Don't let anyone tell you differently, but deadheading is a job! It seems like an endless job if you want to keep your garden in bloom throughout the summer. We all know about cutting off the spent roses (however, with Knockout, you do not have to prune except for looks and size). The salvias and nepetas need to be deadheaded if you want rebloom. May Night is everywhere, and it seems that not many people know that it has to be cut back if you want it to bloom throughout the summer. The rebloom on most plants will never equal the initial bloom. I cut my May Night back about four times in a season. I have several other low growing salvias that benefit from the same practice. Nepeta, I deadhead less or more often depending upon the plant, Walker's Low once during the season, Six Hills Giant, once and Souvenir d'Andre Chaudron stem by stem.
Certain phlox will bloom most of the summer if deadheaded, one of the most prolific after pruning is Blue Paradise. I have tried it with David (although my daughter has better luck with David reblooming profusely) Franz Schubert and Eva Cullum with less than spectacular results. Deadheading Echinacea (coneflowers) is important if you want to extend the blooming time. I went to a workshop some years ago where the speaker was Tracy DiSabato-Aust who wrote The Well-Tended Perennial Garden (planting and pruning techniques) great book for a beginning pruner as I was. Some plants do not benefit from deadheading, most daylilies, lillies, asters and peonies are in this category, and I am sure there are many more. My motto is, if in doubt, deadhead - it can't hurt and it will give you a neater looking plant.