Dahlias, native to the highland plateaus of Mexico, are tropical plants with large, ornately shaped and colored blooms. They grow from "bulbs" called tubers, which are firm, swollen horizontal stems. Dahlias grow their finest when soil is warm, moist and fertile and sunny daytime temperatures are between 70 and 80 degrees Fahrenheit and nighttime temps are cool, between 45 and 55 degrees. In U.S. Department of Agriculture hardiness zones 7 and colder, lift the tubers out of the ground and overwinter them indoors.
Mid to Late Spring
After all danger of frost has passed in mid or late spring and soil temperatures have warmed to at least 55 degrees, plant the dahlia tubers. According to the American Dahlia Society, this corresponds to about the same time gardeners plant tomatoes outside in their region. In most areas of the United States, dahlia tubers may be planted as late as mid-June to provide ample time for plants to grow and produce flowers before the first autumn frost.
End of Rainy Season
In frost-free tropical or subtropical regions like Hawaii or southernmost Florida, the year rotates between wet and dry seasons. Since dahlias grow best in cool to warm temperatures when sunny weather and lower humidity dominate, this corresponds to planting tubers at the end of the rainy season. The cooler, drier months from autumn to spring supply an abundance of sunshine with cooler temperatures that permit dahlia plants to develop strong stems and larger sized flowers. It may be difficult to buy dahlia tubers in autumn, so purchase them in spring and store them in a cool, dry place until early fall when they can be first planted. Then, plan on lifting them after their growing season winds down in late spring and store the dormant tubers indoors across the hot, sultry and rainy summer months.
Early Indoor Planting
if you live in a cold winter region or a tropical region with summer season inhospitable to dahlia planting, planting tubers early indoors before transplanting outdoors is an option, according to the American Dahlia Society. For example, in the temperate United States, determine the date of the expected last spring frost and count back 30 days. Plant tubers in containers, and place them in front of a sunny window. Allow them to begin growing, and then transplant them outdoors after the last spring frost. Likewise, in tropical regions, count back 30 to 45 days from the usual end of the summer rainy season, and start the tubers in containers indoors. Transplant them outdoors in autumn once humidity drops and daytime temperatures are no longer much above 80 degrees.