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Dahlias prefer full sun, so will thrive best in the sunniest part of your garden. When dahlias don’t have enough sunlight, they tend to grow taller and leggier, and produce fewer blooms.
Here is a good source of information on planting, siting, and care:
http://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?kempercode=a445