The lowest set of leaves are known as the "cotyledon" leaves--the very first set of leaves that appear on the stem. Now, count 3 sets of leaes ABOVE the cotyledon leaves, and pinch above them. In other words, pinch above the third set of true leaves.

For absolutely fabulous information about growing dahlas, go down from the top until you come to the posting "Dahlias in pots" by "proudgm". Open it, and the 5th posting inside this post is a post by "poochella" in which at the bottom of his post is a reference to the American Dahlia Society. Read it, download it, and print it. It is authentic, detailed, and accurate. You could not ask for a better source, nor a better article! ENJOY !

Good Info can be found in the link below.

Here is a link that might be useful: Topping Dahlias