Dahlias grow quickly and can rapidly bring vivid blossoms and lush foliage to a backyard flower patch. Gardeners typically start dahlias directly in the ground, but you must wait until the danger of frost is past. If you're in a rush, you can germinate dahlias indoors to give your plants a head start on the growing season. Start the dahlias three to four weeks before the last frost date in your area, and then transplant them outdoors for an instant flower garden.
Get a compartmentalized seed tray from a nursery or garden store. For the best results, choose a tray with compartments measuring 4 inches deep.
Fill the seed tray with a commercially prepared sterile seed starter soil mix. Or make your own mix by combining equal parts of compost or peat moss, garden loam and perlite or sand.
Sow the dahlia seeds. Place one seed in each of the seed tray's compartments, buried 1/2 inch below the soil surface.
Sprinkle water on the seed tray to evenly moisten each compartment.
Seal the top of the seed tray with plastic kitchen wrap, and set the tray in a warm area out of direct sunlight. For the fastest dahlia seed germination, the University of Minnesota recommends putting the tray in an area that's 70 to 80 degrees F.
Remove the plastic wrap once the seeds germinate. Dahlias typically sprout within a week.
Prepare the outdoor gardening area as soon as your area's last frost date passes. For optimal dahlia establishment, the University of Minnesota suggests breaking up the soil to a depth of 10 inches, then mixing in 1 lb. of 5-10-10 fertilizer for every 50 square feet of gardening space.
Transplant the seedlings into the ground. Dig a hole in the prepared soil that's the size of each of the seed tray's compartments. Slide the seedling out of the seed tray and into the ground. Separate each transplant by approximately 36 inches to account for the dahlia plant's mature size.