The American holly (Ilex opaca) flowers in the spring on old wood, meaning on branches produced in the previous year (or possibly, earlier). It is generally recommended that hollies should be pruned in the winter when the tree is dormant. Thus, if the tree is pruned too vigorously then it will not produce many flowers and berries the following year. Therefore, as a general rule, hollies should be pruned as little as possible. Another point to note is that young holly trees will not flower until they are three to five years old.

Courtesy of NYBG Plant Information Service.