The non-destructive evaluation of plant root growth is a challenge in root research. In the present study we aimed to develop electrical impedance spectroscopy (EIS) for that purpose. Willows (Salix myrsinifolia Salisb.) were grown from cuttings in a hydroponic culture in a growth chamber. Root growth was monitored at regular intervals by a displacement method and compared with the EIS parameters of the plants. To measure its impedance spectrum (IS) (frequency range from 40 Hz to 340 kHz) each plant was set in a measuring cell filled with a solution of the hydroponic culture. The IS was measured using a two-electrode measuring system. A silver needle electrode was connected to the stem immediately above the immersion level and a platinum wire was placed in the solution. The measurements were repeated twice weekly for a root growth period of one month. The IS of the entity consisting of a piece of stem, roots and culture solution were modelled by means of an electric circuit consisting of two ZARC-Cole elements, one constant-phase element, and a resistor. On the plant basis, an increase in root volume by growth correlated with a reduction in the sum of resistances in the ZARC-Cole elements (mean Pearson’s correlation coefficient r = –0.70).