Tree peonies are usually propagated by grafting, but you can grow them from seed. Do not be disappointed when the seed you plant does not produce the same flower as its parent. The new plant will be a totally new variety.
Tree peonies aren't difficult to grow, but not all seeds from the mother plant are viable. Also, tree peonies require a two-step germination process. First, the seed requires warm-moist stratification, the process of simulating natural winter conditions. During that stage, an immature root emerges. This root requires cold temperatures followed by another warm period.
According to "Making More Plants, the Science, Art and Joy of Propagation" by Ken Druse, "It could be two years before a leaf appears, but with warm and cold stratification, leaves may show in six months."
When tree peony seedpods begin to open, about one month after bloom time, you may collect the seeds. Remove the seeds before they mature and drop to the ground, or cover a seedpod with a brown paper bag tied to the stem to collect seeds as they drop. You also may let nature sow the seeds for you, letting the seeds drop and moving resulting plants to another location later in their development.
A tree peony seed may be planted in a pot of soil and the pot sunk into the ground. This method protects the seed from squirrels and chipmunks and allows the seed to undergo a natural stratification process.
If you wish to try your luck with indoor planting, scrape the seed against a rough surface, or nick the seed with a knife before planting to help break down the seed's tough outer coat. Plant the seed about 1-inch deep in moistened potting mix or garden soil that contains no amendments, cover the pot with a plastic bag, close the plastic bag and put the pot in a well-lit, 60 to 70 F location for 90 days. Then place the pot in a 31 to 42 F room or refrigerator for another 90 days. Remove it from the cold location, and put it in a well-lit, warm area for another 90 days. By that time, you should see some visible growth unless the seed is not viable.
Plant the pot where it will receive at least four to five hours of sunlight each day in a loamy, loose soil with good drainage. Tree peonies grow best in soil with a pH of 6.5 to 7.5. The seedling may die back during summer, but it should reappear the following year.