Answer: To keep hollies thick and dense and looking nice, they should be pruned back every year or two. I use hedge shears to lightly go over the entire plant, removing from two to six inches. Now that yours have turned into trees I don't think you can encourage new branches lower down on the main stem, but you can help the foliage thicken by shearing the tips of the branches off. You can also reduce the height of the trees by about 12". You'll end up with neat and trim lollipop shapes if you prune in this manner. Best wishes with your hollies.
Q&A Library Searching Tips