Just pick off the seed pod....the thing that the flower came out of...
If you just pick off the blossom, it will continue to form seeds and stop flowering
BUT...about mid season petunias sometimes get leggy and you need to cut the legs back, maybe by half. But don't do all the legs at once or it will stop blooming.
Linda C

What about Supertunias? Do you need to deadhead them or are they like the Wave Petunias? I have been deadheading them, but two of the plants are at the top of a slope and difficult to reach without stepping on other plants. I put them there so they would spread downhill (looks nice!) but deadheading is like playing twister trying to stay off other plants.

I'm not sure what the difference is between supertunias, wave petunias and callibrachoa.....all are closely related...but callibrachoa doesn't seem to need deadheading as much as the others.
Linda C

I've had several different kinds of calibrachoas and I don't deadhead them. There's just too many flowers. I do cut back like Linda mentioned. I do the same with the waves also. If they did need deadheading, I wouldn't get them. I just don't have the time to do all those flowers plus my other annuals that do need deadheading.

Kat

Why would you want to deadhead petunias? I've never done it, and they always look great. You're creating too much work for yourself....

Now, cutting back mid-summer is another issue. Petunias can get leggy/ratty looking, so about mid to end of July I cut them back by about 2/3 to 1/2 (I tend to vary the amount of each stem so it doesn't look to "haircut-y").

The difference between petunias that have been dead headed every few days and those that have not is night and day. If you dead head them they will get those side branches with flowers and not become long stringy plants that need cutting back.
Linda C

Oh. Didn't know that. I wouldn't do it anyway, I just don't have the time for that. I'm still quite happy with my non-deadedheaded petunias :)

From one who grows hundreds of Wave and Tidal Wave petunias, they do NOT need deadheading at all, at any time of the season.

I have always deadheaded my petunias. But i just pull off the spendt flower, I don't touch the stem. Mine don't get leggy, I guess gramma always did it so I followed her advice. What happens if you don't deadhead? Do they still keep blooming? Curious but if there is no need I guess I will stop.

From what I've read, the Wave kind of petunias don't need the deadheading because they don't go to seed until the days get short. Going to seed will slow down the blooming. I've had some non wave petunias and they do go to seed. I have a couple right now and those I do deadhead and pinch back the stems on them every so often. Since I only have 2, it's not a problem for me. My wave ones I just clip back some stems sometimes and that's it.

Kat

Maidinmontana, you are not deadheading...just pulling off the petals. The same as pulling off the petals on a daisy, does nothing to keep the plant blooming.
Linda C