Hello,

This time of year yellowing is likely due to some drying from exposure to wind and sun, especially branches exposed on the South-southwest sides.

There are a couple insects that can damage leaves and brown sections of shrubs, and voles or other critters can gnaw the base of shrubs causing branches to brown and die.

Winter kill can be a problem when irrigation systems get winterized and the shrubs do not get enough water in the fall before the ground freezes. However, that isn’t a likely problem this winter, as yet, since we have had a wet, relatively warm winter( unless tour shrubs are located in a very dry section of your landscape that doesn’t receive much natural rainfall). Plants often need screening over winter to survive without browning:

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

https://hort.extension.wisc.edu/articles/winter-burn/

 I suggest you have a professional diagnose the shrubs. One way to do this is hire a certified arborist to come out and examine them. This is a professional who has taken training in care, diseases, pests of all woody plants, and passed certification tests. He/she will come on site and give a complete diagnosis and a plant care plan. Find certified arborists by zip code here—-

www.treesaregood.org

A second way is to clip a couple sample branches containing brown and some still green leaves and send them to MSU Plant Diagnostic lab. The form, fee schedule and instructions are here- https://www.canr.msu.edu/pestid/

In the mean time, see if there is a way you can screen the sides exposed to the prevailing winds and sun this winter, such as using cut evergreen boughs loosely  tied up against the sides of the shrubs. Or, deciduous shrub or tree branches with burlap loosely fastened around them. The ground being frozen, you won’t be able to stake the screen material into the ground, as is usually done in the fall. In spring, if it is unusually dry, give them a deep watering when ground is thawed.

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

Once you have a positive diagnosis of the branches, you will know what actions you need to take in spring. Most boxwood will recover from winter burn given time, either by pruning out the brown areas and letting them regrow, or by letting the dormant buds on the branches sprout new leaves.

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