Common petunias die at about 30 degrees F. A few cross-bred and specialty petunia varieties die at 35 degrees F If your garden does not feature common petunias, its best to play it safe and assume the plants will freeze at 35 degrees.
Petunias are annuals, so they naturally die after one year. However, it's still important to prevent freezing, because petunias germinate and propagate seeds into your garden during their natural death cycle.
Petunias have a simple, shallow root system, ideal for potting. If you expect inclement weather, dig up your petunias and place them in indoor pots.