  The phase separation boundary of isotropic t-J ladders is analyzed using
density matrix renormalization group techniques. The complete boundary to phase
separation as a function of J/t and doping is determined for a chain and for
ladders with two, three and four legs. Six-chain ladders have been analyzed at
low hole doping. We use a direct approach in which the phase separation
boundary is determined by measuring the hole density in the part of the system
which contains both electrons and holes. In addition we examine the binding
energy of multi-hole clusters. An extrapolation in the number of legs suggests
that the lowest J/t for phase separation to occur in the two dimensional t-J
model is J/t~1.
