Despite their remarkably similar appearances, Calibrachoa and Petunias belong to different genera, which is the primary scientific distinction. Petunias are classified under the genus Petunia. Calibrachoa, on the other hand, belongs to the genus Calibrachoa. For many years, Calibrachoa was actually classified within the Petunia genus. It was through advanced cytological and DNA analysis that botanists determined a significant genetic difference: Petunias have 14 chromosomes, while Calibrachoa have 18. This fundamental genetic divergence is why they were separated into distinct genera, even though both reside within the same plant family, Solanaceae (the Nightshade family), which also includes tomatoes, potatoes, and tobacco.
To the casual observer, the flowers look nearly identical. However, upon closer inspection, key morphological differences become apparent. The most reliable distinguishing feature is the flower structure. Petunia flowers possess five fused petals that form a distinct, often larger, trumpet-shaped bloom. The throat of the flower, inside the trumpet, can be a different color and is often quite prominent.
Calibrachoa flowers are smaller and produce a multitude of blossoms, earning them the common name "Million Bells." Their flowers also have five fused petals, but they are more salverform (like a flat platter) with a very short, narrow tube, making the flower face appear almost flat when fully open. The foliage also differs subtly; Calibrachoa leaves are typically smaller, narrower, and somewhat stickier than the broader, more velvety or slightly hairy leaves of most Petunia varieties.
The growth habit of these two plants is a major differentiator for gardeners. Most modern Petunias are categorized into types like Grandiflora (large flowers, prone to damage from rain), Multiflora (smaller but more numerous, rain-resistant flowers), and Spreading or Wave types (vigorous, ground-covering habits). They can be upright, mounding, or trailing.
Calibrachoa exhibits a consistently trailing or semi-trailing growth habit. Their stems are more herbaceous and less vigorous than the spreading Petunia types, making them ideal for hanging baskets and container edges where they form dense mounds of color. Furthermore, Calibrachoa are generally considered "self-cleaning," meaning they do not require deadheading (removal of spent blooms) to continue flowering profusely. Many Petunia varieties, particularly the large-flowered Grandifloras, require regular deadheading to prevent them from diverting energy into seed production and to maintain a tidy appearance.
The reproductive strategies of these plants also highlight their botanical separation. In the wild, Petunias are often pollinated by insects, including bees, and some species are even moth-pollinated, having evolved flowers with longer tubes to accommodate specific pollinators.
Calibrachoa, in contrast, are predominantly self-pollinating. Their flower structure is adapted for this, with the anthers (pollen-producing parts) positioned very close to the stigma (pollen-receiving part) to ensure fertilization without an external pollinator. This difference in reproductive biology is a key evolutionary factor that contributes to their status as separate genera.
