Flexible aquatic vegetation exists widely in nature and serves multiple hydro-environmental functions mainly through fluid–structure interactions. The waving motion of vegetation arrays, known as Monami, is predominantly governed by Kelvin–Helmholtz (KH) instability, and its characteristic scales, such as wave height and wavelength, are still being explored. In this paper, the interactions between a large array of flexible vegetations and a laminar boundary-layer flow are investigated using direct numerical simulation. The parameters used are the Reynolds number Re = 400, mass ratio beta=1.0, bending rigidity gamma=0.04–0.22, and gap distance d=0.4–1.6. A low frequency in Monami is found to be related to the fluctuation frequency of the onset position of the KH instability, which leads to the identification of four different Monami modes: regular Monami, quasi-regular Monami A, quasi-regular Monami B, and irregular Monami. The influences of the bending rigidity and gap distance on the Monami modes, KH instability onset position, and Monami characteristic scales are discussed. It was found that the causes of spatial and temporal variations in the characteristic scales of Monami vary depending on the mode. In the regular Monami mode, these variations result from the evolution of the KH vortex. In the quasi-regular Monami A mode, they are strongly affected by the shifting of the onset position of the KH instability. In the other two modes, these variations are caused by a combination of the fluctuation in the KH instability onset position and the complex interaction between vortices.