It may be worrisome to observe your emerald green arborvitae (Thuja occidentalis) turning yellow. There are a number of reasons why these plants change color. The location of the yellow leaves, timing of yellowing and growing conditions can offer clues about the condition of your plant.
Fall Needle Drop
Just as deciduous trees shed their leaves in fall, many evergreens also drop yellowing leaves at the end of the growing season. This is particularly true of arborvitae and white pine. Older leaves positioned toward the inside of the tree turn yellow and drop, while new growth at the branch tips remains green.
Arborvitae is susceptible to a number of pests, and damage typically shows as yellowing leaves. For example, larvae of leaf miner injure the growth tips, turning them yellow and eventually brown. Phytopthora root rot causes leaves and roots to turn yellow. Different types of blight will result in yellow spots on young leaves or yellowing of older leaves on low branches.
Yellowing leaves during the growing season usually indicate a stressful environment for the plant. Change of leaf color and leaf drop occurs during water stress in both drought and flooded conditions. Transplanting a shrub can also cause stress symptoms.