No matter what type of dahlia you wish to grow, all dahlias originate from central Mexico, in the region around Mexico City. Dahlias thrive in a slightly acidic soil, with a pH level of around 6.5 to 7. Dahlias may be started via seeds, tubers, or cuttings. They are not frost-hardy, and may be started indoors and hardened off (gradually exposed to outdoor weather until outdoor hardiness is achieved) as the weather becomes consistently above freezing.
Test your soil. The most reliable, accurate and thorough way to do this is to contact your county extension office and submit a soil sample for analysis. This usually costs very little money, but is well worth it.
Choose an area that gets full sun, and then dig in compost in a 1-to-1 ratio in the area to be planted. Dig to a depth of 12 inches, as dahlias feed heavily and require a lot of space. Amend soil according to soil test results. Agricultural lime can lower soil pH, while sulfur can raise it. Dahlias also like potassium, so if your potassium levels are low, dig in potash in the amounts recommended by your soil test and county extension office, advises University of Minnesota Extension. Amounts needed vary greatly by location and soil type. Prepare soil as soon as ground has thawed enough to be worked.
Sow seeds in a flat to a depth of 1/2 inch. Potting mixture should be a third potting soil, a third sand, and a third vermiculite or perlite for good drainage. Mist soil lightly, but do not water heavily. Cover with plastic wrap and set in a warm place, out of direct sunlight. Seeds should sprout in about a week.
Start tubers in the same potting mixture. Lay them horizontally in a pot or a sideways milk carton, eyes facing up, and covering them only lightly with slightly moist soil. Do not water until sprouts have appeared. Sprouts will appear in five to six days.
Start cuttings in the same potting mixture, planting their bases to a depth of about an inch. Water the soil, not the plants, as cuttings need to form roots.
Harden off seedlings when daytime temperatures are above 55 degrees F, and seedlings are above 8 inches tall, recommends the Dahlia Society of California. Bring them outdoors during the warmest part of the day, then bring them back indoors. Leave them outside longer the next day, and so on. As temperatures stay consistently warm overnight, you can begin leaving them outdoors around the clock.
Transplant hardened-off seedlings after all danger of frost has passed. Space holes about 30 inches apart, and dig each one to a depth of 6 inches. Sprinkle a teaspoon of bone meal in the bottom of the hole, then gently transplant each plant into a hole. Cover lightly with soil. Apply a balanced 10-10-10 fertilizer (the number represents the nitrogen-phosophorus-potassium, or NPK levels) once every three weeks according to package instructions. Water the soil near the plants, but do not water the plants themselves.