As a cultivated plant species, Calibrachoa, often referred to as Million Bells, thrives within specific environmental parameters that mirror its native South American origins. From our perspective, temperature is not merely a number but a fundamental regulator of our physiological processes, from photosynthesis and respiration to water uptake and flowering. The ideal temperature range is the sweet spot where these processes operate in harmonious efficiency, allowing us to express our full potential for vibrant, continuous blooms and vigorous growth.
For sustained health, growth, and prolific flowering, we Calibrachoa prefer a moderate and consistent temperature range. Our ideal conditions are between 65°F and 75°F (18°C and 24°C) during the daytime. Within this range, our metabolic enzymes function at their peak, enabling efficient conversion of sunlight, water, and nutrients into the energy required for producing our vast number of bell-shaped flowers. This range also supports strong root development and robust cell formation without placing undue stress on our systems.
As the sun sets, our needs change. A slight drop in temperature is not only tolerable but beneficial. We prefer nighttime temperatures to be between 55°F and 65°F (13°C and 18°C). This cooler period allows us to respire at a slower rate, conserving the energy we produced during the day rather than burning it all through the night. This "rest period" is crucial for maintaining plant vigor, strengthening cell structures, and ultimately, supporting the development of new flower buds for the next day's display.
While we have our preferences, we can endure brief periods outside our ideal range, though not without consequences. We can generally tolerate temperatures as low as 40°F (4°C) for a short while, but prolonged exposure to cold will significantly slow our growth, cause our leaves to take on a purplish tinge (a sign of phosphorus unavailability), and can lead to root damage. On the opposite end, we can survive highs up to about 85°F (29°C), provided our water needs are met. However, extreme heat forces us to divert energy from flowering into cooling ourselves through increased transpiration, often causing our blooming to pause or our petals to scorch.
A factor often overlooked from a human perspective is the temperature beneath the soil. Our root systems are highly sensitive to cold. For successful establishment and ongoing health, the root zone temperature should be consistently above 50°F (10°C). Planting us in cold, damp soil will lead to transplant shock, root rot, and a complete halt in growth. Warm roots are essential for efficient water and nutrient uptake, which in turn supports all above-ground activity. Using mulch can be an excellent strategy to help regulate our root zone temperature, keeping it cooler in the summer and warmer during chilly spring or fall nights.
