Calibrachoas (Calibrachoa x hybrida) produce colorful 1-inch flowers that crowd the tips of their cascading shoots. Hardy only in U.S. Department of Agriculture plant hardiness zones 9 through 11, most people treat them as annuals. Because they seldom produce seeds, they bloom tirelessly all summer without deadheading. Calibrachoas can be propagated from softwood stem cuttings, but you can now buy seeds for the Kabloom series.
Propagate From Seed
Always buy your seeds from commercial sources rather than gathering them from plants, because seeds collected from hybrids won't produce seedlings identical to the calibrachoas from which they were harvested. Sow the pelleted seeds in late winter or early spring, using a flower pot or other container that has drainage holes.
Pack the flower pot to 1/2 inch short of its rim with damp and sterile seed-starting mix. After spacing the seeds about 1 inch apart on the surface of that mix, press them into it without covering them, and stretch plastic wrap over the top of the pot.
Set the pot on a sunny windowsill -- or under the center of a grow light programmed to run for 14 to 16 hours per day -- where the temperature remains between 70 and 75 degrees Fahrenheit. Check the potting mix daily and water it if the surface starts to dry. The seeds should germinate within five to 14 days if kept moist. Remove the plastic wrap, leaving the pots in the warm conditions for five to 10 days after they sprout before moving them to a cooler spot with temperatures between 55 and 65 F.
Fertilize the seedlings once every two weeks with an iron-enriched plant food intended for acid-loving plants, when they each have two sets of leaves, but begin with a half-strength mixture. For a 20-3-19 fertilizer, use 1/4 teaspoon of the crystals in 1 gallon of water for the first two feedings, then increase the amount to 1/2 teaspoon in 1 gallon.
Transplant the seedlings, after they have reached 6 weeks of age, into individual 4-inch pots filled with a well-draining, acidic potting soil -- such as that marketed for cactus and citrus plants. Alternatively, plant three seedlings, evenly spaced, in an 8-inch pot for a fuller look. About two weeks after you transplant them, pinch off the tips of their shoots to encourage the plants to branch out.
Take the pots outside for a few hours per day, placing them in shade and gradually exposing them to more outdoors hours and brighter light until they are receiving full sun, at least six hours of it per day. This helps adapt the plants to outdoor conditions. To avoid raising their soil's pH, which can cause yellow leaves, water the plants with rain water rather than hard tap water whenever their soil is dry 1 inch down.
Propagate From Cuttings
Wipe the blades of your pruning shears with a cloth soaked in rubbing alcohol to sterilize them. Snip three 3-inch pieces from the tips of a healthy calibrachoa plant. Strip the leaves from the lower 1 1/2 inches of each cutting, and pinch off any flowers or buds so only leaves remain.
Fill a 6-inch terra cotta flower pot to within 1 inch of its rim with a damp mix of equal parts peat moss and perlite. Poke three 1 1/2-inch-deep, evenly spaced planting holes in the mix with a pencil.
Insert the bare stem of a cutting into one of those holes, so its lowest leaves are just above the surface of the damp mix, tamping the mix tightly around the stem. Repeat this process with the other two cuttings.
Cover the pot with an inverted clear plastic zip-top bag, supported with short stakes or plastic straws to hold the bag away from the cuttings. Zip the bag shut beneath the pot and place it under a grow light or in an area where it receives bright, indirect light but no direct sunlight. If you keep temperatures between 70 and 75 F, new growth should appear on the cuttings within about six weeks, indicating they have rooted.
Ease the bag off the pot for a couple hours once you see new growth. Replace the bag later. Remove the bag for another couple hours each day until the plants have adapted to the lower humidity. You then can transplant and grow the calibrachoas as you would seedlings.