The situation with your tree may be as simple an issue as underwatering. However, there are a host of other common causes of browning of the branches of an Arborvitae.

Understanding all the causes can help you take the right steps to restore its health. Following are some common culprits:

• Water Stress: Both overwatering and underwatering can cause browning. The roots need the right balance of moisture.

• Pests: Insects like spider mites can damage foliage and lead to browning.

• Diseases: Fungal infections can affect the tree’s health. Look for signs of rot or spots.

• Environmental Factors: Extreme heat, cold, or wind can stress the plant, leading to discoloration.

Watering Issues
Water stress is one of the leading causes of browning. Arborvitae trees prefer well-drained soil but also requires consistent moisture.

Signs of Overwatering
If your tree gets too much water, the roots can suffocate and decay. Here are signs of overwatering:

• Yellowing leaves that fall off
• Soft or mushy bark
• Visible mold or fungus on the soil

Signs of Underwatering
On the flip side, underwatering can lead to drought stress. Recognizing these signs early is crucial:

• Brittle leaves that curl or dry out
• Brown tips on foliage
• Soil pulling away from the edges of the pot or ground

Pest Infestation
Pests are another common issue that can lead to browning. Some pests that affect Arborvitae include:

• Spider Mites: These tiny creatures suck sap from the leaves, causing discoloration.
• Scale Insects: They appear as small bumps on the bark and can weaken the overall health.
• Bagworms: These caterpillars can defoliate branches, leading to browning.

Diseases and Fungal Infections
Fungal infections can be a significant problem for Arborvitae. If you notice browning, check for signs of disease:

• Cankers: These are dead patches on branches caused by fungal pathogens.
• Root Rot: Often caused by overwatering, leading to unhealthy roots.
• Leaf Blight: Fungal infections that cause leaf drop and browning.

Environmental Factors
Environmental stressors can also impact the health of your Arborvitae. Consider the following factors:

• Wind: Strong winds can dry out foliage and break branches.
• Temperature Fluctuations: Sudden changes can shock the plant, leading to browning.
• Sun Exposure: Too much direct sunlight can scorch leaves.

How to Fix The Browning

Correct Watering Practices
To avoid watering issues, follow these tips:

• Check soil moisture before watering. The top inch should be dry.
• Water deeply but infrequently. This encourages deep root growth.
• Use mulch to retain soil moisture and regulate temperature.

Pest Control Methods
If pests are responsible for browning, take these steps:

• Inspect your tree regularly for signs of insects.
• Use insecticidal soap or neem oil for control.
• Introduce beneficial insects, like ladybugs, to manage pest populations.

Treating Diseases
For fungal infections, act quickly to minimize damage:

• Remove affected branches to prevent spreading.
• Apply fungicides as recommended on the product label.
• Avoid overhead watering to reduce moisture on foliage.

Mitigating Environmental Stress
Taking care of environmental conditions can go a long way. Consider these tips:

• Provide windbreaks to protect from harsh winds.
• Mulch around the base for temperature regulation.
• Consider relocating the tree if it is not thriving in its current spot.

Possible helpful links:
https://www.montana.edu/extension/Full_HTML_Pubs/a-guide-to-pests-problems-and-identification-of-ornamental-shrubs-and-trees-in-montana/plant-id/arborvitae.html

https://homegarden.cahnr.uconn.edu/factsheets/arborvitae/

https://extension.oregonstate.edu/news/arborvitae-stands-tall-easy-care-hedge

You may benefit from consulting with a Certified Arborist for an assessment of your tree who would be able to make an on-site review of the tree and possibly recommend appropriate action. A Certified Arborist is recommended because they must pass a test and must acquire continuing education. You can locate such individuals at http://www.treesaregood.org/findanarborist/findanarborist.aspx