The growing season for peonies, both tree and shrub varieties, varies somewhat depending on location, but is generally understood to be when the ground in not frozen. This is most often calculated by first and last frost dates. For most locations, this works out to be the majority of the spring and fall months, and all of the summer.
Peonies, like most flowering plants, are dormant during the winter. When the ground is frozen and cold, the roots are usually unable to retrieve nutrients. Instead, the roots typically stockpile needed sugars and minerals during the fall, then cease new growth until the spring.
Areas that do not have defined winters or that only rarely freeze, particularly the tropics or southern coastal regions of Europe and North America, often support longer, sometimes year-round peony growing seasons. Peonies are native to harsher climates, but will sometimes grow in warmer temperatures, too. There is usually still some period of dormancy during the coldest time of the year, but not always.
The height of the growing season for peonies is usually in the fall. When the plants store up their nutrients, they usually experience a burst of growth designed to propel them through the winter and into a new year of growth. Even in warmer climates, fall is usually the recommended time for planting peonies so that they will have time to adjust to their surroundings before blooming.
In the spring, the growing season for peonies resumes, often with a burst of nutrients from the roots. It is sometimes tempting to plant during this season, but this is often when the roots are the most fragile. Most horticulturists recommend that peonies only be planted in the spring if they have been started indoors and their root systems have had the winter to get established and solidify themselves.
Peonies require relatively little care once planted, and are generally considered to be a very self-reliant plant. Occasional mulching in the spring is sometimes recommended, depending on soil quality. The plants rarely need to be trimmed, and need only be sheltered from the cold in extremely harsh habitats. Winter is usually only a dangerous season for peonies in very cold environments, as the plants are generally relatively cold-resistant.
Grafting tree peonies usually follows the same schedule as planting peonies. A graft from one tree to another can be a very effective way of stimulating growth and increasing bloom production, but must be done with some care. In most climates, the best season for peonies to be grafted is mid-to early fall. This will give the new graft a chance to adapt and benefit from the autumnal rush of nutrients, and it will have the winter to heal and prepare to support spring blooms. Caring for peonies that have been grafted can be somewhat time-consuming, but if done in the right part of the growing season, is usually easy to master.