The term “dead heading’ refers to the practice of removing faded or dead flowers from plants.  When this is done, the plant will usually respond by producing new flowers, thus prolonging the flowering season. Simply pinch off the flower with your fingers or use a pruner  or strong scissors to cut just below the flower head and above the next set of leaves. If the plant is very large and bushy then you can shear the whole plant with a hedge shears. Dead-heading can be a very tedious job so try to do some every day or a few times a week at least. However, the rewards are worth the effort.

Courtesy of NYBG Plant Information Service