Petunia varieties have a dominant growth habit determined by their genetics. I suggest reading the descriptions about a specific plant before purchasing, so that you won't be disappointed.

In most instances, petunias are either mounding or trailing, or something in between. Some judicial snipping and pinching can alter a trailing variety's tendency to develop long, hanging stems.

Does that answer your question?

Even if you allow a mounding petunia plant to reach several inches tall and the stems are falling all over themselves, that doesn't make it a trailing type.

Read that label and use them for what their growth habit suggests.