Also known as trailing petunia, calibrachoa or million bells (Calibrachoa x hybrida) is a group of garden hybrids that resemble, and are closely related to, the ever-popular petunia (Petunia spp.). As the plant's common name suggests, million bells produces a prolific number of small flowers throughout the growing season, in bright shades of violet, yellow, white, pink and blue. Million bells will produce highest number of flowers in a bright, sunny spot, though it will reluctantly tolerate partial shade.
Million bells must be planted in full sun if it is to reach its full flowering potential. Missouri Botanical Garden notes it will tolerate very light shade, but flowering will decrease significantly as the amount of shade increases. When the sun is blocked by clouds, the flowers will close. They also close at night and on days with wet weather. Extended periods of wet or cloudy weather may delay flowering altogether.
Million bells is a frost-tender perennial that is often grown as an annual. As a perennial, million bells may be grown in U.S. Department of Agriculture plant hardiness zones 9 to 11. Some varieties may survive temperatures as low as 15 degrees Fahrenheit, according to Floridata. Million bells is quite tolerant of heat, taking the full brunt of the summer sun as long as it is watered regularly. Container plants will need to be watered more often than those planted in the ground.
Million bells prefers a well-draining soil with a pH less than 6.0. In poorly draining or consistently flooded soil, the plant may develop root rot. Because many million bells varieties are patented and produce few or no seeds, the plant is more expensive than many other annuals used in flowers beds. Million bells is better used in a hanging basket or container, because you won't need to buy as many plants to fill the space.
If you cannot provide enough sun for million bells, consider a plant that will grow well in shade. Primroses (Primula vulgaris), which grow in USDA zones 4 to 8, produce their best flowers in partial shade or filtered sunlight. Primroses require moisture retentive, organic soil. Pansies (Viola x wittrockiana), which grow in USDA zones 6 to 10, also grow well in partial shade. Pansies prefer moist, well-draining soil covered with a layer of organic mulch.