Holly trees normally develop a pleasing and roughly symmetrical shape all on their own. However, should you require a clipped shrub look, you can trim the bushes at any time from spring through late summer. Trimming in the fall may stimulate fresh growth that will not have time to harden sufficiently before winter. The nicest result is achieved by individually pruning out selected branches. You will notice over time that the plant branches and grows denser at the site of each cut, so trimming actually makes it grow thicker.