First It Is Important to understand that Dahlias grow from tubers and not bulbs.
Bulbs, such as lilies, daffodils, tulips, all can be planted in wet cold soils in the autumn. Dahlias must be planted in warm, well drained soils, in April/May time. Dahlias are generally planted about the same time you would plant your vegetable patch.
Plant seeds about1/2inch deep in a pot, pack, or flat. Cover the seeds with more of the soil medium or pure vermiculite and add water until the soil is thoroughly moistened. Placing a film of polyethylene or a pane of glass over the container will keep the surface moist and enable the seeds to germinate undisturbed. Place the container in a warm location (not in direct sunlight) and allow seeds to germinate. Dahlias normally germinate in 5–7 days at temperatures between 70–80 degrees F. When the new shoots are first visible, remove the plastic or glass. From here, it’s pretty much your decision if they go into pots or the garden. Either way–handle gently using the lowest set of leaves as your “handle.” Pre-punch a hole in the soil to accommodate the new plant.
With luck, in another 3-5 weeks you’ll be rewarded with bright colorful flowers attractive to birds, bees and butterflies.
Packet. App. 50 seeds.