Both Grandiflora and Multiflora petunias are cultivated varieties (cultivars) derived from complex hybridization within the Petunia genus, primarily involving Petunia axillaris and Petunia integrifolia. They belong to the Solanaceae family, sharing this lineage with tomatoes, potatoes, and tobacco. The key difference lies not in their species but in the selective breeding objectives that have emphasized distinct morphological traits, resulting in their classification into different horticultural groups. Grandiflora types were bred for sheer flower size and visual impact, while Multiflora types were selected for prolific flowering and environmental resilience.
The most apparent difference from a plant morphology perspective is the size and structure of the flowers. Grandiflora petunias produce very large, showy blooms that can measure between 4 to 5 inches (10 to 13 cm) in diameter. These flowers often feature dramatic, ruffled, or fringed edges, making them the centerpiece of any planting. In contrast, Multiflora petunias produce a significantly higher number of flowers, but each individual bloom is smaller, typically measuring 1.5 to 3 inches (4 to 7.5 cm) in diameter. The flowers are simpler in form, with smoother margins, and are engineered by breeders for quantity over individual size.
The growth habit and overall architecture of the plants are adapted to support their respective flowering strategies. Grandiflora petunias often have a slightly more sprawling but less dense habit. Their energy is directed towards producing fewer, but much larger, floral structures, which require robust stems and substantial vascular resources to support. Multiflora petunias exhibit a more compact and densely branched growth form. This bushy architecture supports the development of a massive number of flowering nodes. The plant's energy is distributed across a multitude of smaller blooms, resulting in a solid blanket of color.
A critical physiological difference lies in their tolerance to environmental stressors, particularly rain and water. The large, delicate petals of Grandiflora petunias are highly susceptible to damage. Heavy rain can tear the blooms, and persistent moisture can lead to rot and fungal diseases like botrytis (gray mold), often leaving the plant looking tattered. Multiflora petunias, with their smaller, sturdier, and more numerous flowers, are significantly more resilient. Their smaller floral surface area and stronger attachment to the stem allow them to withstand rain and wind with minimal damage. This makes them a more reliable choice for gardeners in areas with unpredictable summer weather.
From a reproductive ecology standpoint, the two types employ different strategies. A Grandiflora petunia invests heavily in a few, highly conspicuous flowers to attract pollinators from a greater distance. However, if these few large flowers are damaged, the plant's ornamental and reproductive output suffers noticeably. The Multiflora strategy is one of redundancy and persistence. By producing a continuous and overwhelming abundance of flowers, the plant ensures that even if some blooms are lost to weather or pests, plenty remain to maintain the display and attract pollinators, guaranteeing a consistent performance throughout the growing season.
