Hello Sandra,

Thanks for reaching out to UVM Extension Master Gardener Helpline.

As a general rule you should never take more than a third of a tree out when pruning. If you are planning to take that much off of the holly, you would ideally wait until late winter or early spring – May would be right on the edge there, but it's probably better than doing so now which could risk stressing the plant and winter injury. If the tree is very overgrown and you plan to take out more than a third of it, you'd be best doing so over the course of a few growing seasons.

That said, the other rule of pruning I sometimes ascribe to is "prune when the pruners are in your hand." If you're able to make a few strategic, larger cuts now or in December, that should be fine. A lot of people trim or lightly prune in December to use the branches around the holidays.

Start with the crossing, dead or diseased branches then start thinking about the overall shape you'd like to achieve for some larger cuts.

I've included a few pruning resources below.

Hope that helps!

Resources:

• https://extension.umn.edu/planting-and-growing-guides/pruning-trees-and-shrubs
  
  
• https://extension.okstate.edu/fact-sheets/pruning-ornamental-trees-shrubs-and-vines-2.html
  
  
• https://www.udel.edu/academics/colleges/canr/cooperative-extension/fact-sheets/pruning-evergreens/#:~:text=When%20pruning%20American%20holly%2C%20always,pruning%20during%20the%20growing%20season.