Petunias are self-seeding annuals, so they germinate and propagate seeds naturally at the end of their yearly life cycle. This means you could find your garden overrun with petunias within just a few years, if you don’t cull them. If you have extra petunias, you can start new plants to transplant to other parts of your garden using "cuttings." Petunia cuttings grow new roots once stalks are cut and planted.
Difficulty: Moderately Easy
Use a scissors or pruning shears to cut off a healthy petunia stalk from the main plant. Cut away the stalk as close to the main plant as possible, near a Y-shaped junction.
Slice the base of the petunia stalk using a thin kitchen knife. Make sure your slice extends about half-way up the stalk. This slit in the stalk’s exterior allows new root growth from the recently cut petunia.
Dig a hole large enough to accommodate about half the cutting. Most petunia cuttings are about 3 to 4-inches tall, so your hole only needs to be 1 1/2 to 2 inches deep. Make sure you dig your hole in a spot in the garden that gets at least six hours of direct sunlight daily.
Place the cutting in the hole and fill the hole with soil.
Water the cutting, ensuring the soil remains moist, but not overly wet or muddy. The cutting should begin sprouting roots almost immediately, and new flower buds can appear in as little as one to two weeks.