The Dahlia genus, Dahlia spp., originated in the mountainous regions of Mexico and Central America. This tuberous perennial was first cultivated by the Aztecs before being introduced to Europe in the late 18th century. Since its introduction, the flower has become a favorite among gardeners and breeders, celebrated for its late-season blooms and virtually limitless color palette. The dahlia’s journey is a story of explosive diversification, resulting in an incredible range of shapes and sizes.
Defining the Scope of Dahlia Variety
Determining a fixed number for dahlia varieties is impossible because the total count is constantly expanding. The genus Dahlia contains around 42 accepted wild species, such as Dahlia coccinea and Dahlia imperialis. The vast number of dahlias known today comes from cultivated varieties, known as cultivars.
The number of named cultivars is massive and fluid, with new ones introduced every year. The Royal Horticultural Society (RHS) maintains the official register, which contains over 63,000 different cultivated names. This figure continues to grow annually. The actual number of unique dahlias in circulation is potentially even higher, as not every hybrid created by small-scale breeders is formally registered.
The Official Classification System
To manage the enormous volume of cultivars, a standardized classification system groups them based primarily on their bloom form and petal arrangement. This system helps breeders, growers, and judges organize the flowers into manageable categories, of which there are typically 14 recognized groups. The distinct visual characteristics of the bloom are the most important criteria for placement within the system.
One of the most common forms is the Decorative dahlia, characterized by fully double flowers with broad, flat petals that are evenly arranged to hide the central disc. In contrast, the Cactus and Semi-Cactus dahlias feature pointed, narrow ray florets that are rolled inward for more than half their length, giving the flower a spiky appearance. Semi-Cactus types have broader petals that are rolled less severely than the true Cactus forms.
The classification system includes several other unique forms:
- Ball and Pompon dahlias are identifiable by their spherical, fully double blooms. Pompon dahlias are smaller, tightly quilled spheres, while Ball dahlias are larger.
- Anemone-flowered dahlias have a raised, dense center of elongated disc florets surrounded by one or more rings of flat ray petals.
- Collarette dahlias feature a single outer ring of flat petals surrounding an inner ring of shorter, differently colored petaloids, forming a “collar.”
- Waterlily dahlias display a symmetrical, double flower with a flat to saucer-like shape, featuring broad, rounded ray florets.
These classifications provide a structure for appreciating the visual breadth of the genus, which also includes size classifications like the massive Dinnerplate types and the smaller Mignon Singles.
The Role of Hybridization in Expanding Numbers
The rapid diversification of the dahlia is rooted in a unique genetic structure that facilitates hybridization and selective breeding. Modern cultivated dahlias are highly polyploid, meaning they possess multiple sets of chromosomes, often described as octoploid with eight sets. Most other familiar plants are diploid, carrying only two sets of chromosomes.
This polyploidy provides a vast genetic reservoir, allowing for a wide range of trait combinations and greater phenotypic variation in the offspring. When breeders cross two different dahlia cultivars, the polyploid nature means that the genes for color, form, and size can be recombined in complex and unpredictable ways. This leads to the creation of new forms that would be impossible in diploid plants. This genetic flexibility is the fundamental reason why the number of dahlia varieties has exploded into the tens of thousands.