Some perennials, such as coreopsis and columbine, are sold in six-packs like annuals because they grow quickly from seed. More often, perennial plants are grown from root divisions or cuttings and are sold individually in peat or plastic pots. As with annuals, look for compact, stocky plants with a bushy crown of leaves. Avoid plants with brown leaves, plants coming into flower, and plants that have weak, gangly stems.
To remove a perennial from its pot, soak the soil and slide out the rootball. If this proves difficult, cut the sides of the pot, which will make it easier to remove the plant without tearing its roots. Tease the roots apart and plant so that the rootball fits snugly into the hole. Firm the surface with a trowel or with the sole of your shoe to make good soil contact around the roots; then water. In the absence of rainfall, newly planted perennials need watering almost every day until they are well established.