Astilbe flower seed is usually sown indoors in early spring and transplanted outside when the weather is warmer. Use a loose, soft soil mix, and plant the Astilbe seeds on the surface, pressing them into the soil. Make sure that the soil drains well and is able to keep the flower seeds moist, but not wet, at all times. For the best results, add in a little fertilizer that has a high concentration of nitrogen. Keep the Astilbe plants inside until the seedlings reach about 2 inches in height. Transplant after last frost.