In return, this allows the it to become more fuller and have more blossoms. This is done due to the fact that dahlias tend to grow in only one branch. One can top or stop dahlias by cutting the main stem just a couple of inches from the top. One can also use there fingers to snip off the top of the branch. Topping or stopping dahlias can be done upon planting them but also when you dead head later in the summer.