Several experiments in the near future will test dark energy through its effects on the linear growth of matter perturbations . It is therefore important to find simple and at the same time general parametrizations of the linear growth rate . We show that a simple fitting formula that generalizes previous expressions reproduces the growth function in models that allow for a growth faster than standard , as for instance in scalar-tensor models . We use data from galaxy and Lyman- \alpha power spectra to constrain the linear growth rate . We find \gamma = 0.6 { } _ { -0.3 } ^ { +0.4 } for the growth rate index and \eta = 0.0 _ { -0.2 } ^ { +0.3 } for the additional growth parameter we introduce .