Peonies are any members of the Paeonia genus and are flowering plants that generally produce spectacular blooms in the summer. Most peonies have herbaceous stems that you would usually propagate through division. However, you can also propagate a peony with a woody stem by grafting it to the rootstock of another peony. Several peony species have woody stems and are known collectively as "tree peonies." They aren't true trees, but merely peonies with tree-like characteristics due to their woody stems.
Select the scion for the peony graft. This is a twig or shoot from the tree peony that you wish to graft and should contain at least two shoot buds. The scion shouldn't have any bloom buds because it will try to produce a flower after you graft it, which will weaken the young peony.
Choose the rootstock for the graft. This will be a root from a mature peony that's already established and is usually an herbaceous peony. The rootstock can be up to 8 inches in length and should be no thicker than your finger.
Sterilize the pieces of the graft in early August. Soak the scion and rootstock in a solution of 90-percent water and 10-percent household bleach. Rinse both parts thoroughly after an hour, and use rubbing alcohol to sterilize your tools and work surface.
Cut the scion from the tree peony by making two cuts at a 30-degree angle on opposite sides of the scion. Trim a thin piece from the top and bottom of the rootstock to remove the bleach solution that soaked into the root. Cut a cleft on the top of the rootstock that matches the shape of the scion.
Place the cut end of the scion into the rootstock's cleft so that the two pieces are flush with each other. Secure the joint by wrapping a grafting band around it and tuck the end of the band under the last wrap. Cover the grafting band with parafilm to form a watertight seal. Put the graft in a plastic bag along with a piece of moistened paper towel. Seal the bag and store it for four to six weeks in a warm location.
Plant the graft so that the joint is covered by only an inch or two of soil. Apply mulch in the late fall and remove it in the early spring before the graft begins to grow.