Hardy perennials, grown outdoors or in cold frames can theoretically be planted whenever you can work the soil. Actively growing plants from greenhouses are best planted after the danger of frost has passed in the spring, or no less than one month before the first autumn frost. Plants that are marginally hardy, or are situated in less than optimal sites are usually best planted in May or June, because it allows them to become established before the first winter season.
To prepare the planting site, remove large rocks, old roots and dead plant material. Then loosen the soil with a spading fork, add two to three inches of sphagnum peat moss or another well-composted organic matter, and thoroughly incorporate in the soil.
Most perennials are available in one gallon, four inch or smaller pots. To plant, remove the perennial from its pot and place it in a hole. Make sure the top of the rootball is flush with the overall soil level. Gently firm the soil mix around the rootball to eliminate large air pockets. Next, thoroughly water your newly planted perennials. Keep them adequately watered, and don’t allow them to dehydrate for at least a month in summer weather.
Container-grown perennials may be rootbound. When this occurs, loosen the rootball by untwining or scoring it with your hands or a sharp knife.