an inch of water each week until they have established residence in their new locations.
A few perennials need special care for dividing and/or replanting, like bearded irises and peonies, so do a little research to make sure any special directions are followed.
Plan on dividing most perennials every three or four years to keep the garden vibrant and vigorous. Use those divisions to create new beds, fill gaps in existing borders or share them with fellow gardeners. Remember, we all love to get plants for free!
• Diana Stoll is a horticulturist, garden writer and speaker. She blogs at gardenwithdiana.com.