Tree peonies, Paeonia suffruticosa, are a variety of peony that grows particularly large and often takes the shape of a shrub. The plant has woody stems like a shrub that can reach a length of 10 feet (3 meters) under ideal conditions. Since tree peonies were originally native plants found in China, they often are sold by the name of Chinese tree peonies, and are the national flower of China. Cultivation of the plants to improve upon them was first widely done during the Tang Dynasty, from 618 to 907 AD, and between the 15th to 19th centuries, they spread to other nations such as Japan, France, and the US. The plant can produce a variety of different colored and sized flowers, usually in red, pink, or yellow tones.
Most types of peonies have fragile stems that will die back and should be cut off when the winter season approaches, but tree peonies are hardier and their stems should be allowed to winter over. They are a perennial plant that will come back every year, and grow best in well-drained, fertile soil. They can tolerate a wide range of lighting conditions, but the flowers that they produce will endure the longest if the plants are given partial shade during the sunniest part of the afternoon. The main caution with growing tree peonies in a northern region that experiences harsh winters is that they be planted with the graft union, which is a bulb-like structure at the base of the main stem, 2 inches (5 centimeters) or more below the surface of the ground.
Planting tree peonies should be done with sufficient space in mind for them to spread out as they grow. The typical size for many popular strains is a height of up to 5 feet (1.5 meters) and a bushy width of around 3 feet (0.9 meters). They also tend to be hardy from zones 4 to 7, which in North America stretches from the Canada-US border south to an area of northern Texas. In Europe, hardiness zones 4 to 7 dominate in nations like Germany, Poland, and Denmark, and along the coastal regions of Spain. The species can also be grown in warmer climates up to hardiness zone 9, however, as long as they are planted in areas where they have partial shade throughout the day.
Caring for tree peonies includes establishing them in soil with a neutral pH level and being patient, as they are slow to mature and can take several years before they bloom. Once they begin to bloom, an individual plant can produce up to 50 flowers around late spring and early summer. They require a significant amount of fertilizer or compost to remain healthy and thrive. If they are going to be transplanted before a frost arrives, it is recommended that this be done six weeks ahead of time to give them time to settle into new ground before it freezes.
Several pests are problems for tree peonies, including Japanese beetles, nematodes, and diseases like ringspot virus and tip blight. Good air circulation around the plants will prevent many diseases from establishing themselves. Powdery mildew is a common disease for peonies, however, that usually occurs in late summer to early fall. The smaller, herbaceous strains of peonies are more prone to powdery mildew and it can be controlled in tree peonies by removing the white-coated leaves before winter arrives and treating the plant with a biofungicide. Tree peonies are fairly hardy flowering plants that are deer resistant, and, if cared for well, can live for up to 50 years.