Deadheading keeps plants that are self-seeding from overtaking the garden bed. It also promotes the formation of a bushier plant and encourages more blooms. These benefits are long lasting in perennials, which flower year to year.

Why It Is Important

Plants form flowers because they are going to fruit or produce seeds. This is an instinctive behavior that the plant undergoes in order to perpetuate itself. Just like animals, there is a natural need to keep the species alive and viable and seeds are the main method of propagation for most plants. Plants whose natural life cycle dictates the production of seed for future generations can be encouraged to bloom again if we remove the flowers before they produce useful seed. The challenge for the species to survive inflicts this urge in plant biology. Seed production uses up much of the plant's energy and removing spent flowers allows the plant to redirect this energy to the formation of new buds.
Flowers with many petals harbor insects in their folds and deadheading can prevent pest populations from building up and harming the plant's vitality. Cleaning off the spent blooms in plants such as Camellias and Roses will also keep the petals from blowing all over the garden. For certain flowers, such as roses, deadheading will also encourage taller plants with fuller bloom centers and thicker flower displays. Of course, some roses are self-cleaning and do not need the practice, while others like Rugosa form beautiful hips which add a lovely accent to the plant and cause no problems if left to grow.
Seed pods and heads add a great deal of interest to the fall garden. In plants like irises and sedum, the pods and seed clusters persist well into the cold season and are unique features of the plant. Leaving these on is the gardener's choice and will not cause the plant to loose energy in the long run. Many perennial and wild flowers produce prolific seed and will choke an area of the garden the next season if left on. They can become a nuisance especially those like Joe Pye weed, which will float seed to areas of the landscape best left to other types of plants. Harvesting the seed before it disperses allows you to plant species of annuals the next year and provide a source of free specimens. Again, it is up to the individual gardener and the manner in which he/she keeps the landscape space to decide if deadheading seed heads and pods is a necessary chore.

Best Methods

There are many ways to deadhead. Most flowers should simply be snipped off individually or popped off the stem. Some plants such as roses need to have the blooms pruned off down to the first or second 5 leaf set.
Some rosarians disagree and believe bushier plants result from snapping the rose off at the stem but this leaves and unsightly brown stem in a week. Petunias grow numerous blooms on long stems. Individually tackling all those dead flowers is a waste of time. The best approach is to cut back the stem and allow the plant to produce a new flush of growth. Geranium deadheading requires the removal of the entire stem which breaks off easily at the base of the branch. Plants with clusters of flowers can be particularly annoying as not all the flowers in the group will die and bloom at the same time. You can neurotically pick your way through all the dead ones or simply wait until the bulk of the cluster has passed its prime and then snap off the entire stem. You will lose a few blooms but the process will promote more and it is more time efficient.

Should I Deadhead?

Many of the flowers we grow don't require deadheading, but will perform better with the practice. Something to think about is that some species such as Rudbekia and sunflowers provide important seed food for birds.
Any time you remove the spent flowers from a plant that might produce a fruit you are reducing opportunities for wildlife. If you hate deadheading and can't stand the sight of brown flowers choose a self-cleaning variety or one whose new blooms will cover up the old flowers. Ageratum and Anvitalia are good examples of those which will bloom over the spent flowers. Sterile varieties are also a good choice as they will bloom continuously in the vain hope they can produce seed, but they never do. There are many new cultivars and species from which to choose, but keep in mind they do not attract pollinators to your landscape.