Dahlias grow everywhere and anywhere; plant them in the late spring after the soil has warmed, and prepare to be wowed come summer, with buckets of blooms in wild colors.
Each of our Dahlias are sold as a whole, labeled, clump of tubers. You can plant the entire clump as-is, or you can divide the tubers and plant them out individually (each tuber will create its own dahlia plant, but dividing the tubers takes time and practice, planting out the whole clump is the easier way to do it!).
Dahlias want full sun, warm soil (wait to plant them in the spring until your gardener friends are planting out their tomatoes; Dahlias are sensitive souls and really suffer if it gets cold), and a little support for their big blooms.