pruning a long established holly hedge
Have cleared a 20 ft run of ivy for 8 holly trees (18 ft high). Not a lot of foliage for the first 8 ft. I need to thicken the trees to return a 50 year old hedge.
Sounds like you might be talking about a rejuvenation pruning. This article should tell you more: https://www.gardeningknowhow.com/ornamental/shrubs/holly/trimming-holly-bushes-how-to-prune-holly-bushes.htm
If this seems too drastic to use on all the bushes at once, you can prune just a few branches at a time.