In , a new species, Dahlia sambucifolia , was successfully grown at Holland House , Kensington. Whilst in Madrid in , Lady Holland was given either dahlia seeds or tubers by Cavanilles. And in colour as bright as your cheek. More significantly, he sent seeds to botanist Carl Ludwig Willdenow in Germany.
Willdenow now reclassified the rapidly growing number of species, changing the genus from Dahlia to Georgina ; after naturalist Johann Gottlieb Georgi. He combined the Cavanilles species D. Since when Cavanilles first flowered the dahlia in Europe, there has been an ongoing effort by many growers, botanists and taxonomists, to determine the development of the dahlia to modern times.
At least 85 species have been reported: approximately 25 of these were first reported from the wild, the remainder appeared in gardens in Europe. They were considered hybrids , the results of crossing between previously reported species, or developed from the seeds sent by Humboldt from Mexico in , or perhaps from some other undocumented seeds that had found their way to Europe.
Several of these were soon discovered to be identical with earlier reported species, but the greatest number are new varieties. Morphological variation is highly pronounced in the dahlia. William John Cooper Lawrence , who hybridized hundreds of families of dahlias in the s, stated: "I have not yet seen any two plants in the families I have raised which were not to be distinguished one from the other.
In , all species growing in Europe were reclassified under an all-encompassing name of D. In William Smith suggested that all dahlia species could be divided into two groups for color, red-tinged and purple-tinged. In investigating this idea Lawrence determined that with the exception of D.
The genus Dahlia is situated in the Asteroideae subfamily of the Asteraceae , in the Coreopsideae tribe. Within that tribe it is the second largest genus, after Coreopsis ,  and appears as a well defined clade within the Coreopsideae. Sherff , in the first modern taxonomy described three sections for the 18 species he recognised, Pseudodendron , Epiphytum and Dahlia. Further species continue to be described, Saar describing 35 species. To date these sectional divisions have not been fully supported phylogenetically ,  which demonstrate only section Entemophyllon as a distinct sectional clade.
The remainder of the species occupy what has been described as the variable root clade VRC which includes the small section Pseudodendron but also the monotypic section Epiphytum and a number of species from within section Dahlia. Outside of these three clades lie D. Horticulturally the sections retain some usage, section Pseudodendron being referred to as 'Tree Dahlias', Epiphytum as the 'Vine Dahlia'. The remaining two herbaceous sections being distinguished by their pinnules , opposing Dahlia or alternating Entemophyllon.
Sections including chromosome numbers , with geographical distribution; . Only Pseudodendron D. There are currently 42 accepted species in the genus Dahlia ,  but new species continue to be described. The naming of the plant itself has long been a subject of some confusion. Many sources state that the name "Dahlia" was bestowed by the pioneering Swedish botanist and taxonomist Carl Linnaeus to honor his late student, Anders Dahl , author of Observationes Botanicae. However, Linnaeus died in , more than eleven years before the plant was introduced into Europe in , so while it is generally agreed that the plant was named in in honor of Dahl, who had died two years before,  Linnaeus could not have been the one who did so.
It was probably Abbe Antonio Jose Cavanilles , Director of the Royal Gardens of Madrid, who should be credited with the attempt to scientifically define the genus, since he not only received the first specimens from Mexico in , but named the first three species that flowered from the cuttings. Regardless of who bestowed it, the name was not so easily established. Petersburg , Russia. It was not until , in a published article, that he officially adopted the Cavanilles' original designation of Dahlia.
Dahlia is found predominantly in Mexico , but some species are found ranging as far south as northern South America. Dahlia is a genus of the uplands and mountains, being found at elevations between 1, and 3, meters, in what has been described as a "pine-oak woodland" vegetative zone. Most species have limited ranges scattered throughout many mountain ranges in Mexico .
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The most common pollinators are bees and small beetles. Slugs and snails are serious pests in some parts of the world, particularly in spring when new growth is emerging through the soil. Earwigs can also disfigure the blooms. The other main pests likely to be encountered are aphids usually on young stems and immature flower buds , red spider mite causing foliage mottling and discolouration, worse in hot and dry conditions and capsid bugs resulting in contortion and holes at growing tips.
