Dahlia is named in honor of Dr. Andreas Dahl. This genus is in the compositae family and is usually tuberous rooted with species from Mexico, Central America and Columbia.
Dahlia species and related genera was the subject of the Pacific Bulb Society topic of the week in July 2003. It was introduced by Johannes-Ulrich Urban. Click for his 
Introduction
Dahlia Cultivars is our wiki page where pictures of some commonly named and unnamed Dahlias that are popular garden plants are pictured.
Dahlia coccinea An extremely common species in most of the Mexican altiplano. The colors of the flowers are variable and range from orange to red with every tone in between. It prefers cool conditions and can be seen growing at a altitude between 1500-2500 meters above sea level in oak or pine-oak forest. It likes growing in rich, forest humus but on a few occasions it can be seen growing as a lithophyte or even as an epiphyte. It blooms from August-October near the end of the rainy season. Photo by Dennis Szeszko taken in-situ in Mexico State at 1500 meters above sea level at the ecotone between oak forest and deciduous jungle.
Closeups of flowers by Arnold Trachtenberg
Dahlia imperialis known as a tree dahlia, grows to 9 m. In some climates it is wiped out by frost before it blooms. Photo by Rob Hamilton
Flowering started here on the 6-7 November 2005; if no severe or winds frosts intervene flowering will last until Christmas. The height is about 4m (15ft). The first image was taken of an old plant agains a 6m (20ft) high south facing wall of a wine cave, where it receives water from the roof. The plants was staked against the wall in August. Here we remove all shoots in the early part of the season , which are used for propagation. The second image shows the field nursery. Planted out in June 2005 very close together; they support each other, while the outer stems tend to fall over. Photos by Lauw de Jager.
And yet another view of the common lavender Dahlia imperialis (Tree Dahlia), these on the North Coast of California, U.S. in late November. The plants (the first photo shows three growing together) reach between 10 (3m) and 15 feet (4.5m) tall. They are in the southeast part of the yard with a dog pen and a large Douglas Fir behind. Plants on the left side are budded and ready to bloom, barring no gale force winds (pinched back earlier this fall they've bushed out nicely and are withstanding rain and stormy weather, so far) and no hard frost. The second picture shows a close-up of the flowers. If the plants make it safely through December, they can bloom into April when I chomp them down to the ground. Photos by Susan Hayek.
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