It's that time of year to consider pinching or topping your dahlias. This is the process of cutting off the top portion of your plant to allow for greater plant growth and to multiply the production of flowers.
Your plants should be 12 to 18 inches tall when pinching. It would be best to come up to the third or fourth lateral (laterals are sets of leaves growing off the stem), then cut the top portion off.
If your plant is already producing buds, be brave and make that cut. This will help your plant produce so many beautiful flowers that you'll be glad you did this. If you have more than two stems attached to your tuber, the extra stems should also be removed.
See below for a video on Pinching dahlias.