  We show that the growth of a unimodular random rooted tree $(T,o)$ of degree
bounded by $d$ always exists, assuming its upper growth passes the critical
threshold $\sqrt{d-1}$. This complements Timar's work who showed the possible
nonexistence of growth below this threshold. The proof goes as follows. By
Benjamini-Lyons-Schramm, we can realize $(T,o)$ as the cluster of the root for
some invariant percolation on the $d$-regular tree. Then we show that for such
a percolation, the limiting exponent with which the lazy random walk returns to
the cluster of its starting point always exists. We develop a new method to get
this, that we call the 2-3-method, as the usual pointwise ergodic theorems do
not seem to work here. We then define and prove the Cohen-Grigorchuk co-growth
formula to the invariant percolation setting. This establishes and expresses
the growth of the cluster from the limiting exponent, assuming we are above the
critical threshold.
