Deadheading (removing spent blossoms) should be done throughout the growing season. By midseason, if you have not taken the time for this task many of your plants probably need a good cleanup.
Most flowering annuals, perennials, and some flowering shrubs will benefit from removing the dead blooms. Some continue to produce new blooms whether or not you deadhead, but many will stop producing flowers altogether unless the seed heads are removed.
In addition to encouraging additional blooming, deadheading refreshes your garden. By midseason many annuals and perennials become overgrown, gangly and just plain messy looking. Dead blooms blanket your garden with unappealing brown, and shriveled flowers becoming quite unsightly.
A thorough midseason deadheading and some trimming of overgrown and damaged foliage will bring new life into your garden. Fertilize at this time as well, and in no time at all new foliage growth and bright new blossoms will appear for a fresh and colorful redecorating of your garden beds.