Diseases affecting dahlias include powdery mildew , grey mould Botrytis cinerea , verticillium wilt, dahlia smut Entyloma calendulae f. Dahlias are a source of food for the larvae of some Lepidoptera species including angle shades , common swift , ghost moth and large yellow underwing. Dahlias grow naturally in climates which do not experience frost the tubers are hardy to USDA Zone 8 , consequently they are not adapted to withstand sub-zero temperatures.
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However, their tuberous nature enables them to survive periods of dormancy , and this characteristic means that gardeners in temperate climates with frosts can grow dahlias successfully, provided the tubers are lifted from the ground and stored in cool yet frost-free conditions during the winter. When in active growth, modern dahlia hybrids perform most successfully in well-watered yet free-draining soils, in situations receiving plenty of sunlight.
Taller cultivars usually require some form of staking as they grow, and all garden dahlias need deadheading regularly, once flowering commences. The inappropriate term D. 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.
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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 guide. Many national societies developed their own classification systems until when the International Horticultural Congress agreed to develop an internationally recognised system at its Brussels meeting that year, and subsequently in Maryland in This culminated in the 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. In many cases the bloom diameter was then used to further label certain groups from miniature through to giant. There are now more than 57, registered cultivars ,  which are officially registered through the Royal Horticultural Society RHS.
The original registry published about 14, cultivars adding a further by and in there were 18, The official RHS classification lists fourteen groups, grouped by flower type, together with the abbreviations used by the RHS;   . Earlier versions of the registry subdivided some groups by flower size. Groups 4, 5, 8 and 9 were divided into five subgroups A to E from Giant to Miniature, and Group 6 into two subgroups, Small and Miniature. Dahlias were then described by Group and Subgroup, e.
As of The RHS uses two size descriptors . Sizes can range from tiny micro dahlias with flowers less than 50mm to giants that are over mm in diameter. The groupings listed here are from the New Zealand Society. In addition to the official classification and the terminology used by various dahlia societies, individual horticulturalists use a wide range of other descriptions, such as 'Incurved' and abbreviations in their catalogues, such as CO for Collarette.
Some plant growers include their brand name in the cultivar name. In , several new species were reported with red, purple, lilac, and pale yellow coloring, and the first true double flower was produced in Belgium. One of the more popular concepts of dahlia history, and the basis for many different interpretations and confusion, is that all the original discoveries were single flowered types, which, through hybridization and selective breeding, produced double forms.
But two of the three drawings of dahlias by Dominguez, made in Mexico between —77, showed definite characteristics of doubling. In the early days of the dahlia in Europe, the word "double" simply designated flowers with more than one row of petals. The greatest effort was now directed to developing improved types of double dahlias.
During the years to several people claimed to have produced a double dahlia. In Henry C. Andrews made a drawing of such a plant in the collection of Lady Holland, grown from seedlings sent that year from Madrid. The first modern double, or full double, appeared in Belgium; M. Donckelaar, Director of the Botanic Garden at Louvain , selected plants for that characteristic, and within a few years secured three fully double forms. Up to this time all the so-called double dahlias had been purple, or tinged with purple, and it was doubted if a variety untinged with that color was obtainable.
In , scented single forms of dahlias were first reported in Neu Verbass, Austria. A new scented species would not be introduced until the next century when the D. The exact date the dahlia was introduced in the United States is uncertain. According to Edward Sayers  "it attracted much admiration, and at that time was considered a very elegant flower, it was however soon eclipsed by that splendid scarlet, the Countess of Liverpool". However 9 cultivars were already listed in the catalog from Thornburn, Sayers stated that "No person has done more for the introduction and advancement of the culture of the Dahlia than George C.
Thorburn, of New York, who yearly flowers many thousand plants at his place at Hallet's Cove, near Harlaem.
Hogg, Mr William Read, and many other well known florists, have also contributed much in the vicinity of New York, to the introduction of the Dahlia. Indeed so general has become the taste that almost every garden has its show of the Dahlia in the season. In Thomas Bridgeman, published a list of double dahlias in his Florist's Guide.