Karl Foord, UMN Extension Educator
I just finished photographing the peony collection at the Minnesota Landscape Arboretum and was amazed at the variety of flower types and colors. Peony flower types fall on a continuum starting with single and progressing through semi-double, Japanese, and double. The single flower has a central cluster of carpels surrounded by a ring of stamens surrounded by a ring of petals, a basic flower structure. The following flower types all have modifications of this basic structure. The semi-double flower may have a fertile center surrounded by several layers of petals. "Japanese" peonies have modified stamens (staminoides) which may have some functional anther material and pollen, but the stamen filaments are now more like petals. In double peonies all flower parts are petals.
To identify peonies, a fairly comprehensive list is available at http://www.paeo.de/ - (Carsten Burkhardt's Web Project Paeonia). If you are interested in buying peony bulbs suppliers can be found through the plant info tab on the Minnesota Landscape Arboretum's website. (http://plantinfo.umn.edu/ and http://www.arboretum.umn.edu/ hit the plant info button top center of page). I did a search for peony and got 1297 search result(s) for Paeonia on this site.
Photo credits: Karl Foord