Soil moisture control on evapotranspiration is poorly understood in ecosystems experiencing seasonal greening. In this study, we utilize a set of multi‐year observations at four eddy covariance sites along a latitudinal gradient in vegetation greening to infer the ET ‐θ relation during the North American monsoon. Results reveal significant seasonal, interannual and ecosystem variations in the observed ET ‐θ relation directly linked to vegetation greening. In particular, monsoon‐dominated ecosystems adjust their ET ‐θ relation, through changes in unstressed ET and plant stress threshold, to cope with differences in water availability. Comparisons of the observed relations to the North American Regional Reanalysis dataset reveal large biases that increase where vegetation greening is more significant. The analysis presented here can be used to guide improvements in land surface model parameterization in water‐limited ecosystems.