We dig comfrey roots all year long (as long as the ground is not frozen!) The strain we have is Symphytum x uplandicum or Russian Comfrey.
Russian Comfrey is sterile which means it does not make viable seed. Often thought of as invasive, the plant really doesn’t present a problem until the root is disturbed. Once cut, the root pieces make new plants and this can lead to Comfrey overload in the garden. You can plant these roots or turn them into medicine when you get them.