Answer: This would be a good time to prune them, early summer. You can reach into the tree and selectively prune off the overly long stems. This will allow some air and sun to reach into the plant while preserving the overall outline. By doing this the plant should stay healthier and will not develop an outer core or shell of foliage concealing bare interior branches. You can also selectively trim in the winter time, however to some extent this will stimulate more vigorous growth in the spring. Here is some pruning information you may find helpful. You may need to cut and paste the complete url into your browser ot make it work correctly.
Here is an illustration/description of how to trim a boxwood by thinning, the principle is the same for holly.
Pruning generally prevents or limits berry formation. The reason is that the berries develop where female flowers bloomed, and the flowers bloom from buds on old wood that grew the year before. If you prune in fall or winter you remove buds, if you prune in spring you remove flowers, if you prune in summer you remove immature berries. Also, the pollen is transported by flying insects moving from a male to a female plant. If the weather is bad when they bloom, if the insects are killed by a pesticide application, if the male plant has been pruned, or if some other untoward event such as a summer drought occurs, there will be no or few berries. I hope this answers your question.
Q&A Library Searching Tips