Perennials can be purchased in a number of ways. The most common way is plants in quart, one or two gallon containers. These plants are already growing and afford the gardener the flexibility to select and plant through the growing season. Another way is bare root or packaged plants. These are obtained through mail order or at garden centers and are sold as dormant material. These are available for spring planting only. If these materials are received at a time that they can not be planted immediately, keep the plants cool and keep the roots moist. They can be held for several weeks this way, thus assuring their survival prior to planting.
Most perennials are best planted in the spring. However, with the availability of material in containers, the planting season often extends well into the summer and early fall with autumn planting continuing until the first of October. The earlier perennials are planted the better the root system will be when the plant enters the winter. Late fall plantings can sometimes result in frost heaving and loss of perennials.
Containerized perennials should be planted at the same depth they were grown in the container. Planting too high results in plants drying out and too low invites crown rots. Some perennials such as bleeding heart, iris and peony need shallow planting in order to flower properly. Containerized plants should be watered before planting and bare root perennials should be soaked in water for one hour prior to planting in order to rehydrate the plants.
Most perennials are transplanted in the spring as growth starts or in the late summer or early fall. It is usually best to wait until the plants have flowered and then cut back by half just prior to moving. If plants are moved out of season, they may need to be shaded for several days to allow them to recover.