When the dahlia first starts growing, it will develop 4 sets of leaves. Pinching off the top of the plant above the fourth set of leaves will encourage branching, resulting in a fuller plant and therefore more flowers. At this point in their growing cycle, the dahlias are sprouting rapidly, so they'll recover quickly. You can continue to remove new growth throughout the growing season to encourage even more flower production. New growth is easy to remove with just your forefinger and thumb, although you can use a sharp tool if you prefer. More flowers take up more of the dahlia's energy, resulting in smaller flowers. To get the biggest dahlia blooms, restrict the number of flowers on each plant, so the energy can be directed towards growing just a few large flowers. If you prefer to have more flowers and you aren't as worried about size, just focus on cutting back new growth occasionally. As the dahlia branches out, you may see as many as 10 flowers on a single plant. The practice of removing faded flowers, also called deadheading, encourages more blooms. You may even see your dahlia bloom all the way through the fall if you remove faded flowers through the season. Since dahlias have tough, stringy stems, it's easiest to remove the flowers with sharp scissors, pruning shears, or a knife. Cut back the flower stem to where it meets a main stem. When all the petals have completely fallen off the flower, it can be hard to tell the difference between a new flower bud and the closed calyx left behind from the flower, also known as the dead head. However, the dead head is cone-shaped, whereas a bud is rounded. While removing dead heads will encourage new buds to begin blooming, removing the buds will delay flower development.

Summary:
Pinch the tips off of dahlias once they reach 16in (41cm). Limit each plant to 3 to 5 flowering stems if you want giant blooms. Allow up to 10 flower stems if you don't mind smaller blooms. Cut off flowers with scissors once they have started to fade. Do not remove rounded buds when you're deadheading.