Dahlias are among the tallest flowers to grow in the home garden, with some varieties reaching around 6 feet high with 12-inch-wide blooms. You can also use shorter dahlia varieties, if desired. While you can grow most dahlia flowers seed, it is easiest to plant them from tubers (their underground root structures). Dahlias require a growing space that receives at about 5 to 6 hours of sunlight. Plant tubers after the last frost in the spring.
Till the soil bed about 12 inches deep and mix in 2 to 4 inches of organic matter that is high in nitrogen, such as compost or manure, to enrich the soil and create soil that drains water well. The organic matter will also release nitrogen over time, which dahlias need to thrive. Also, incorporate 2 lbs. of fertilizer for every 100 square feet. Choose a fertilizer, such as one labeled 5-10-15 or 5-10-10.
Dig holes between 8 and 10 inches deep and wide enough for the tubers to fit. Space dahlias between 2 and 4 feet apart, depending on the size of the variety.
Backfill the soil until it is 6 inches deep and the top of the tuber (when planted) is only about 3 inches beneath the soil. The reason for the extra digging in Step 2 is simply to help loosen the soil for the roots to better grow.
Set the tubers in the holes on their sides with the eyes facing up. Backfill the soil only to cover the tubers with 1 inch of soil. Then, lightly pack it down until it is slightly firm, but not packed tightly. This should remove the large pockets of air, which may later cause root rot.