The Garden Bug: Planting dahlias
By Brenda Weaver
Dahlia tubers can go directly into the ground in the spring when there is little chance of frost, but they can also be planted as late as mid-June in most parts of the country. They require good drainage and partial to full sun.
Put one tuber in each hole, several inches deep, with the "eye" facing up. The tubers should be separated by about 2 feet to give each plant room to grow. Install one stake for each tuber as you plant it.
Source: www.dahlia.org