“Petunias are popular and reliable flowering ornamental plants,” according to the University of Rhode Island. These annuals bloom from spring up to the first frost, emit an attractive scent, grow well in flowerbeds or pots and come in numerous colors and varieties. Growing petunias from seeds is often difficult because of the small seed size and light requirements. However, according to the Pacific Northwest Extension Publication, it's possible to propagate petunias from stem cuttings in two to three weeks.
Fill a pot equipped with draining holes full of a growing medium that provides both water preservation and adequate aeration. Combine a mixture of half sand (perlite, vermiculite) and half peat (sphagnum) moss.
Add enough water to moisten the growing medium. Use a pencil to create an adequate amount of holes for each petunia cutting. Space each hole far enough away to ensure the cuttings (or leaves on the cuttings) do not touch.
Collect stem cuttings from petunias in the spring when the plant's growth is highest, or any time the petunia is actively growing. Cut the petunia stem from just underneath a node. Cut off a 2-to-6 inch section from a healthy petunia plant for each cutting.
Cut off 1/2 to 2/3 of the petunias leaves, beginning from the base of the cutting going up. Remove any blossoms or buds, and cut all bigger leaves in half.
Put some rooting hormone in a small plastic bag. Dip each stem cutting into the hormone and shake the excess off. Place the petunia cutting in a hole and gently press the soil around it. Repeat for each cutting.
Place three or four sticks that are taller than the cuttings around the outside of the pot. This will keep the bag from touching the cuttings. Place a plastic bag over the entire pot to keep moisture around the cuttings. Put the pot in an area with indirect sunlight.
Check the growing medium's moisture level every other day. Mist with a spray bottle when needed to keep it medium damp.
Test for root growth from the petunia stem cuttings in two to three weeks. Carefully use your fingers to reach beneath each petunia stem and lift cautiously. Keep cuttings in place for one or two additional weeks, if you find little or no root growth.
Gradually limit the amount of humidity around the cuttings after roots form. Open the bag a bit more every day to expose the petunia stem cuttings to the outside environment.