How do I take dead flowers off of the petunia? They seem to get tall

Pinch back to a side shoot if there is already more than one branch from the root. If there is only one growing stem, pinch back a few inches from the ground, leaving necessary leaves to feed the new growth...then feed it with a fertilizer for blooms. It should then form other branches and produce a mound of flowers rather than on long stem.

Just pinch off the spent blooms. I do this with just about any flowering plant. I do a walk around every day and pinch dead blooms off plants and once a week I make sure the all the flowers are neat and tidy.

If I get your meaning, you want to keep them compact when they start getting "leggy"? I read once that you can pinch off up to 1/3 of the plant at a time and then give it time to recover before doing more trimming. They do get straggly midway thru! Give it a try. It will prolong their garden life for a while.

Here is a video to help you - https://www.youtube.com/watch?v=JmB4K1NoV24, good luck