When your beautiful Japanese peonies begin to age, established clumps may become overcrowded. This typically occurs only about every 10 years or so in healthy peonies. You'll want to divide them by then because bloom production will suffer if you don't. It's a simple process to dig and divide tubers of healthy mature peonies, and won't take you long at all. Plant them in fertile soil, and they'll thrive and bloom for you for up to 50 years. Propagate in early fall so that tubers will have plenty of time to gain strength and establish robust root systems for the winter.
Cut foliage of mature peonies back to the ground in September. Carefully use a spading fork to dig their roots out of the ground. Shake the loose soil from them and wash the rest of the soil off so that you can easily see the root structures.
Examine the tubers closely for signs of damage or disease. Discard any that are not perfect. Use a clean, sharp knife to cut the tuber so that each new plant will have at least three to five healthy eyes with about as many thick roots attached to them.
Dig a hole about 18 inches deep and wide. Work compost and peat moss into the soil and fill the hole about halfway with it. Work a handful of 5-10-5 fertilizer into the soil in the hole.
Situate the tuber in the center of the hole so that the uppermost eye will be no more than 1 ½ to 2 inches below the soil surface when planted. Pat a little soil around the tuber and add a couple of inches of water to the hole. Allow the water to soak in. Cover the tuber with backfill and press it firmly into place. Soak the planting site well. Space multiple peonies at least 3 to 4 feet apart.
Cultivate the area lightly to discourage weeds until frost, but not deep enough to disturb your tuber. Mulch 2 to 3 inches deep with coarse peat moss, straw, wood chips or shredded bark when frost threatens.
Remove the mulch around April 1, when your new peonies are 3 or 4 inches tall. Feed a 5-5-5 organic fertilizer according to the packaging instructions and water it in well. You can expect your new Japanese peonies to bloom in three or four years.