As a retailer, I'd normally tell people what your other answerers have said, but...Depending on your growing zone and the variety of petunia, they sure can! We grow and sell petunias as annuals, but many of them are free seeding annuals, which means they'll leave seed on the ground for next year's babies. Even cascading and vining types of petunias will reseed themselves if you let them. Many of the folks who bought Purple Wave petunias from us in the early years after the Waves were developed have stopped buying them and have let them seed themselves in beds. Other colors don't give such good results, but the Purple Waves breed true and propagate themselves profusely (We're in Zone 5.).
As to old fashioned types, they're the best at self-propagation of all. When we first moved back to the family farm, we had petunias sprout in a new bed where we had installed holly bushes. We sure didn't plant them there.
There are also perennial petunias, and native wild petunias, even here in Zone 5, but they're not what you would normally find in a garden center.
Do petunias come back every year?
maybe the old timey kind, but the new versions don't
Reply:No, they do not. You'll have to start them every spring. They are annual plants, which means they will live one season.
Reply:In most climates, Petunias are annuals. I have had them come back on their own in my garden, but it is not reliable. If they have a good winter, (I am in zone 8) and they like where they are, (good soil, and protection from frost) they simply grow on their own. I don't find them to be as robust as those fresh plants I put in new each spring, but they are still a welcome surprise.
Reply:No, they are half hardy annuals.