quality:corruption
History

The inappropriate term D. variabilis is often used to describe the cultivars of Dahlia since the correct parentage remains obscure, but probably involves Dahlia coccinea. In 1846 the Caledonia Horticultural Society of Edinburgh offered a prize of 2,000 pounds to the first person succeeding in producing a blue dahlia. This has to date not been accomplished. While dahlias produce anthocyanin, an element necessary for the production of the blue, to achieve a true blue color in a plant, the anthocyanin delphinidin needs six hydroxyl groups. To date, dahlias have only developed five, so the closest that breeders have come to achieving a "blue" specimen are variations of mauve, purples and lilac hues.

By the beginning of the twentieth century, a number of different types were recognised. These terms were based on shape or colour, and the National Dahlia Society included cactus, pompon, single, show and fancy in its 1904 guide. Many national societies developed their own classification systems until 1962 when the International Horticultural Congress agreed to develop an internationally recognised system at its Brussels meeting that year, and subsequently in Maryland in 1966. This culminated in the 1969 publication of The International Register of Dahlia Names by the Royal Horticultural Society which became the central registering authority.

This system depended primarily on the visibility of the central disc, whether it was open-centred or whether only ray florets were apparent centrally (double bloom). The double-bloom cultivars were then subdivided according to the way in which they were folded along their longitudinal axis: flat, involute (curled inwards) or revolute (curling backwards). If the end of the ray floret was split, they were considered fimbriated. Based on these characteristics, nine groups were defined plus a tenth miscellaneous group for any cultivars not fitting the above characteristics. Fimbriated dahlias were added in 2004, and two further groups (Single and Double orchid) in 2007. The last group to be added, Peony, first appeared in 2012.

In many cases the bloom diameter was then used to further label certain groups from miniature to giant. This practice was abandoned in 2012.