The main reasons to divide perennial plants are to make them smaller to fit a space and to create more plants. Division can also help rejuvenate sad and overgrown plants.
When the following plants make themselves known this spring, go ahead and divide them. It's a good idea to prepare pots or dig and prepare the new planting hole before you start. That way the roots will be protected and not left in the wind and sun.
Aster, Bee Balm (Monarda), Blanket Flower (Gaillardia) , Blazing Star (Liatris), Catmint (Nepeta), Chrysanthemum, Coral Bells (Heuchera), Coreopsis, Cornflower, Daylily, (Hemerocallis), Ferns, Hardy Geranium, Hardy Zinnia (Heliopsis), Hosta, Obedient Plant (Physostegia), Coneflower (Rudbeckia), Ornamental Grasses, Perennial Salvia hybrids, Phlox paniculata, Purple Coneflower (Echinacea purpurea), Shasta Daisy, Speedwell (Veronica), Spiderwort (Tradescantia virginiana), Stonecrop (Sedum), Wormwood (Artemisia species) and Yarrow (Achillea species).