Temporally growing modes of the linearized equations of motion for homogeneous shear flows in the beta‐plane are considered. A new upper bound on their rate of growth is derived. This bound is related to the necessary criterion for linear instability derived by Fjørtoft [1]. As a flow stabilizes due to increased beta‐effect or decreased basic‐state vorticity gradient, the upper bound on the growth rate decreases to zero. For more stable flows this newly derived bound is tighter than that derived by Høil