I think you are right. Planting the peony tree when it is not in the dormant stage is a huge risk. But I think the lack of moisture is what may have killed it. In my case, I was probably water it too much while it was in the pot and caused the root to rot. And when I planted it in hard clay soil, I even put it in greater risk of rotting. So it eventually suffocated and died. Tree peonies are finicky. I think they are much harder to grow than herbaceous.
