When Should flowers be cut back?
Annuals focus their energy on growing stems tall enough to outcompete the neighbors. In general, it’s wise to begin pruning after the first burst of blooms and stop pruning when the season is done. Once they start to struggle in the heat, they’ll benefit from a good cut back and then fertilization. This will allow the heat-lovers to be free to perform, while also setting set up your cool-temperature plants to perform again when the days cool as you head into fall.
Deadheading plants, and removing spent/dead flowers, on a regular basis pushes blooms and builds a nice, full appearance. Removing dead flowers, broken stems, and fading leaves forces new growth. Some plants actually need spent flowers removed before they will rebloom at all. Plants with single stemmed flowers should be cut to the base of the stem. Flowers that bloom on a stalk should see the stalk cut back as the blooms fade so the focus is on the lower blooms.
Cutting back annuals such as Coleus, Zinnias, Snapdragons, and Petunias creates a more lush and full plant. Using clean pruners cut above a place on the stem where you see new small leaves beginning to form. These little leaves will push out new stems making the plant take on more vertical and horizontal growth.
Deadhead the plants often, remove faded flowers and don't let seed pods form.
Many modern hybrids have been developed to be self-cleaning. Their faded flowers seem to disappear as new buds form and open.
If you have plants that start to look ragged in midsummer, don't be afraid to prune them back by several inches. For example, petunias can get long and leggy and will look better if they are cut back by half, encouraging them to send out new growth. Coleus will grow straight up if not pinched regularly until it fills out.
If you are going away for a week or two in the summer, prune your annuals just before leaving and they'll be back in bloom when you return.