  We study analytically the initial value problem for a self-interacting
(massive) scalar-field on a Reissner-Nordstr\"om spacetime. Following the
no-hair theorem we examine the dynamical physical mechanism by which the
self-interacting (SI) hair decays. We show that the intermediate asymptotic
behaviour of SI perturbations is dominated by an oscillatory inverse power-law
decaying tail. We show that at late-times the decay of a SI hair is slower than
any power-law. We confirm our analytical results by numerical simulations.
