Calibrachoa, commonly known as Million Bells, is an annual flowering plant prized for its prolific, petunia-like blooms and cascading growth habit. These plants are popular for hanging baskets and container gardening, offering a season-long display of vibrant color. While commercial growers propagate them through vegetative cuttings for uniformity, home gardeners can successfully grow Calibrachoa from seed. This method presents unique challenges, as the seeds are demanding, requiring specific environmental controls for successful germination. Understanding these precise requirements is necessary for successful seed propagation.
Preparing for Seed Starting
To maximize successful cultivation, preparation must begin several months before the outdoor growing season. Gardeners typically start Calibrachoa seeds indoors 10 to 12 weeks before the last expected spring frost date. This extended indoor period allows the slow-growing seedlings enough time to develop into robust transplants.
Shallow seed trays or small cell packs are ideal choices for the initial sowing. These containers must possess adequate drainage holes to prevent waterlogging and fungal issues. Before use, sterilize all containers with a diluted bleach solution to eliminate residual pathogens.
The growing medium should be a sterile, fine-textured seed-starting mix that is lightweight. Standard potting soil is often too heavy and coarse, potentially hindering tiny root systems. Pre-moistening this mix thoroughly before sowing ensures a uniform moisture level throughout the tray for consistent germination.
Sowing and Germination Requirements
The process of sowing Calibrachoa seeds differs from that of many common flowers. Due to their minute size, seeds should be distributed sparingly across the surface of the pre-moistened mix to avoid overcrowding. Gently press the seeds into the medium using a flat object to ensure firm seed-to-soil contact, but do not bury them.
Germination in Calibrachoa is photoblastic, meaning the seeds are dependent on light to break dormancy. Consequently, the seeds must remain completely uncovered on the surface of the growing medium throughout the germination period. Covering them, even lightly, will block the necessary light stimulus and prevent sprouting.
Maintaining a consistently high temperature is required for successful emergence. Optimal germination temperatures fall within the narrow range of 75°F to 80°F (24°C to 27°C) and should be held constant. Achieving this stability often necessitates the use of a thermostatically controlled heat mat placed beneath the seed tray.
Moisture management is delicate; the surface must never dry out, but the seeds cannot sit in standing water. Covering the tray with a clear plastic dome or plastic wrap creates a humid microclimate, retaining high moisture near the surface. Under these conditions—light, warmth, and humidity—emergence usually occurs within 10 to 21 days.
Cultivating Young Seedlings
Once green seedlings appear, immediately adjust the environment to prevent damping off. Promptly remove the humidity dome and increase air circulation around the young plants using a small fan. This transition helps the seedlings acclimate while maintaining a warm ambient temperature.
The light requirement now shifts to fueling photosynthetic growth. Place seedlings under strong artificial grow lights, such as LED fixtures, for 14 to 16 hours per day. Position the light source just a few inches above the foliage to prevent the plants from stretching and becoming weak.
Transition to a bottom-watering technique: place the tray in a shallow reservoir for 15 to 20 minutes until the medium wicks up moisture. This method keeps the stems and soil surface drier, deterring fungal disease.
The initial nutrient supply is depleted once the true leaves develop. The first application of fertilizer should occur once seedlings have two sets of true leaves, using a highly diluted, balanced liquid feed. Administer this initial feeding at one-quarter to one-eighth strength to avoid burning the root systems.
As the plants grow, they will become crowded, signaling the time to “prick out” the seedlings for individual potting. Carefully lift each seedling by a leaf, never the stem, and transplant it into its own 3-inch pot. Fill these pots with a slightly richer, sterilized potting mix. Providing individual space prevents competition and allows each plant to establish a stronger root ball.
Outdoor Planting and Established Care
Before permanently moving Calibrachoa outdoors, they must undergo hardening off. Over seven to ten days, slowly expose the plants to increasing amounts of direct sun, wind, and cooler temperatures. This step prevents transplant shock and sun scald, allowing plant tissues to develop protection against the elements.
Final planting should occur only after all danger of frost has passed. Place the hardened plants into a location with well-drained soil rich in organic matter. Calibrachoa prefers a slightly acidic soil pH (5.5 and 6.5) for optimal uptake of micronutrients. In containers, space plants 6 to 12 inches apart to ensure good air flow.
These plants are heavy feeders, requiring consistent nutrient replenishment to support continuous flowering. A regular feeding schedule with a balanced, slow-release granular fertilizer or a diluted liquid feed administered every two weeks will maintain vigor. Water established plants deeply when the top inch of soil feels dry, ensuring the entire root zone is saturated.
Pinching back the tips of the stems early in the season encourages lateral branching. This pruning technique leads to a bushier, more compact, and floriferous trailing habit throughout the summer. While Calibrachoa is self-cleaning and does not require deadheading, removing spent blooms can redirect energy into producing new growth.