|Dahlia x hybrida|
30 species, 20,000 cultivars
Dahlia (UK // or US //) is a genus of bushy, tuberous, herbaceous perennial plants native mainly in Mexico, but also Central America, and Colombia. A member of the Asteraceae or Compositae, dicotyledonous plants, related species include the sunflower, daisy, chrysanthemum and zinnia. There are at least 36 species of dahlia, with hybrids commonly grown as garden plants. Flower forms are variable, with one head per stem; these can be as small as 2 in (5.1 cm) diameter or up to 1 ft (30 cm) ("dinner plate"). This great variety results from dahlias being octoploids—that is, they have eight sets of homologous chromosomes, whereas most plants have only two. In addition, dahlias also contain many transposons—genetic pieces that move from place to place upon an allele—which contributes to their manifesting such great diversity.
The stems are leafy, ranging in height from as low as 12 in (30 cm) to more than 6–8 ft (1.8–2.4 m). The majority of species do not produce scented flowers or cultivars. Like most plants that do not attract pollinating insects through scent, they are brightly colored, displaying most hues, with the exception of blue.
The dahlia was declared the national flower of Mexico in 1963. The tubers were grown as a food crop by the Aztecs, but this use largely died out after the Spanish Conquest. Attempts to introduce the tubers as a food crop in Europe were unsuccessful. 
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 1778, more than eleven years before the plant was introduced into Europe in 1789, so while it is generally agreed that the plant was named 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 1789, but named the first three species that flowered from the cuttings.
Regardless of who bestowed it, the name was not so easily established. In 1805, German botanist Carl Ludwig Willdenow, asserting that the genus Dahlia Thunb. (published a year after Cavanilles's genus and now considered a synonym of Trichocladus) was more widely accepted, changed the plants' genus from Dahlia to Georgina; after the German-born naturalist Johann Gottlieb Georgi, a professor at the Imperial Academy of Sciences of St. Petersburg, Russia. He also reclassified and renamed the first three species grown, and identified, by Cavanilles. It was not until 1810, in a published article, that he officially adopted the Cavanilles' original designation of Dahlia. However, the name Georgina still persisted in Germany for the next few decades.
"Dahl" is a homophone of the Swedish word "dal", or "valley"; although it is not a true translation, the plant is sometimes referred to as the "valley flower".
Spaniards reported finding the plants growing in Mexico in 1525, but the earliest known description is by Francisco Hernández, physician to Philip II, who was ordered to visit Mexico in 1570 to study the "natural products of that country". They were used as a source of food by the indigenous peoples, and were both gathered in the wild and cultivated. The Aztecs used them to treat epilepsy, and employed the long hollow stem of the (Dahlia imperalis) for water pipes. The indigenous peoples variously identified the plants as "Chichipatl" (Toltecs) and "Acocotle" or "Cocoxochitl" (Aztecs). From Hernandez' perception of Aztec, to Spanish, through various other translations, the word is "water cane", "water pipe", "water pipe flower", "hollow stem flower" and "cane flower". All these refer to the hollowness of the plants' stem.
Hernandez described two varieties of dahlias (the pinwheel-like Dahlia pinnata and the huge Dahlia imperialis) as well as other medicinal plants of New Spain. Francisco Dominguez, a Hidalgo gentleman who accompanied Hernandez on part of his seven-year study, made a series of drawings to supplement the four volume report. Three of his drawings showed plants with flowers: two resembled the modern bedding dahlia, and one resembled the species Dahlia merki; all displayed a high degree of doubleness. In 1578 the manuscript,entitled Nova Plantarum, Animalium et Mineralium Mexicanorum Historia, was sent back to the Escorial in Madrid; they were not translated into Latin by Francisco Ximenes until 1615. In 1640, Francisco Cesi, President of the Academia Linei of Rome, bought the Ximenes translation, and after annotating it, published it in 1649-1651 in two volumes as Rerum Medicarum Novae Hispaniae Thesaurus Seu Nova Plantarium, Animalium et Mineraliuím Mexicanorum Historia. The original manuscripts were destroyed in a fire in the mid-1600s.
In 1787, the French botanist Nicolas-Joseph Thiéry de Menonville, sent to Mexico to steal the cochineal insect valued for its scarlet dye, reported the strangely beautiful flowers he had seen growing in a garden in Oaxaca. In 1789, Vicente Cervantes, Director of the Botanical Garden at Mexico City, sent "plant parts" to Abbe Antonio José Cavanilles, Director of the Royal Gardens of Madrid. Cavanilles flowered one plant that same year in his Icones plantarum, then the second one a year later. In 1791 he called the new growths "Dahlia" for Anders Dahl. The first plant was called Dahlia pinnata after its pinnate foliage; the second, Dahlia rosea for its rose-purple color. In 1796 Cavanilles flowered a third plant from the parts sent by Cervantes, which he named Dahlia coccinea for its scarlet color.
