I suggest a slow-release plant food such as Osmocote at planting time to provide continuous nutrition as the plant grows. Then, when the dahlia plant is half the expected height, you can use stronger or quicker-acting plant food to promote blooms, such as Miracle Grow, made for flowering plants. The long-term answer is to improve your soil so you will not need to fertilize so much. Adding organic matter such as compost or getting a soil sample to see if your soil is too acidic or too basic can help you unlock natural nutrients in the soil. Dahlias need warm soil to grow quickly, so a raised bed or large container in a sunny site will speed up growth. Keep growing, Marianne Binetti