When it comes to flowers, the term "deadhead" has nothing to do with the followers of the 1970s rock band the Grateful Dead. The term refers to removing the spent flower--the "dead head"--to spur the plant to produce new blooms. How you deadhead a plant depends on how that plant blooms and the overall growing habits of the plant. In the case of dahlias, blooms form in clusters of three. Many gardeners prefer to allow all blooms to open but some prefer to "disbud" or cut the secondary buds to force energy to the central bloom. Either way, deadheading the faded blooms improves the appearance of dahlia plants.
Look for blooms with faded colour and discoloured edges. Outside petals begin to brown and the flower loses colour as it ages. Eventually, petals begin to fall, leaving the flower shrivelled and unattractive.
Clip the stem to the flower head to deadhead the dahlia bloom. Typically the bloom is suspended on a four- to six-inch stem. Cut the stem of the dahlia flower where it joins the main stalk of the plant. Use care not to cut the stems of the developing buds unless you wish to disbud.
If you wish to disbud, cut the secondary buds. They form on their own stems.
If dahlias are not deadheaded on a regular basis, they may stop blooming entirely.