  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.
