There are several reasons to deadhead flowers. Most flower heads are easily snipped away with a sharp pair of garden shears. For tougher-stemmed flowers, use hand pruners. The easiest way to keep this up is to have scissors in hand when strolling through the garden. If deadheading is done routinely, it is not a time-consuming chore.
Deadhead for Rebloom
Some flowers bloom a second time in one season. Remove the flower heads as soon as they look wilted, brown or dry. The energy the plant normally expends to shed these blooms will be spared. This will result in larger flowers upon rebloom. It may also coax the plant to produce a larger number of flowers.
Deadhead to Rejuvenate
Pruning away dead areas on any plant has a rejuvenating effect. Removing spent flower heads as soon as they fade will also add vigor. The cell activity is boosted as the plant attempts to heal from the wound. New healthy leaves and flowers may emerge from the cut area. This is helpful even if the plant is not a repeat bloomer. By waiting too long to deadhead, you could remove tissue that is developing for next year's bloom season.
Deadhead for Control
Once flower heads mature, they begin to produce seed. In most species the seed starts out fleshy and green. This stage will not pose a problem. Once seeds change to a bronze or brown, they are reaching the ripe stage. Before long the plant will disperse the seed. Some seed casings pop open, and the wind will carry the seed away. Other seed may be eaten by birds and dispersed after it is digested. To avoid unwanted seedlings in your garden, cut the flower heads before the seed reaches maturity.
Deadhead for Appearance
Some of the most glorious blooming plants can look bad when the flowers get shriveled and brown. Shrubs like camellia are just as valuable as foliage plants as they are for their attractive blooms. When you remove the unsightly dead flowers, the plant can take its place as an evergreen background shrub.
You may want to save seed for the following year. Some plants drop or otherwise disperse seed so quickly it is easy to miss. Placing something beneath the plant may be necessary to catch the seed. Plants such as coneflower have obvious seeds. The seed can be allowed to dry within the flower until it is easily pulled off the cone. Then the entire flower head can be removed. Others, such as poppy, drop their petals and the seed head remains standing. Plants like sweet pea develop a seed pod once the flowers die back. Refrain from deadheading plants when you want to collect seed. Wait until you have harvested the seed before snipping off the flower heads.
When to Deadhead Early
Sometimes flowers must be removed before they mature. This is necessary to keep plants from going dormant. Annual herbs like basil or cilantro complete their cycle when they produce seed. As soon as the flower buds appear, they need to be removed. This will allow the plant to go on producing edible leaves all summer.