Good Morning Scott,

There are many factors that can cause an arborvitae to brown, including improper planting, lack of water, too much water, or too much nitrogen. Your question was addressed on Saturday morning with Joe Boggs on "In the Garden with Ron Wilson", at the 10:26 minute. The Buggy Joe Boggs Report 4/20/2024 - In The Garden with Ron Wilson | iHeart. They discussed the reasons for the bottom browning on an arborvitae tree.  I hope you're able to recognize your problem in their discussion. 

I have also included two other resources that may be of some help.

The Mighty Arborvitae (psu.edu)

Winter Burn – Wisconsin Horticulture

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