Let T be a rooted tree, and V(T) its set of vertices. A subset X of V(T) is called an infima closed set of T if for any two vertices u, v is an element of X, the first common ancestor of u and v is also in X. This paper determines the trees with minimum number of infima closed sets among all rooted trees of given order, thereby answering a question of Klazar. It is shown that these trees are essentially complete binary trees, with the exception of vertices at the last levels. Moreover, an asymptotic estimate for the minimum number of infima closed sets in a tree with n vertices is also provided.