The Garside group, as a generalization of braid groups and Artin groups of finite types, is defined as the group of fractions of a Garside monoid. We show that the semidirect product of Garside monoids is a Garside monoid. We use the semidirect product $\mathbb Z\ltimes G^n$ of the infinite cyclic group $\mathbb Z$ and the cartesian product $G^n$ of a Garside group $G$ to study the properties of roots and powers of elements in $G$. The main result is an estimate of the growth of the minimal word-length of powers of elements in Garside groups, when the generating set is the set of simple elements. A direct application is that the set of translation numbers in Garside groups is discrete. It gives an affirmative answer to the question of Gersten and Short for the case of Garside groups. The original question is for biautomatic groups. And we show that the root extraction problem in a Garside group $G$ can be reduced to a conjugacy problem in $\mathbb Z\ltimes G^n$. Using the algorithm for the conjugacy problem in Garside groups, the root extraction problem is decidable for any Garside group.

Title has been changed from "Growth of minimal word-length in Garside groups" To appear in Journal of Algebra