  We show that a graph manifold which is a Z-homology 3-sphere not homeomorphic
to either the 3-sphere or the Poincar\'e homology 3-sphere admits a horizontal
foliation. This combines with known results to show that the conditions of not
being an L-space, of having a left-orderable fundamental group, and of
admitting a co-oriented taut foliation, are equivalent for graph manifold
Z-homology 3-spheres.
