  We propose and numerically validate a new fitting formula that is
sufficiently accurate to model the growth of structure in Horndeski theories of
modified gravity for upcoming Stage IV and V large-scale structure surveys.
Based on an analysis of more than 18,000 Horndeski models and adopting the
popular parameterization of the growth rate $f(z) = \Omega_{M}(z)^{\gamma}$, we
generalize the constant growth index $\gamma$ to a two-parameter
redshift-dependent quantity, $\gamma(z)$, that more accurately fits these
models. We demonstrate that the functional form $\gamma(z)=\gamma_0+\gamma_1z^2
/ (1+z)$ improves the median $\chi^2$ of the fit to viable Horndeski models by
a factor of $\sim40$ relative to that of a constant $\gamma$, and is sufficient
to obtain unbiased results even for precise measurements expected in Stage IV
and V surveys. Finally, we constrain the parameters of the new fitting formula
using current cosmological data.
