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The best time to prune boxwoods is in the late fall, early winter or early spring. By pruning at these times, you are not encouraging the shrub to put out new growth, which will not have time to mature and handle the coming winter temperatures.

Boxwoods do experience dieback in severe winters. Prune out dead growth or broken branches throughout the year. Light pruning/or shearing (to shape the shrub) can be done in spring and early summer.

Here are some references in helping you in pruning your boxwoods:

https://www.mastergardenersmecklenburg.org/pruning-boxwoods.html

https://www.canr.msu.edu/news/tips_on_tree_and_shrub_pruning

https://www.udel.edu/academics/colleges/canr/cooperative-extension/fact-sheets/pruning-woody-plants/

If you must shape them now, be prepared to trim any dead tips that might occur on the shrub, from any winter damage. Evergreens continue to need an 1” of water per week. So if we have a dry fall, remember to supplement with additional water, up until the ground freezes (in Michigan that can be up to late October). Evergreens never go completely dormant, so keeping them fully hydrated helps reduce winter dieback and maintains their health.