To promote your Dahlia to bloom more, it requires some trimming. This technique is called “deadheading.” This simple pinching technique is used to discard older blooms that have passed. You can use your fingers to gently pinch off older flowers, or if you prefer to cut the dead materials you can use floral snips.
The best way to deadhead summer blooming bulbs is to cut the old blossoms close to the plant's newest growth. By trimming out the old blossoms, it tells the plant to stop sending energy to the dead blossom, and it will promote new buds to appear.
Dahlias, Begonias, Cannas, and Callas are all summer blooming bulbs that require deadheading.