Tree peonies (Paeonia suffruticosa) tend to be rarer in gardens than regular ones, and are really astonishing when you see one in bloom. They look so exotic, but they are hardy here in Canada, to Zone 4. Unlike herbaceous peonies, which have stems that tend to flop, sending the flower heads crashing to the ground after a rain, tree peonies have woody upright stems that hold the flowers up high. And the flowers are exquisite. Tree peony planting tips from Canadian Gardening:
Place each tree peony in a hole that’s 60 centimetres deep by 60 centimetres wide. Put a handful of bone meal in the hole and plant the bud union 10 to 15 centimetres below the soil. Fertilize with compost or mix and enrich them with additional bone meal. Firm the soil and water well. Plant them at least 1.2 metres apart.