Perennials grown in containers may be planted at any time during the growing season, but it is best to get them in the ground in early to mid-spring or in the fall. Cooler temperatures during these seasons will allow them to develop a good root system. Avoid plantin gin midsummer, when hot temperatures and strong sun may cause them stress.
First dig a hole as deep and about twice as wide as the container the perennial came in. Set the soil aside as you dig. Remove the plant from the container and examine the roots. If the rootball is very constricted with soil, make several half-inch cuts around the outside of the ball to encourage the roots to grow outward.
Place the plant in the hole. You want it to sit at the same level as it sat when it was in its pot. You may need to take it out again and make a small mound of soil in the bottom of hole to place it on, in order to lift its crown to the right height. A plant sitting slightly too high in the ground is better off than a plant sitting too deep. Once the plant is in a good position, place the soil back in the hole. Push the soil into a sort of collar encircling the plant, about 12 inches from the main stems. This will keep water from running away from the plant; instead it will sink down into the root zone. Water your newly planted perennial well.