Prepare a rooting mixture by combining peat moss, sandy soil and plant food. These should be mixed together in equal portions for best results. Moisten the mixture and place into a rooting tray.
Cut a section of the petunia stem in September when the plant is starting to seed and blooming is nearing completion for the year. The cutting should be taken from near the top of the petunia where the leaves are still vibrant and soft.
Place the root end of the cutting into a rooting hormone to begin the growth of new roots.
Push the end of the cutting into the prepared potting mixture and press soil around the stem to stabilize.
Water the petunia cutting with a light spray of water. Do not allow the water to pool in the potting mixture.
Place the cutting in an area where the temperatures will remain above 45 degrees during the summer months.
Water the cutting enough to maintain the moisture level of the soil.