I have a new young Peony Tree in a large 15 inch tub. I hope I can plant it in the Garden at some stage once it is well established - can I do this? if so at what age?
Can I Move a Peony Tree?
You can plant your peony in the garden at any time, as long as you're in an area zoned for peony This article has some tips for peony growing: https://www.gardeningknowhow.com/ornamental/flowers/peony/peony-care.htm
This will help you determine your hardiness zone: http://www.trebrown.com/hrdzone.html