Mexican petunias (Ruellia brittoniana) are considered invasive plants in some areas since they easily spread seed and grow. Because pruning encourages more blooms, some experts, such as those at the University of Florida Nassau County Extension, do not recommend pruning Mexican petunias. However, if you want to encourage fuller plants that bloom more and you have them under control (e.g., they're not near your neighbor's yard), you can certainly prune them, if desired. Otherwise, just allow Mexican petunias to die back in the fall and then regrow in the spring.
Cut off dead, damaged and diseased Mexican petunias anytime. Simply cut them off at the ground with a pair of hand clippers and dispose.
Wait until the late fall when the plant begins to die from a hard frost. This is the time to prune Mexican petunias, if desired.
Prune them to the ground. Use hand clippers or, if you have a lot, a pair of pruning shears will work as well. Discard the cuttings so you do not attract small animals and bugs to hibernate or take home in your garden bed.