This paper explores the interaction between flow field and plant motion in open-channel flows with submerged flexible vegetation. Experiments were performed in a 10 m-long and 40 cm-wide tilting flume. Flows with submerged vegetation were studied under a range of conditions by varying lengths of flexible vegetation models (5.0, 7.0, 9.0 and 10.5 cm). Flow velocity components and the tip motion of waving plants were measured simultaneously using particle image velocimetry (PIV) and particle tracking velocimetry (PTV) techniques. The applied techniques are capable of investigating the interaction between flow field and plant motion. We examined quantitatively how plant motions were coupled to strong oscillations in flow velocity associated with the "monami" phenomenon as well as its vertical extent. A strong correlation between flow velocity and plant motion was observed in the mixing-layer zone (near the vegetation edge) only. Analysis of cross-correlation and coherence functions enabled estimation of the phase lag between the turbulent flow field and plant motion. The experiments revealed that the number of flexible vegetation elements waving at the same time depends on the length-scales of the turbulence structure in the mixing-layer zone.