Deadheading and cutting back are two pruning methods most frequently applied to perennials and annuals. They are also applicable to shrubs and vines, especially those that are flowering.
Depending on the hardness of the stem, deadheading is the removal of declining flowers from a plant with fingers, scissors, or hand pruners. Deadheading prevents a plant from setting seed or producing fruit, and in many cases encourages the plant to produce more flowers. As an added benefit, deadheading makes plants appear tidier.
The most straightforward way to deadhead is to remove only the faded flowers. Once all the flowers on a stem have been removed, cut the stem back to a bud or leaf, or to its base if it's leafless, such as a Daylily. A variation of deadheading is disbudding, which is the removal of smaller flower buds. This concentrates a plant's energy on the remaining flowers. There won't be as many flowers, but those that remain will be larger.
The only time you want to avoid deadheading is when you want to produce fruit or seed, or even leave an attractive seedhead to remain for winter interest. Some of the more interesting seedheads belong to the Echinaceas, and some of the hips of roses. Also, seeds left on plants are often used to feed birds and small critters.
This term refers to the removal of part of a plant's top growth. The amount removed depends on the type of plant, time of year, and intended result.
In spring, cutting back is done to remove the dead growth from the previous season. Generally, a perennial is cut back to a few inches above the ground. Shrubs may be cut back by about 1/3 if they have summer blooms. Spring-blooming shrubs would have blooms on the growth from the previous season so they should not be cut back until the bloom is finished.
In summer, perennials and annuals are often cut back to encourage a more dense bloom and less height in the plant. For example, the Chrysanthemums in nurseries have very dense blooms because they have been cut back; contrast these to many chrysanthemums in the garden, which were not cut back.
In the fall, some perennials are cut back in preparation for winter. Other perennials can wait for cutting back until next spring. As a general rule, if a plant has interesting or strong enough stems that will not collapse or be damaged under the weight of snow, they could be left intact until next spring. If the plant has weak stems, then it may be best to cut it back in the fall.
Almost all plants benefit from some type of pruning, so don't be afraid to venture out with your garden clippers. Your plants will benefit, and your garden will be more interesting as a result.