People in northern temperate climates will probably think twice about growing dahlias because you have to dig them up each fall, store the tubers carefully over the winter, and then plant them in the late spring when the ground temperature reaches 60 degrees. And yet dahlias are incredibly popular, with dahlia societies in major cities around the world.
The origins of this cultivated plant are in the highlands of Mexico and Central America. It was important to the Aztecs and the Toltecs, who may have been responsible for spreading it southward through Central America into northern South America. Europeans first encountered it among the Aztecs, who may have been growing them to eat them and perhaps to use as medicine. The conquistadores brought along botanists who found the Aztecs to be growing what is now called the tree dahlia (Dahlia imperialis), which can get to be 20 feet tall and has hollow stems. It was called acocotli or “water cane”.
A portion of the medicinal lore of the Aztecs was preserved in the Badianus Manuscript written in 1552 by Juannes Badianus, an Aztec student in themselves Colegio de Santa Cruz de Tlatelolco in Mexico City. An illustration in this book shows a red, eight-petaled flower that resembles a plant later described as D. coccinea.
Later in the 16th century Philip II of Spain sent Francisco Hernandez to Mexico to discover medicinal plants in Mexico. Hernandez produced Rerum Medicarum Novae Hispaniae, which includes illustrations of several apparent Dahlia species, including one that is a doubled-flower, suggesting it has been cultivated.
Jose Antonio Cavanilles, director of the Royal Botanic Gardens in Spain, was responsible for sending back to the Old World the first dahlia plant materials that could be grown. In 1789 he sent seeds back to Europe. By 1791 they had produced plants. He called them Dahlias after Swedish botanist Andres Dahl and called his first species, a doubled-flowered plant, D. pinnata. By 1796 he had named D. rosea and D. coccinea.
As successive generations of seeds are sown, an enormous variety of flowers is produced. These are given species names at first, but eventually it is realized that these are hybrids. Much later it will be discovered that dahlias are polyploids (octoploids in this case) with multiple sets of chromosome pairs in each cell. In addition, transposons, genes that readily migrated from chromosome to chromosome, are quite common in dahlias. These qualities allow for tremendous variation in each generation that is cultivated.
Over the years dahlias have been classified and reclassified. At one point the amount of variability in the group caused German botanists to begin an entirely new genus, Georgina, later shown to simply be more hybrid dahlias.
Much of the variation in the flower shapes is derived from reshaping of the petals. In some varieties they curve up and then down longitudinally; in others they curve down and then up; in yet others the petals roll up latitudinally. This sort of variation produces 14 groups of cultivars, many of them named for their resemblance to other flowers (e.g. peony, orchid, anemone, and waterlily) and some for their overall shape (e.g. pompon and ball).
Their enormous variety and many different colors (almost everything but blue) contribute to their popularity. In addition, they flower for months on end. Their origins in the Mexican highlands mean that they cannot make it through the winter if the frost penetrates more than 6 inches into the soil. Storing the tubers in the winter requires that they be kept cool and not too moist (a bit like some begonias or gladiolus).
They are sensitive to herbicides and fertilizers, so the soil they’re planted in shouldn’t be amended with potting soil. Dahlias prefer good old-fashioned dirt.