I did a search of the patent office and found one plant that has a patent, Singing in the rain II. I do not know if it is any different than Singing in the rain. I could not get the document to open. Do a search for Singing in the rain and contact that Don Smith to see if his others have been patented. Most that are in commerce are becoming fairly common.

Here is Don's website. Just be aware that he is very scientific and may lose you in the conversations. He had several of the mediteranian peonies several years go but I believe he has since destroyed his stock.

Here is a link that might be useful: Don Smith Newton MA

Very interesting link. There's a big difference between registering a variety and patenting it.