  We solve a problem posed by Blasco, Bonilla and Grosse-Erdmann in 2010 by
constructing a harmonic function on $\mathbb{R}^N$, that is frequently
hypercyclic with respect to the partial differentiation operator
$\partial/\partial x_k$ and which has a minimal growth rate in terms of the
average $L^2$-norm on spheres of radius $r>0$ as $r \to \infty$.
