Ohio State University calls the dahlia one of the most spectacular flower plants available to backyard gardeners. The hardy plants add dense foliage and vibrantly colored blossoms to any landscape. Though you can buy started dahlia plants at most nurseries, sowing dahlia seeds is often more economical and saves you the trouble of having to transplant seedlings.
Harvest dahlia seeds, unless you've purchased a seed packet from a garden store or nursery. Wait for the flowers on an existing dahlia plant to wilt and die, leaving behind a bulbous flower head. Cut off the flower head in September or October when the flower head has turned brown, according to Ohio State University. Break open the flower heads, and spread out the seeds in a shallow tray. Let them dry for several days, then store them in a sealed container until you're ready to plant them.
Prepare the garden area once the outdoor temperature has reached a minimum of 70 degrees F. Though dahlias are hardy and will grow in any type of dirt, they'll thrive best in loose garden loam that receives six hours of sunlight daily. Break apart the soil to a depth of 6 inches with a spade, then mix in several inches of organic matter like peat moss, compost or aged manure, according to Oregon State University.
Plant the dahlia seeds. Bury each seed 1/2 inch under the soil surface, according to the University of Minnesota, and space each seed by 9 to 12 inches apart, according to Ohio State University,
Water the dahlia seeds twice daily or as needed to keep the soil moist. The seeds will germinate within a week, according to the University of Minnesota.