• Don’t be in a hurry to plant; dahlias will struggle in cold soil. Ground temperature should reach 60°F. Wait until all danger of spring frost is past before planting. (Try planting them a little after the tomato plants go in.)
• Select a planting site with full sun. Dahlias grow more blooms with 6 to 8 hours of direct sunlight.
• Dahlias thrive in rich, well-drained soil. The pH level of your soil should be 6.5-7.0, slightly acidic.
• Plant your dahlias 2-3 feet apart.
• Plant tuber with the eye facing up, about 6 to 8 inches deep. The crowns should be just below soil level.
• Medium to large flowering dahlias will require support. Place stakes (5-6 feet tall) at planting time and tie stems to them as the plants grow. Most dahlias grown for cut flowers reach 4-6 feet so staking is important.

 http://www.semds.org/wp-content/uploads/2017/05/Time-to-plant-tubers-and-plants.pdf