The sight of a cedar tree turning brown can be alarming, especially since these evergreens are valued for their consistent color and privacy screening. This discoloration signals a response to stress, ranging from a normal process to a serious health issue. Understanding the specific pattern of the browning is the first step in diagnosing the problem, as the location and timing help differentiate between a natural cycle and a threat from the environment, pests, or disease.
Normal Seasonal Needle Drop
Not all browning is cause for concern. Cedars, particularly the common Thuja species (arborvitae), regularly shed their oldest inner foliage in a process called “flagging.” This occurs primarily in late summer or early fall as the tree prepares for winter, conserving energy by dropping needles that are no longer efficient. The browning is concentrated on the older needles deep within the canopy, closer to the trunk, where sunlight is limited.
This natural shedding directs the tree’s resources toward the newest, outermost growth. The change involves inner needles turning yellow or orange-brown before dropping, leaving the branch tips green. If the browning is restricted to these older, shaded needles and happens predictably in the autumn, it is a healthy sign of growth regulation. The process is often more pronounced following a year of abundant growth or environmental stress like drought.
Environmental and Cultural Stress Factors
When browning occurs outside of the normal seasonal cycle or affects the outer growth, the cause is often related to the tree’s environment and care practices. Water management is a frequent factor, as cedars possess shallow root systems sensitive to both dry and overly wet conditions. Drought stress causes the tree to shed needles rapidly to reduce transpiration, and this browning often begins at the tips of the outer branches and progresses inward.
Conversely, excessive moisture from overwatering or poor soil drainage can be damaging, leading to a breakdown of root function due to a lack of oxygen. Saturated soil promotes root rot, which appears as random browning patches across the tree. Other abiotic stressors include winter injury, where cold, dry winds pull moisture from the foliage faster than frozen roots can replenish it, resulting in browning on the exposed side. Chemical damage from de-icing road salts or herbicide runoff can also cause needle browning, often seen near the base or edges facing a contaminated source.
Common Pests and Fungal Infections
Browning that spreads rapidly or displays unique visual characteristics may point to an infestation or disease. Spider mites are common pests that feed on the foliage, causing the needles to take on a stippled or bronze appearance. Their presence is often confirmed by fine, silky webbing visible on the branchlets. Bagworms pose a threat as their larvae consume the foliage, creating distinctive, spindle-shaped silk bags that hang from the branches, resulting in localized dieback.
Fungal pathogens also cause damage, often indicating underlying cultural problems like poor air circulation or excessive moisture. Tip blights, such as those caused by Didymascella thujina, manifest as small, circular black or brown fruiting bodies on the affected needles, leading to browning and premature needle drop. Root rots, including Phytophthora and Armillaria, are a consequence of consistently waterlogged soil, causing branches to turn brown and die. This is sometimes accompanied by white, fan-like fungal growth beneath the bark at the tree’s base.
Diagnostic Steps and Treatment Options
Determining the cause of browning requires a systematic approach that begins with careful observation. Check the soil moisture by digging a few inches down; if the soil is dry, deep, slow watering is necessary to address drought stress. Soggy soil indicates a drainage issue that requires aeration or reduced watering. To check for spider mites, hold a white sheet of paper beneath a branch and shake it; tiny, slow-moving specks confirm an infestation treatable with horticultural oil or insecticidal soap spray.
If the browning is not seasonal or water-related, inspect the branches for signs of pests or disease, such as bagworm silk bags or fungal blight dots. Pruning affected branches helps remove sources of infection and improves air circulation within the canopy, which is a practical step against many fungal issues. For severe, rapidly spreading browning or suspected root rot, consulting a certified arborist is advisable. Systemic issues like bark beetle infestation or advanced root rot often require professional diagnosis and specialized treatments, such as targeted fungicide applications or, in extreme cases, tree removal.