The process of deadheading can be time consuming but it is well worth the effort. A plant has basically three stages.
We spend a lot of time ensuring our plants grow well in our gardens. Many hours are spent finding the right plant for our garden schemes. Our reward is the bloom of flowers. We get to enjoy the beauty of their shape, the brilliance of their colors and the sweetness of their fragrance. Deadheading will allow you to enjoy the flowering stage of your plants for an extended period of time. I think that is worth the extra time it takes to deadhead a flowering plant.
It is important to know what type of flowering plants you have in your garden. Do you have annuals, perennials or did you plant bulbs? The type of flower your planted or inherited will give you some idea about how effective deadheading will be for that plant.
Annuals: Most annuals will greatly benefit from deadheading
Perennials: Many perennials will benefit from deadheading but some won't
Bulbs: They will not benefit
What is the benefit? Well it is really our benefit. They will continue to flower for a longer period of time. You won't harm a plant by deadheading it. If in doubt, deadhead and see how the experiment goes. Sometimes we inherit flowers that were planted by the previous owners of our homes. See what happens if you deadhead them.
Strictly speaking, you deadhead by removing the spent flower. If you leave the head, the plant begins to produce seeds. If the plant believes it has enough seed heads it will slow or stop flower production. Deadheading interupts this process. If you remove the spent flower heads the plant will respond by producing more flowers. Many flowering plants 'panic' when deadheaded and produce even more flowers. The goal of the plant is to reproduce. The goal of the gardener is to enjoy the flowers.
There are four basic ways to deadhead.
Pinch or cut the flower off the plant right where the flower meets the stem (Petunia)
Pinch or cut the stem of the flower off all the way down to the base of the plant (Daisies)
Shear the plant back when dealing with small flowers (Moonbeam Coreopsis)
Pinch or cut the flower back to the first set of leaves (Marigolds)
They all involve removing the flower. If the plant has a long stem, cut it back to the base of the plant. If it has a short stocky stem cut it back to the first set of leaves. Plants that get hundreds of little flowers like 'Moonbean Coreopsis' are best sheared back an inch or two (like a hair cut). Plants like petunias that are sort of bushy can just have the flower head pinched off. The reason you cut back flowers on stems differntly is so you don't have the eye-sore of the stem with no flower sitting there.
Some plants may not easily fall into one of the above categories. Mums for instance have a ton of flowers and you could snip each flower off if you don't want to cut back foliage. I find this too time consuming and shear the whole plant back just enough to remove all the blooms when most of the flowers are spent.
Deadheading = More Flowers
Also keep in mind deadheading works for garden vegetable plants. Pea and bean plants are good examples of deadheading. Instead of removing the flower, you are removing the seed pod. By harvesting pea pods and beans, you will get more production out of your vegetable plants. If you left the pods on the plants they would mature and brown. The seeds would be set and the plant will stop producing.