How will I know when I need to divide?
Your plants will usually "tell" you when they need to be divided! Here are several signs to watch for:
The center of the plant is beginning to decline while the outside edges are growing well
The plant is not blooming well or has stopped producing blooms altogether
The plant just isn't performing like it used to
Your plant is outgrowing its space and crowding out the plants beside it.
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.
When is the best time to divide plants?
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).
Some plants can be divided most anytime
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.
Plants that shouldn't be divided
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.