If you want to encourage more side branches which will bear flowers, "top" the plant by snapping off the center of the uppermost growth at about 15" tall, or 3-4 leaf sets. It will produce a fuller plant, not shorter than it is destined to be genetically.

Just reach in with thumb and forefinger nails and snap off the center tip between two leaf nodes. This is easiest when the plant is cool and fullest of moisture: evening or early a.m. The only thing not to do is harm the little new growths on either side of your breaking point. You won't. It's easy as pie and a good way to get green thumbnails.

The link offers two very detailed methods of topping dahlias. It's a bit involved for my needs, but may be helpful.

Here is a link that might be useful: Dahlias.net

I read from Swans that when the plant has about 3 to 4 set of leaves that we are supposed to cut the whole top off leaving about an inch or more and than plant and cover the whole thing.?