In 1798, Cavanilles sent D. Pinnata seeds to Parma, Italy. That year, the Marchioness of Bute, wife of The Earl of Bute, the English Ambassador to Spain, obtained a few seeds from Cavanilles and sent them to Kew Gardens, where they flowered but were lost after two to three years.
In the following years Madrid sent seeds to Berlin and Dresden in Germany, and to Turin and Thiene in Italy. In 1802, Cavanilles sent tubers of "these three" (D. pinnata, D. rosea, D. coccinea) to Swiss botanist Augustin Pyramus de Candolle at University of Montpelier in France, Andre Thouin at the Jardin des Plantes in Paris and Scottish botanist William Aiton at Kew Gardens. That same year, John Fraser, English nurseryman and later botanical collector to the Czar of Russia, brought D. coccinea seeds from Paris to the Apothecaries Gardens in England, where they flowered in his greenhouse a year later, providing Botanical Magazine with an illustration.
In 1804, a new species, Dahlia sambucifolia, was successfully grown at Holland House, Kensington. Whilst in Madrid in 1804, Lady Holland was given either dahlia seeds or tubers by botanist Antonio José Cavanilles. She sent them back to England, to Lord Holland's librarian Mr Buonaiuti at Holland House, who successfully raised the plants. A year later, Buonaiuti produced two double flowers. The plants raised in 1804 did not survive; new stock was brought from France in 1815. In 1824, Lord Holland sent his wife a note containing the following verse:
"The dahlia you brought to our isle
Your praises for ever shall speak;And in colour as bright as your cheek."
Mid gardens as sweet as your smile,
In 1805, German naturalist Alexander von Humboldt sent more seeds from Mexico to Aiton in England, Thouin in Paris, and Christoph Friedrich Otto, director of the Berlin Botanical Garden. 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. pinnata and D. rosea under the name of Georgina variabilis; D. coccinea was still held to be a separate species, which he renamed Georgina coccinea.
That same year, 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.  Many of the species of dahlias then, and now, have single flowered blooms. coccinea, the third dahlia to bloom in Europe, was a single. But two of the three drawings of dahlias by Domenguez, made in Mexico between 1570–77, 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 1805 to 1810 several people claimed to have produced a double dahlia. In 1805 Henry C. Andrews made a drawing of such a plant in the collection of Lady Holland, grown from seedlings sent that year from Madrid. Like other doubles of the time it did not resemble the doubles of today. 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. By 1826 double varieties were being grown almost exclusively, and there was very little interest in the single 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 1843, scented single forms of dahlias were first reported in Neu Verbass, Austria. D. crocea, a fragrant variety grown from one of the Humboldt seeds, was probably interbred with the single D. coccinea. A new scented species would not be introduced until the next century when the D. coronata was brought from Mexico to Germany in 1907.
The exact date the dahlia was introduced in the United States is unknown. In 1840 Thomas Bridgeman, supplied a catalog of "all the choicest varieties available." He stated that a list and description of about 100 choice seedlings of 1838 and 1839, which had been purchased in England and grown in the garden of Mr. G. C. Thornburn of Astoria, N.Y. had been furnished to him by that gentleman and would be offered for sale in 1840. To this list he added about two hundred fifty varieties, "most of which he had under cultivation in his own garden."
"Stars of the Devil"
In 1872 J.T. van der Berg of Utrecht in the Netherlands, received a shipment of seeds and plants from a friend in Mexico. The entire shipment was badly rotted and appeared to be ruined, but van der Berg examined it carefully and found a small piece of root that seemed alive. He planted and carefully tended it; it grew into a plant that he identified as a dahlia. He made cuttings from the plant during the winter of 1872-1873. This was an entirely different type of flower, with a rich, red color and a high degree of doubling. In 1874 van der Berg catalogued it for sale, calling it Dahlia juarezii to honor Mexican President Benito Pablo Juarez, who had died the year before, and described it as "...equal to the beautiful color of the red poppy. Its form is very outstanding and different in every respect of all known dahlia flowers.".
This plant has perhaps had a greater influence on the popularity of the modern dahlia than any other. Called "Les Etoiles de Diable" (Stars of the Devil) in France and "Cactus dahlia" elsewhere, the edges of its petals rolled backwards, rather than forward, and this new form revolutionized the dahlia world. It was thought to be a distinct mutation since no other plant that resembled it could be in the wild. Today it is assumed that D. juarezii had, at one time, existed in Mexico and subsequently disappeared. Nurserymen in Europe crossbred this plant with dahlias discovered earlier; the results became the progenitors of all modern dahlia hybrids today.
