The most critical factor for successfully dividing a mature Paeonia lactiflora is timing, which is dictated by the plant's natural growth cycle. The ideal window for division is in the early autumn, typically from late August through October, depending on your geographical location. During this period, the plant is entering its seasonal dormancy. The intense heat of summer has passed, and the cooler soil temperatures encourage root development rather than top growth. The plant has completed its annual cycle of flowering and foliage growth, and its energy is concentrated in its storage roots and "eyes" (the reddish buds for next year's growth). Dividing at this time minimizes shock, allowing the plant to establish a robust root system in its new location before the ground freezes, without the added stress of supporting stems and leaves.
Begin by cutting the existing foliage down to ground level. This directs all the plant's energy downward to the root system. Using a sharp spade, dig a wide circle around the plant, carefully lifting the entire root clump from the ground. Gently wash off the soil with a hose to expose the root structure and, most importantly, the crown and its eyes. This visibility is crucial for making clean, purposeful cuts.
Using a sharp, sterilized knife or a handsaw, divide the large root mass into sections. Each division must have a minimum of three to five healthy, plump "eyes" and a substantial portion of thick, fleshy storage roots to provide the necessary energy for regeneration. These storage roots are the plant's carbohydrate reserves, and without an adequate supply, the division will fail to thrive. Smaller divisions with only one or two eyes may take many years to flower again, as they must first rebuild their energy stores.
Replant the divisions promptly to prevent the delicate roots from drying out. Prepare a planting hole that is wide and deep enough to accommodate the root system without crowding or bending the roots. A key physiological requirement for peonies is planting depth. The eyes (buds) must be positioned no more than 1-2 inches below the soil surface. Planting too deep is a primary reason for a mature plant's failure to bloom, as the buds cannot gather enough energy from the sun to initiate flowers. Backfill the hole, water thoroughly to settle the soil and eliminate air pockets, and apply a light layer of mulch to insulate the roots over winter.
It is vital to understand that a newly divided peony is allocating its resources almost exclusively to re-establishing its root system. Do not expect flowers in the first spring after division; the plant is in a recovery phase. It may only produce smaller-than-usual or even no blooms as it directs energy underground. By the second growing season, the plant will be more established, and you should see improved growth and possibly a few flowers. By the third year, the division will have matured into a strong, flowering plant, having successfully completed its vegetative recovery and returned to its normal reproductive cycle.
