Petunia cuttings are easily rooted in the spring when stems are soft and green. It’s important to understand the difference between patented and trademarked petunias. If a petunia cultivar is patented, you cannot legally root a cutting taken from it. Instead, you have to buy seeds. You can, however, root a cutting from a trademarked cultivar. A tag on the nursery plant will give its status.
Understanding Petunia Basics
Hundreds of species, subspecies, varieties and named cultivars of petunias exist. They grow as perennials in U.S. Department of Agriculture plant hardiness zones 10 and 11 and as annuals elsewhere.
A patented petunia in a nursery has a tag giving the name of the cultivar and a patent number. The tag might also say "patent pending" or PPAF, “plant patent applied for.” You might also see PVR, standing for “plant variety rights." Avoid rooting cuttings from these plants.
If a petunia is trademarked, protecting the name of the petunia cultivar only, you can legally root cuttings from it. A trademarked plant will have a tag with an R inside a circle. A tag marked TM means someone has claimed the cultivar name but not yet registered it. For instance, you can legally plant cuttings of the “Wave” series of spreading petunias because “Wave” is trademarked, not patented. “Cascadia,” “Supertunia” and “Surfinia” petunia series are also trademarked and may be propagated by cutting.
Prepare for collecting petunia cuttings in late winter. Sterilize a single-edged razor or sharp knife by soaking the cutting blade for five minutes in a mix of 1 part household bleach to 3 parts of water and letting air dry. Cut back the tops and branches of a petunia until they are roughly 1 inch long.
When the trimmed petunia begins growing again in spring, use the sterilized razor or knife to collect cuttings of soft, green new growth 2 to 4 inches long. Pinch off the top bud and remove the lower leaves. Leaves lose water from their pores through transpiration, a form of evaporation, and cuttings do not have roots yet to obtain water. Removing leaves keeps moisture loss to a minimum.
Fill a 4-inch plastic pot with a mix of 1 part vermiculite, 1 part perlite and 2 parts of peat. Use a tray if you are rooting a large number of cuttings and make sure the tray or container has bottom drain holes or the cuttings will rot. Dip the bottom of the cutting in IBA, a root hormone sold under several brand names. The root hormone will stimulate the growth of roots.
Poke a hole in the mix with a pencil and insert the cutting into the hole deep enough so at least one node is below the surface. Gently firm the mix around the cutting and water the mixture thoroughly. Cover the pot with a plastic bag or a tray with a dome to prevent moisture from evaporating. Place the cutting in moderate light with a temperature between 65 and 70 degrees Fahrenheit. A window facing north will do.
Cuttings should develop roots within a month. Once that has happened, remove the plastic bag or dome. Keep the soil moist but not soggy. Gradually expose the plants to different temperatures, and slowly increase the amount of direct sunlight, allowing the plant to thicken and become firm.
When the seedling has dark-colored, thick leaves and a stocky stem, move it outdoors into direct sun but out of the wind for a few hours daily for seven days. Since the transplant is still developing roots, keep the soil moist. Gradually increase its time outdoors.