Dahlia seeds, cutting, and tubers may all be started in pots successfully. Good drainage is key to mixing soil for dahlias. When started in pots, they will need to be hardened off before transplantation outdoors. Hardening off gradually exposes plants started indoors to outdoor life, including temperature and wind stresses. Bringing young plants outdoors for an hour or two at a time during the day as temperatures increase helps them acclimate to outdoor life without the shock of overnight temperature dips.
Mix your soil. Mix 1/3 commercial potting soil, 1/3 sand, and 1/3 vermiculite or perlite to create a soil with good drainage qualities for your dahlias. Fill the pots about 3/4 of the way with this mixture.
Sow seeds to a depth of 1/2 inch in the pots. Spacing does not matter at this point, because you can thin out any seedlings that are growing too close together later. Mist soil lightly with water, but do not drench. Cover with plastic wrap and set aside in a warm room, out of direct sunlight. Seeds should sprout in about a week.
Plant tubers, with eyes facing up, in shallow trenches in the soil of each pot. Plant only one per pot, and cover with only about 1/2" of soil at most. Do not water until sprouts appear. Set aside in a warm room, out of direct sunlight. Sprouts should break ground in 5-6 days.
Plant cuttings so their stalks are about 1/2 inch below ground. They will need to develop roots. When watering, make sure to water the soil, not the plant itself.
Water carefully, especially with the tubers. Rot is a potentially serious problem with dahlias, so you will need to monitor their water needs carefully. While indoors, water them sparingly when soil seems dry.
Apply a 10-10-10 fertilizer after plants reach 8 inches tall. The numbers represent a fertilizer's nitrogen, phosphorus and potassium (or NPK) levels. Dahlias do best with a balanced fertilizer.
Harden your dahlias off a few weeks before you plant to transplant them outdoors. Begin by bringing them outdoors for an hour in the middle of the day. Bring them back inside for the rest of the day. Repeat the process daily, upping the amount of time spent outdoors, until the plants are able to stay outdoors for a full 24 hour cycle. Do not attempt to leave them outdoors if nighttime temperatures in your area are below 50 degrees F.