  In this paper we investigate the past evolution of an anisotropic Bianchi I
universe in $R+R^2$ gravity. Using the dynamical system approach we show that
there exists a new two-parameters set of solutions that includes both an
isotropic "false radiation" solution and an anisotropic generalized Kasner
solution, which is stable. We derive the analytic behaviour of the shear from a
specific property of $f(R)$ gravity and the analytic asymptotic form of the
Ricci scalar when approaching the initial singularity. Finally we numerically
checked our results.
