Certified GKH Gardening Expert
First of all, do not store any seed in the freezer, as this will normally kill them. The refigerator or an airtight container in a dark location would be better. That being said, peonies are usually propagated by grafting or root division. Many peonies are infertile and do not produce seeds. However, for those that do, collect the seeds as soon as the pods begin to open. Place them in a sealed plastic bag with barely moist vermiculite or soilless germination mix. Place the sealed bags in a warm area and ensure they remain moist (but not wet). Leave them in the warmth until the roots appear, which can take anywhere from a few weeks to several months.
Once the roots appear and are about an inch long, move the bag to a cool location, such as a refrigerator, for about 10-12 weeks, after which you can pot them up in a soilless germination mix. Note: seeds can also be sown directly in the ground provided the conditions for dormancy and growth are present. However, it will usually take until the second spring before shoots appear.