As you gain experience with gardening, you may eventually want to change the location of some of your plants. Almost all perennials can be moved at nearly any time by lifting them with a good clump of soil intact around the roots. In early spring and in the fall, the plants can be dug without soil on the roots, but when the plants are in growth, they cannot establish new hair roots in time to keep up with the demands of the tops. Plants can be dug and divided even after they've begun to grow in the spring, but the longer you wait, the more care they will need to reestablish themselves. If a plant is divided and reset with no wilting of the tops, they're fine. If they do wilt, they should be watered again or even shaded with a board or open box until new roots can start in two or three days. You might try cracking a piece off the side of some plants just after the ground warms, transplanting these to new spots (leaving the main part of the plant undisturbed).
Plants like Dianthus have a central stem and cannot be split, and Astilbes have such a hard center that they'd have to be dug in order to cut or divide them. Some plants, such as Coreopsis and Shasta Daisies, will seed themselves. These new offspring can be saved once you learn to recognize the seedlings.