Petunias are fast-growing, summer bloomers that are available in a vast selection of varieties, including a rainbow of bright colors, double and single types, big blooms or demure miniatures, and upright or trailing types. Although petunias are usually propagated by seed, vegetative propagation is a fast and inexpensive way to grow a new petunia plant from an existing plant.
Cut a 2- to 6-inch stem tip from a healthy petunia plant, using pruners or a knife with a sharp, clean blade. Make the cut just below a leaf. Remove any blooms or buds from the stem.
Pinch off the leaves from the bottom one-half to two-thirds of the petunia stem. Dip the cut end of the stem in powdered rooting hormone.
Plant the stem cutting in a container filled with commercial potting soil. If the container doesn't have drainage holes in the bottom, poke three or four holes with a sharp knife. Water the potting soil with a spray bottle until the soil is damp, but not soggy.
Cover the container with a clear plastic bag and secure the bag with a rubber band. If necessary, place a few small sticks in the bag to keep the plastic from touching the petunia leaves.
Put the petunia stem cutting in a bright spot away from direct, hot sunlight which can scorch the stem. The stem should root in two to three weeks. Tug lightly on the stem. The stem has rooted if you feel a slight resistance.
Remove the container from the bag. Place the new petunia plant outdoors in a shady, protected area for three or four days.
Remove the petunia from the container and transplant the petunia to its permanent home. Petunias should be planted in full sunlight and well-drained soil.