The common reasons why arborvitae are turning brown include water stress (both too little and sometimes overwatering), environmental stressors like drying winds and high temperatures, winter burn due to freezing temperatures and lack of water uptake, pest infestations (such as spider mites and bagworms), fungal diseases, root or trunk damage, transplant shock from recent planting, and overcrowding that limits sunlight and airflow. More specifically:

• Too little water or drought stress causes browning because the plant cannot maintain hydration, especially in dry or hot conditions.
• Winter burn happens when the ground is frozen, preventing roots from replacing moisture lost through the leaves exposed to sun and wind.
• Pests suck sap from arborvitae leaves, weakening and browning the foliage.
• Root damage or transplant shock disrupts the tree's water and nutrient uptake, causing browning.
• Inside branches may naturally brown and shed due to shading from dense outer foliage.
• Overcrowding and insufficient sunlight lead to unhealthy growth and browning.

Prevention and care advice includes regular watering (about 1 inch per week), protecting from harsh winter winds and sun with burlap wraps, mulching to retain soil moisture, avoiding root and trunk injury, ensuring proper planting spacing and sunlight, and monitoring and treating for pests or diseases as needed. Pruning should be delayed until it is clear which branches can recover from browning, such as after winter burn.