Despite their remarkably similar appearances, Calibrachoa and Petunia belong to different genera within the Solanaceae, or nightshade, family. The Petunia has been a garden staple for much longer, with its genus, Petunia, being formally described in the 19th century. Its origins trace back to South America. Calibrachoa, on the other hand, was initially classified within the Petunia genus but was reclassified into its own separate genus, Calibrachoa, in the 1980s based on advanced genetic and morphological studies. This reclassification highlights a fundamental difference at the DNA level, confirming they are distinct botanical entities even though they share a close common ancestor.
To the casual observer, the flowers are the most obvious point of comparison. Both offer trumpet-shaped blooms, but key distinctions exist. Petunia flowers are generally larger, often reaching 3 to 4 inches in diameter in standard varieties, with a classic, sometimes flared, trumpet shape. Their petals are typically fused into a single, large tube. Calibrachoa flowers are significantly smaller, usually about 1 inch across, earning them the common name "Million Bells" due to the prolific number of petite, bell-shaped blooms they produce. The foliage also differs; Petunia leaves are broader, larger, and somewhat sticky or hairy to the touch. Calibrachoa leaves are smaller, narrower, and less sticky, with a more glossy appearance.
The growth habit of these two plants is a major differentiator for gardeners planning their containers and landscapes. Modern Petunias are categorized into several types, including grandifloras (large flowers), multifloras (many smaller flowers), and spreading or groundcover types (like the popular Wave® series). While many have a trailing habit, they often start more mounded before spilling over edges. Calibrachoa exhibits a inherently and aggressively trailing growth habit from the very beginning. Its stems are more herbaceous and less woody than some petunia varieties, and it naturally forms a dense cascade of foliage and flowers, making it ideal for hanging baskets and the edges of containers where it can drape freely.
A critical botanical difference lies in their reproductive systems. Petunias are typically seed-propagated, especially heirloom and species types, though many modern hybrids are propagated vegetatively to maintain specific traits. Calibrachoa, however, produces very few viable seeds. This is why virtually all Calibrachoa plants available in nurseries are propagated asexually through tissue culture or cuttings. This ensures the uniformity and specific characteristics of the hybrid cultivars. From a cultivation perspective, Calibrachoa is often noted for being more self-cleaning, meaning spent flowers drop away naturally without needing deadheading to encourage further blooming, though some modern Petunia varieties have also been bred for this trait.