Since 1789 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 1805, 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 1920s, 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. Constant reclassification of the 85 reported species has resulted in a considerably smaller number of distinct species, as there is a great deal of disagreement today between systematists over classification.
In 1829, all species growing in Europe were reclassified under an all-encompassing name of D. variabilis, Desf. Through the interspecies cross of the Humboldt seeds and the Cavanilles species, 22 new species were reported by that year, all of which had been classified in different ways by several different taxonomists, creating considerable confusion as to which species was which.
In 1830 William Smith suggested that all dahlia species could be divided into two groups for color, red-tinged and purple-tinged(92). In investigating this idea Lawrence(58) determined that with the exception of D. variabilis, all dahlia species may be assigned to one of two groups for flower-colour: Group I (ivory-magenta) or Group II (yellow-orange-scarlet).
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.
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. 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. Planting the tubers quite deep (10 – 15 cm) also provides some protection. 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.
(see also List of Award of Garden Merit dahlias)
Pests and diseases
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. dahliae), phytophthora and some plant viruses. Dahlias are a source of food for the larvae of some Lepidoptera species including Angle Shades, Common Swift, Ghost Moth and Large Yellow Underwing.
Food and medicinal uses
Today the dahlia is still considered one of the native ingredients in Oaxacan cuisine; several cultivars are still grown especially for their large, sweet potato-like tubers. Dacopa, an intense mocha-tasting extract from the roasted tubers, is used to flavor beverages throughout Central America.
In Europe and America, prior to the discovery of insulin in 1923, diabetics—as well as consumptives—were often given a substance called Atlantic starch or diabetic sugar, derived from inulin, a naturally occurring form of fruit sugar, extracted from dahlia tubers. Inulin is still used in clinical tests for kidney functionality.
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- Harvard Arboretum
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- Dahlia.com Growing Tips
- "RHS Plant Selector Dahlia 'Bednall Beauty' (Misc/DwB) AGM / RHS Gardening". Apps.rhs.org.uk. Retrieved 2013-04-29.
- "RHS Plant Selector Dahlia 'Bishop of Llandaff' (P) AGM / RHS Gardening". Apps.rhs.org.uk. Retrieved 2013-04-29.
- "RHS Plant Selector Dahlia 'Clair de Lune' (Col) AGM / RHS Gardening". Apps.rhs.org.uk. Retrieved 2013-04-29.
- "RHS Plant Selector Dahlia 'David Howard' (MinD) AGM / RHS Gardening". Apps.rhs.org.uk. Retrieved 2013-04-29.
- "RHS Plant Selector Dahlia 'Ellen Huston' (Misc/DwB) AGM / RHS Gardening". Apps.rhs.org.uk. Retrieved 2013-04-29.
- "RHS Plant Selector Dahlia 'Fascination' (P) AGM / RHS Gardening". Apps.rhs.org.uk. Retrieved 2013-04-29.
- "RHS Plant Selector Dahlia 'Gallery Art Deco' PBR (Gallery Series) (SD) AGM / RHS Gardening". Apps.rhs.org.uk. Retrieved 2013-04-29.
- "RHS Plant Selector Dahlia 'Gallery Art Nouveau' PBR (Gallery Series) (MinD) AGM / RHS Gardening". Apps.rhs.org.uk. Retrieved 2013-04-29.
- "RHS Plant Selector Dahlia 'Glorie van Heemstede' (SWL) AGM / RHS Gardening". Apps.rhs.org.uk. Retrieved 2013-04-29.
- "RHS Plant Selector Dahlia 'Honka' (SinO) AGM / RHS Gardening". Apps.rhs.org.uk. Retrieved 2013-04-29.
- "RHS Plant Selector Dahlia 'Moonfire' (Misc/DwB) AGM / RHS Gardening". Apps.rhs.org.uk. Retrieved 2013-04-29.
- "RHS Plant Selector Dahlia 'Twyning's After Eight' (Sin) AGM / RHS Gardening". Apps.rhs.org.uk. Retrieved 2013-04-29.
- Williams, Francis M., M.D.; Report on Therapeutics, On the effect of giving levulose and inulin to patients suffering diabetes mellitus, Boston medical and surgical journal, Massachusetts Medical Society, New England Surgical Society, Volume 133, no. 2, 1895, p. 37.
|Wikimedia Commons has media related to Dahlia.|
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- Dahlia species; The National Dahlia Society (U.K.)
- Whitley GR (September 1985). "The medicinal and nutritional properties of Dahlia spp". J Ethnopharmacol 14 (1): 75–82. doi:10.1016/0378-8741(85)90031-5. PMID 3910964.