Peonies (Paeonia spp.) have been cultivated in landscapes around the world for thousands of years and can grow in your own garden for almost a century, without ever needing to be divided. The common garden peony (Paeonia lactiflora), which grows in U.S. Department of Agriculture hardiness zones 3 through 8, and the fern-leaf (Paeonia tenuifolia) and tree varieties (Paeonia suffruticosa), which grow in USDA zones 4 through 8, thrive in a variety of climates and can easily be propagated through the process of division. While these large-bloomed beauties are content to grow where they are, dividing them up periodically will give you more of them to admire. With a sharp shovel and a half hour to spare, you can spread their loveliness around the landscape for other generations to enjoy.
Everything has its season and for dividing perennials, that season is fall. September and October are the best months for dividing peonies, after the long, sunny days of summer have supplied the plants with plenty of sunshine to recharge their food reserves underground. In areas that experience a hard frost early in the autumn, it is best to divide all perennials at least four to six seeks before the ground freezes, making September a good choice. Information on average dates of hard frosts can be found on extension websites for your area.
Similar to irises (Iris germanica), which grow in USDA zones 3 through 10, peonies have specialized root systems (or modified stems) called rhizomes that grow horizontally underground and store energy for the plant during the dormant months each year. These rhizomes send up leaves and stems in the spring and replenish themselves during the summer. By waiting until the days begin to shorten, you ensure that the rhizomes have ample energy to get your peonies started off next spring in their new location.
Tools for the Job
A sharp shovel or spade will do the trick, but a favorite tool for dividing perennials is the sharp, Japanese-style hori hori gardening knife. Depending on the depth and size of your peony clump, you may have to put a bit of pressure on the rhizome to remove it, so a longer-handled tool can be helpful. Use pruners to cut back the existing stems and stalks on your plant before you divide it.
Dividing the Plants
Begin by trimming down the stems and greens of your peony plant. In warmer climates where you have not had a hard frost, you may leave some of the stems for appearance, but cutting back fully makes the division process easier.
When dividing perennials like iris, which tend to form self-strangling clumps, you usually dig up the entire plant, divide it and then replant the smaller clumps. This is not as necessary with peonies, which do not demonstrate the revitalizing effects of separating clumps. Instead, mentally divide the peony into quarters and dig up half. A good way to do this is to think of the clump like a circle and choose slices from opposite sides.
When digging, cut into the rhizome with your tool to divide the clump. This will not harm the plant because new growing tissue will form on each piece of the rhizome. Remove pieces that are at least 3 to 4 inches long. Once you have removed sizable sections, cover the remaining clump back over with soil and press firmly.
Take the pieces you have divided from the larger clump and shake off excess soil. It is best to plant these immediately so that they do not dry out. Choose a sunny location that gets at least six hours of sun daily that has well-draining soil and dig a hole that is large enough to hold the full root system of the newly divided pieces. Cover each rhizome with at least 3 to 4 inches of soil and press firmly. You will not see any new growth until the spring.
Watering with a general purpose fertilizer helps the new peonies establish themselves before winter. Mix up a gallon per new plant using a water soluble formula with equal parts of nitrogen, phosphorous and potassium; this would read 7-7-7 on the label. In colder climates it's a good idea to add a thin layer of mulch to keep new peonies warm.
Once you have completed these steps, which work for dividing all peony varieties, you have nothing left to do but sit back and wait for spring.