One of the terrific things about the Wave Petunia is that it requires very little maintenance and there are no complex pruning procedures. The results of pruning out the leggy late-summer growth will be even more waves of flowers and a beautiful display.
Sterilize Your Pruners - To prune a Wave petunia, it is important to use clean tools in order to prevent any fungal issues from spreading from other plants you have previously pruned. Do this by cleaning your tools with 70% isopropyl alcohol, which is often labeled as "rubbing alcohol".
Cut Petunias Harshly - If you want to trim back the petunia itself, cut mercilessly and with vigor. Cut up to half their length by taking pruners and lopping off at any location along the stem. Compost the cut pieces.
Water and Fertilize Post-Prune - Water petunias immediately following the prune. This is also a good time to fertilize your petunias to encourage new growth. I prefer an organic fertilizer such as Espoma Flower-Tone Plant Food as it is better for the environment.