Soil/Planting
Start seed by spreading potting soil mix in a low, flat tray. Potting soil can be purchased at any garden center, please purchase the plain, cheap potting soil without the added fertilizers. Stay away from any pre-fertilized potting soils such as Miracle-Gro. Sprinkle your dahlia seed over the top of the soil. Then lightly cover with potting soil. Lightly water so the soil is damp and continue to water lightly, keeping the soil damp during sprouting. Sprouting should occur within a 7–14 day period. DO NOT OVER WATER OR KEEP SOIL SOAKING WET. Seeds must be started in a location that is consistently 70+ degrees or more. They should be kept in these conditions throughout germination, they do not need any additional light, other than what Mother Nature gives us during the daylight hours. Keep in mind that unlike vegetable seed, dahlia seed will not all sprout the same day; germination will occur over several days. The usual success rate in dahlia seeds is 50-75%.
Potting
After the young seedlings have reached a height of approximately 3 inches, they will be ready to separate and transplant into small “plugs” or pots. These containers should be filled with the same potting soil used to start your seeds. After potting up the seedlings, water enough to keep the soil dampened while they take root. Continue to water and keep the soil damp as the plants grow. The seedlings will stay in these containers and continue to grow for about 4 to 5 weeks before they are ready for transplanting outside.
Transplanting Outdoors
Before you transplant them outside, it is recommended that you harden off the young seedlings first. They should be moved into a cold frame, or the trays can be set outside during the day and brought in at night for a period of about 7–10 days. This will help condition the young seedlings, and they will go through less shock when transplanted.
Pick a sunny location that has well-drained soil to transplant to. After transplanting your dahlias into your beds, be sure to keep them watered until the roots are established (approximately 1–2 weeks). At this point, you will need to fertilize and care for them the same as you would for tubers planted directly into your beds and it is time to start your summer care regimen. Remember, dahlias require low nitrogen fertilizer. In general, a bloom-type fertilizer is best. Fertilizer is applied for the first time about 3 weeks after planting outside and repeated once a month during the growing season. We recommend fertilizing them every 3-4 weeks if planted directly in the ground and every 2-3 weeks if planted in containers. Over-fertilizing and too high nitrogen fertilizer will produce weak stems, huge green plants, small blooms, or no blooms.