A factor u of a word w is a cover of w if every position in w lies within some occurrence of u in w. A factor u is a seed of w if it is a cover of a superstring of w. Covers and seeds extend the classical notions of periodicity. We introduce a new notion of α-partial seed, that is, a factor covering as a seed at least α positions in a given word. We use the Cover Suffix Tree, introduced recently in the context of α-partial covers (Kociumaka et al, CPM 2013); an O(n log n)-time algorithm constructing such a tree is known. However it appears that partial seeds are more complicated than partial covers - our algorithms require algebraic manipulations of special functions related to edges of the modified Cover Suffix Tree and the border array. We present an algorithm for computing shortest α-partial seeds that works in O(n) time if the Cover Suffix Tree is already given.