From a botanical perspective, the primary distinction between single, double, and heirloom hollyhocks (Alcea rosea) lies in the morphology of their flowers, specifically the structure of their inflorescences and the modification of floral organs. Hollyhocks are herbaceous biennial or short-lived perennial plants in the Malvaceae family, characterized by their tall, upright spikes of showy flowers. The fundamental difference between single and double varieties is a result of a genetic mutation known as the "double-flowered" trait, which alters the development of the flower's reproductive parts into additional petaloid structures.
The single hollyhock represents the wild or species-typical form. Its flower structure is simple and follows the classic Malvaceae plan. A single flower consists of five true petals that form a shallow, open cup or funnel shape. These petals surround the central reproductive column, or androgynophore, which is a fused structure unique to the mallow family. This column bears the male stamens at its base and the female stigma-bearing styles at its tip. This arrangement is highly efficient for pollination, as it readily provides pollen and nectar to visiting insects like bees, ensuring genetic diversity through cross-pollination. The plant's energy is directed towards seed production following successful pollination.
The double-flowered hollyhock is a cultivar that arose through selective breeding for a specific mutation. Botanically, the extra "petals" are not true petals but are stamens that have undergone a homeotic transformation, becoming petaloid structures. In some highly double forms, even the pistils may be modified. This results in a much denser, pom-pom-like flower with many layers of petaloid organs. While this is highly desirable for ornamental purposes, creating a lush, dramatic visual effect, it comes with a reproductive cost. The transformation of stamens into petals often renders the flower sterile or significantly reduces its fertility, as the vital reproductive parts are obscured or nonexistent. This makes double varieties more reliant on vegetative propagation, such as cuttings or division, to persist.
The term "heirloom" is not a botanical classification of flower form but rather a horticultural and genetic designation. An heirloom hollyhock is an open-pollinated cultivar that has been passed down through generations, often for 50 years or more, and remains genetically stable. It can be either single or double-flowered. From a plant science viewpoint, heirlooms are valued for their genetic diversity and resilience, having adapted to specific local conditions over time. They are typically true-to-type from seed, meaning offspring will closely resemble the parent plant, whether it is single or double. This is in contrast to modern F1 hybrids, which are first-generation crosses between two different parent lines designed for specific traits but do not produce identical offspring from saved seeds.
