  Weakly nonlinear hexagon convection patterns coupled to mean flow are
investigated within the framework of coupled Ginzburg-Landau equations. The
equations are in particular relevant for non-Boussinesq Rayleigh-B\'enard
convection at low Prandtl numbers. The mean flow is found to (1) affect only
one of the two long-wave phase modes of the hexagons and (2) suppress the
mixing between the two phase modes. As a consequence, for small Prandtl numbers
the transverse and the longitudinal phase instability occur in sufficiently
distinct parameter regimes that they can be studied separately. Through the
formation of penta-hepta defects, they lead to different types of transient
disordered states. The results for the dynamics of the penta-hepta defects shed
light on the persistence of grain boundaries in such disordered states.
