Arborvitae, belonging to the genus Thuja, are popular evergreen shrubs valued for their dense foliage and use in screens and hedges. When the vibrant green foliage begins to turn brown or yellow, homeowners often worry the plant is failing. This discoloration can signal a routine biological process or a severe, life-threatening problem requiring intervention. Correctly diagnosing the cause requires a systematic examination to distinguish between natural changes and genuine distress.
Separating Normal Shedding From Serious Stress
The most common cause of browning is “flagging,” or seasonal needle drop, which typically occurs in late summer or fall. This natural process concentrates browning on the oldest foliage located deep inside the canopy near the main trunk. The inner needles are shaded out and shed naturally to improve air circulation and reallocate resources.
This inner browning is uniform across the shrub and is not cause for alarm, as the outer tips and new growth remain healthy. Conversely, true pathological stress almost always begins on the outside of the plant, affecting the branch tips first. If browning starts on the exterior, is widespread during spring or summer, or affects only one side, it signals a significant underlying issue.
Key Visual Indicators of Illness and Infestation
When browning begins on the branch tips, this “tip browning” or “dieback” indicates distress rather than seasonal change. The foliage may turn reddish-brown or ash-gray, often starting at the terminal ends of the branchlets before progressing inward. Widespread, uniform discoloration, such as a dull, bronzed, or yellowed appearance across the exterior canopy, suggests a systemic issue.
Look for thinning foliage or distinct bare patches, which indicate non-viable sections. Inspect the foliage for physical evidence of pests, such as fine, silky webbing that points toward spider mites, or small, spindle-shaped sacs that signal bagworm activity.
Pinpointing the Underlying Problem (Environmental Stress vs. Biological Threats)
Environmental problems are often characterized by uniform damage across an exposed area, such as the entire top or one side of the shrub. Biological threats, conversely, present with more specific physical evidence.
Environmental Stressors
Drought stress is a frequent culprit, causing the tips of branches to brown first as the plant sacrifices outer growth to conserve moisture. Winter burn causes browning on the most exposed side, typically the south or west face. This results from the combination of winter sun and wind desiccating the foliage while the roots are frozen and unable to draw water. Salt damage from de-icing agents used on nearby sidewalks and roads causes browning that is usually concentrated on the lower foliage facing the source of the salt spray.
Biological Threats
Diseases like twig blight (Pestalotiopsis or Phomopsis species) cause distinct tip browning that eventually moves down the branch, sometimes leaving tiny black fungal fruiting structures on the dead tissue. Root rot, which is caused by fungi in overly saturated soil, leads to a sudden and generalized decline of the entire plant, where the foliage turns a dull, lifeless color. Pests like spider mites feed using piercing-sucking mouthparts, causing foliage to yellow or bronze before turning brown. Bagworms strip foliage and leave behind their recognizable, tough, silken bags, leading to noticeable defoliation and bare sections.
How to Determine If the Shrub Is Beyond Saving
The definitive test for determining a woody plant’s viability is the “scratch test,” which assesses the health of the cambium layer beneath the bark. Using a fingernail or a small knife, gently scrape a tiny section of bark off a branch that appears dead. If the tissue immediately underneath is bright green and moist, that section of the plant is still alive and capable of recovery.
If the scraped tissue is brown, dry, or brittle, that part of the shrub is dead. When testing a large branch or the main trunk, continue scratching further down toward the ground until green tissue is found, or determine that the entire plant is compromised. If the trunk shows no signs of life, or if major structural damage like a severely cracked trunk is present, the Arborvitae is likely beyond salvage and should be removed.