Deadheading spent flowers keeps more blooms coming
Many plants—and most annual bedding plants like geraniums, petunias, and begonias—will re-bloom if you deadhead them.
Deadheading spent flowers (pruning them off) tricks the plant into producing more flowers. Flowers are the reproductive organs of many plants. So, the lifecycle of a plant is: grow, flower, get pollinated, produce seed, and die. When you deadhead flowers, the pollinated flowers are prevented from producing seeds. Instead, the plant produces more flowers, again with the goal of producing seeds.
Keep deadheading flowers and you will have flowers all summer long. Some annuals with soft stems, like petunias, begonias and impatiens, can be deadheaded with just your fingernail. Others, like roses or geraniums will need to be cut off with your pruning shears, to not risk a ragged break in the stem where disease can settle. A clean cut helps the wound seal and heal up faster.
Deadheading should be done once a week. After you finish deadheading, give your plants a half-dose of Scott's Ultra Bloom (the package says to feed Ultra Bloom to flowers every 2 weeks).
Here is a good article from the Burpee Seed Company about deadheading.
Here is a great guide on deadheading different types of flowers, by the Royal Horticultural Society in the UK.
If you want to keep the seed heads for fall and winter interest, like sunflowers, don’t deadhead them. Just let them be, and cut them in the spring clean-up of the next growing season.