Dahlias are old. They are derived from wild plants found growing in Mexico. The first reports of dahlias originated circa 1570-1577. Dahlia seeds arrived in Spain in 1789, and were successfully flowered in 1790. Dahlias need space, particularly if you're growing the giant versions.
Dahlias are organized and competitive.
There are several organizations around the globe dedicated to the growth, classification, propogation, and exhibition of dahlias. When planting, work a time-released fertilizer into the soil. Consider supplementing with a water-soluable fertilizer throughout the summer every two weeks. Dahlias like sunlight, and need a minimum of half a day with good sunlight. Soaking dahlias every 5-7 days helps them grow better.