Alternate bars are bedforms recognizable in straight or weakly curved channels as a result of riverbed instability. The length and height of alternate bars scale with the river width and the water depth, respectively. During low water stages, alternate bars become exposed and can be colonized by riparian vegetation. The effects of established plants on the morphodynamics of alternate bars have been poorly investigated. In this work, we focus on the effects induced by rigid vegetation on the dynamics and morphology of previously developed alternate bars in a straight channel by means of flume experiments. We investigate three different spatial densities of plants to reproduce scenarios of vegetation establishment. The results illustrate that vegetation alters both the altimetric and planimetric characteristics of bar patterns. In particular, as compared to bare-bed bars, vegetated bars have a higher wave amplitude and scour, and this effect becomes stronger with plant density. Moreover, they exhibit decreasing wavenumbers according to vegetation density. A comparison with previous fundamental work for the planimetric instability of straight channels with bare-bed alternate bars, suggests that the established vegetated bars may promote the transition to river meandering.