. . . if you started with a single daylily and you divided it into three plants after one year, and then divided it and each of its divisions into three plants the following year, and kept doing this every year for 7 years, you would end up with 2,187 daylilies? You can do the same thing with Hosta and many other perennials!
While many good quality perennials seem expensive in the garden center, the cost can easily be justified by a patient gardener who divides his or her perennials. Not only does this increase the number of plants in your garden, but it's also good for the plant, encouraging new root growth and better blooms the following season. Propagating your own plants allows you to expand your gardens inexpensively and carry a common theme throughout your landscape.
Of course, if you want, you can usually divide a plant whenever it is large enough, i.e. has more than one stem such that each division will have roots and a crown.
Fall is an excellent time to divide many perennials because the warm soil, increased likelihood of rain, and fewer insect and disease related problems combine to make perfect growing conditions for your new divisions. New roots will grow all winter and, come spring, will be strong enough to support lush new top growth. These divisions, supplemented with new perennials, can be used to create a new bed or to extend an existing garden.
Early spring is another good time for dividing perennials. February or March before they break dormancy is the best time to divide most fall blooming perennials like asters, Chrysanthemums, Ceratostigma (Plumbago), Helianthus, Japanese anemones, dahlias, and ornamental grasses. Other plants that do better when divided in the spring but after they finish blooming are Dicentra (Bleeding heart) and Primula (Primrose).
Daylilies fall into this category but keep in mind that it is important to cut the foliage back when you divide plants that are actively growing in order to reduce water loss through the leaves.
Certain plants do not do well or cannot be propagated reliably by division. These plants should be propagated by other means such as with cuttings. Included in this group are lavender, Alyssum, Iberis, Santolina, Perovskia, Buddleia, and Caryopteris.