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Monitoring the moisture content (MC) of trees and plants by a non-destructive and wireless methodology is an essential effort towards the early diagnostics and prevention of forest fires. This work presents a MC sensing system comprising a nested split-ring resonator (NSRR) probe — a structure which has been demonstrated to be superior to the traditional SRRs in terms of its sensitivity and compactness — and an antenna to illuminate it. The NSRR is attached to the plant and the near-field interaction between the NSRR and the antenna produces an electromagnetically coupled system whose resonance frequency ( ${f}_{\textit {res}}$ ) is modified by the water content of the medium loading the NSRR. Full-wave simulations are performed to model the system response for different tree species. Results of the experiments conducted in vitro and in vivo are presented. In the in vitro experiments, a freshly cut tree branch sample is allowed to dry at ambient temperature and its MC change is monitored by the system. The in vivo experiments prove that the sensing system can also capture the dynamic MC change of a plant. Calibration sets generated by curve-fitting of the variation of ${f}_{\textit {res}}$ with respect to the MC and other sensing parameters are used to transform the measured ${f}_{\textit {res}}$ to MC information. The results of the simulations and the experiments demonstrate that the proposed sensing system can be utilized as a non-destructive and wireless method for highly-sensitive characterization of the MC of trees and plants.