Topping dahlias diagram and information courtesy of www.easytogrowbulbs.com
Pinching or "topping" dahlias results in shorter, sturdier, fuller dahlia plants that produce more blooms. This is true for the dwarf dahlias, the dinner plate dahlias and every style and size in between.
Pinching back your dahlias should be used in two ways. First, when your plants are 12-16" tall, pinch off the top of the plant just above the 4th set of leaves, as shown in this diagram to the right. Pinching off the top of the plant is referred to as "topping" your dahlia plant.
Top your dahlia plant cleanly, using shears, scissors or your fingers, taking care not to tear the stem.
Removing the top of your dahlia plant temporarily refocuses the plant's energies from producing flower buds, and instead the plant develops more stems and more foliage - capable of producing and supporting many more blooms. You and your garden will benefit from topping your dahlias for months!
At this stage, your dahlias are growing quickly, rapidly recovering from the pruning.
As your dahlia plant grows, and sets flower buds, continue to pinch back new growth for more bloom production. Where multiple buds are formed, pinching back a few buds will result in larger blooms for those that remain.
Cutting dahlias for your vase will have the same effect as pinching back your dahlia plants.
Dead head your dahlias. As dahlia blooms fade, pinch back the spent blooms. Known as "dead heading", removing spent dahlia blooms prevents production of the seeds the plant is producing to aid in reproduction. By removing the spent blooms, you are causing your dahlia plant to produce more blooms in its quest for reproductive seeds. ( for those interested, this post information works for chrysanthemums too )