We study the QCD Adler function in the energy region approximate to 0.7 - 2.5 GeV, in which the non-perturbative effects become dominant. Our analysis is a renormalon-based evaluation using transseries within the resurgence of the Renormalization-Group-Equation and does not require the Operator-Product-Expansion. (C) 2021 The Author(s). Published by Elsevier B.V.