Calibrachoa, often known by the common name Million Bells, is a highly popular flowering plant prized for its stunning display of small, trumpet-shaped blooms. This plant is a favorite for hanging baskets and containers, where its cascading habit creates a prolific waterfall of color from spring until the first hard frost. The definitive answer is that Calibrachoa is generally treated as a tender annual outside of very specific warm climate zones, meaning it usually will not survive the winter outdoors in most locations.
The Core Answer: Annual or Perennial?
The status of Calibrachoa as an annual or perennial depends entirely on the winter temperatures in your location. Botanically, Calibrachoa is a true perennial, but it is native to regions of South America with mild, frost-free climates. This natural preference means the plant is extremely sensitive to freezing conditions, which dictate its survival in gardens globally.
It functions reliably as a perennial, surviving year-round outdoors, only in USDA Hardiness Zones 9 through 11. In these regions, where winter temperatures rarely drop below 25°F (-3.9°C), the root system can persist through the cooler months and regrow the following spring. For the vast majority of gardeners living in Zones 3 through 8, soil-freezing temperatures instantly kill the roots. This forces them to treat the plant as an annual and replant it each spring after the danger of frost has passed.
Preparing Plants for Overwintering
Saving a Calibrachoa plant requires bringing it indoors before the first hard frost of the season arrives. The transition process must focus on reducing the plant’s size and ensuring it is free of outdoor pests before it enters your home environment. This preparation is necessary to prevent introducing common garden insects like aphids or spider mites, which can quickly spread to other indoor plants.
The first step involves a significant pruning, cutting back the long, trailing stems by at least half, or even more drastically to within two to four inches of the soil line. This reduction in foliage helps the plant conserve energy and encourages bushier, more compact growth when it reemerges from dormancy. Before moving the container inside, a thorough inspection for pests is necessary, followed by a preventative treatment. Applying an insecticidal soap or a diluted neem oil solution to all parts of the plant will help eliminate unwanted hitchhikers.
Sustaining Calibrachoa Indoors
Once the Calibrachoa is pruned and sanitized, its indoor survival depends on replicating a cool, resting period, rather than trying to force it into continuous bloom. The ideal indoor environment is a cool, stable location, such as a basement or unheated garage, where the temperature remains consistently between 50°F and 60°F (10°C to 15°C). Too much warmth will encourage weak, spindly growth that will not survive the transition back outside in the spring.
The plant still requires light during this dormant phase. Natural indoor light is often insufficient, so placing the plant under supplemental full-spectrum grow lights for 14 to 16 hours a day can prevent etiolation. Without supplemental lighting, position the plant in the brightest window available, ensuring it receives at least six hours of light.
Watering must be significantly reduced to match the plant’s slower metabolism during the winter months. Allow the top one to two inches of soil to dry out completely between waterings to prevent root rot, which is a common killer of overwintered plants. Fertilization should be completely suspended during this period, as the plant is resting and does not need the nutrients.
In late winter, as spring approaches, you can begin the process of revitalization by increasing the frequency of watering and introducing a light feeding with a balanced liquid fertilizer. The final step is gradually re-acclimating the plant to outdoor conditions after the last expected frost date. This “hardening off” process involves placing the plant outside for increasing durations each day, starting with a few hours in a shaded area, before moving it to its final full-sun location.