It rooted without rooting hormone- just plain water. So I planted it. Easy.
This is what my cutting will develop into once established. I bought these mini white petunias on clearance at a department store in June. They were half off because they had been neglected and dried out. It didn't take long for them to bounce back with a little water and pruning.From now on when I have some cuttings left from pruning, I'll put them in a vase to propagate more petunia plants.
Turns out Petunias, as well as Tomato cuttings, root very easily. Take a look at these videos to see how Halvor in Norway roots his tomatoes and petunias.