Hello,

There are many factors that cause an arborvitae to brown, including improper planting, lack of water, too much water, environmental changes that could affect drainage and lead to root damage, insects, diseases and winter burn.

How you water may have an impact. If an above ground sprinkler hits the branches of the tree, they may break the branches and an attack by an opportunistic fungus such as Phomopsis twig blight. Read more about Arborvitae diseases here: https://apps.extension.umn.edu/garden/diagnose/plant/evergreen/arborvitae/patchyyellowtobrowndiscoloration.html

And here: https://ag.umass.edu/landscape/fact-sheets/arborvitae-needle-blight#:~:text=The%20best%20course%20of%20action,overwinter%20in%20the%20tree%20canopy.

And here: https://pnwhandbooks.org/plantdisease/host-disease/arborvitae-thuja-occidentalis-root-rot

Is the damage on the south side of the tree? If not protected, winter winds can be drying to the foliage that may result in browning. Read more here: https://hort.extension.wisc.edu/articles/winter-burn/

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

Insects such as spider mites may cause damage to arborvitae. Check for mites by placing a white paper under a branch and give it a shake. The moving black dots that fall to the paper are mites. Read more here: https://extension.psu.edu/spruce-spider-mite

Attached is another article about stress related evergreen dieback: https://www.extension.purdue.edu/extmedia/id/id-477-w.pdf

Finally, for a definite diagnosis, you can submit a sample to MSU Extension’s Plant and Pest Diagnostic Lab: https://www.canr.msu.edu/pestid/ or you could contact a certified arborist.

To find an arborist in your area, go to www.treesaregood.org, the website of the International Society of Arboriculture.

Thank you for contacting Ask Extension.