Petunias have gotten more expensive in the last few years, with fancy types sold as singles rather than in pony packs. A lot of the spreading ones, new colors like ‘Black Velvet’ and ‘Black Cat’ (both sold as ‘the only black petunia in the world’), and the gorgeous Dulce Lemonicello (yellow center fading to white edges) and Dulce Flambe (soft shades of tan, cream, rose, mauve and yellow, with two or three colors on each flower) are sold this way, and sometimes cost up to four dollars a plant. That makes it hard to use them as bedding plants! The good news is, you can start petunias from cuttings, save the plants over the winter in the house, and have more plants to set out next spring. The plants will be further along with less trouble than if you started petunias from seeds.
Take cuttings two or three inches long from healthy plants. Make sure there are no insects on the cuttings before you bring them in. Remove all flowers and flower buds. With a sharp knife or a single edged razor, cut ¼” below the bottom node- the growing point that leaves and flowers come out of. Make another cut ¼” above the top node, at a 45 degree angle. Then remove all the leaves except the top two. If those top leaves are large, cut them in half so there is less surface for moisture to transpire from- without roots, the cuttings can’t take up much water.
Fill a four inch plastic plant container with fresh, sterile potting soil- do not reuse soil when starting cuttings or seeds. (It’s usually all right to do so with larger plants, as long as the former dweller in the soil wasn’t diseased or insect ridden). Bring the level of the soil to ½” below the top of the container- this will allow good air circulation. Take a pencil and make a hole in the potting soil; remove the pencil, put the cutting gently into the hole, and firm the soil around it. Never just push a cutting into soil; you can damage the tissue badly enough it won’t root! A four inch container can easily hold five cuttings, more if you’re short of space and doing a lot of cuttings. Water the container until all the potting soil is wet. Do NOT use rooting hormone on soft, green cuttings like these- it will burn the tissue. Put the containers in a cool area with filtered light, like a north window sill.
Petunias will also root- and even over winter- in water. I first learned about rooting petunias when my sister-in-law did this one winter. My personal preference is doing it in soil, though- the roots are less likely to tangle if you’re doing multiple cuttings and it’s less harmful when the cats knock it off the window sill.
In a few weeks, the cuttings will have rooted and will start top growth. When top growth starts, move the plants to a south window. During winter, keep the top growth pinched back so the plants don’t get leggy. If you have room, you can separate the plants and put them in their own containers. In spring, fertilize lightly. You can now let them grow out and take cuttings from those plants, until you have enough to use as free bedding plants!
One thing to remember: If you’ve bought the plants you’re taking cuttings from, check the tag to make sure they aren’t under patent. It’s illegal to propagate those asexually (cuttings, divisions) but all right to use their seeds, but those won’t come true.