There are two times of year that are considered optimal for digging medicine roots, Spring and Autumn. The reason for this is both because it is a folk way that was practiced as an aspect of seasonal living by Earth-based cultures and as part of our understanding of plant phsyiology. Biennial and perennial plants use their roots to store nutrients throughout the Winter. In the Spring the roots retain these rich, essentials until the plant is signaled by the sun and temperature shift that it's time to send it's vital energy upward toward the emergence of sprouts and buds. After the plant has flowered and produced seeds, the lowering of the sun and the first frosts signal it to send it's life force back into the roots. If the plant is biennial (has a two year life-cycle) we can gather the root any time during the Spring or Fall of the first year and the Spring of the second year. For perennials roots are gathered during the Spring or Fall, and, with perennial roots, many plants have certain years that are considered optimal for root harvesting. Such, as with Echinacea, it is generally considered best to harvest it after it's at least 3 years old and after about 7 years old, I have found, the roots become more fiberous and woody so are not optimal.