Arborvitae (Thuja species) are valued in landscapes for providing dense, year-round greenery. The sudden appearance of brown spots signals distress that requires timely diagnosis and action. Understanding the specific reasons behind the discoloration is the first step toward restoring the tree’s health. This guide will help determine if the browning is a natural process or a sign of a deeper underlying issue, and what steps to take next.
Seasonal Needle Drop Versus True Damage
The most common and least concerning cause of browning is a natural process called seasonal needle drop or “flagging.” Arborvitae shed their oldest foliage to prepare for winter and conserve resources. This shedding typically occurs in late summer or fall, causing the inner needles closest to the trunk to turn yellow or brown and eventually drop off.
This process is normal and generally affects the interior of the plant, while the outer canopy remains green. If the browning is uniform throughout the tree’s interior, and the branch tips are still green, the plant is likely undergoing this routine cycle. True damage, by contrast, typically begins on the tips of the branches or appears randomly across the plant’s exterior, often progressing inward.
Environmental Stressors Leading to Browning
Browning outside the natural shedding cycle often points to abiotic, or non-biological, environmental stress factors. These factors prevent the tree from absorbing or retaining adequate moisture, which is reflected in the foliage. One cause is desiccation, commonly known as winter burn, which happens when the ground is frozen and roots cannot take up water. On sunny or windy winter days, the foliage loses moisture through transpiration, leading to browning on the exposed sides of the tree, typically the south or west.
Water stress is a frequent culprit, as arborvitae are shallow-rooted and require consistent soil moisture. Both underwatering (drought stress) and overwatering (which can lead to root rot) disrupt the tree’s ability to transport nutrients and water. Root rot symptoms mimic drought, because the damaged roots cannot function properly, leading to browning and dieback.
External chemical factors also cause localized browning, particularly in trees near roads or walkways. De-icing salts cause salt damage when absorbed by the roots, interfering with water uptake, or when sprayed onto the foliage, causing tip burn. Improper application of fertilizer can also cause chemical burn, stressing the roots and leading to discoloration.
Pests and Fungal Pathogens
When environmental factors are ruled out, the browning may be caused by biological organisms like insects or fungi. Bagworms (Thyridopteryx ephemeraeformis) are a common pest whose larvae construct distinctive, cone-shaped bags from silk and foliage pieces. These caterpillars feed heavily on the needles, causing defoliation and browning that can lead to entire branches dying back.
Another frequent pest is the spider mite, tiny arachnids that feed by sucking the cell contents out of the needles. Damage from mites first appears as a dull green or stippled yellowing that progresses to browning, often accompanied by fine webbing. To check for mites, shake a suspected branch over a piece of white paper; the pests will appear as tiny, moving specks.
Fungal infections, such as Tip Blight caused by pathogens like Phomopsis or Kabatina, also result in brown spots and dieback. These blights typically infect young, succulent growth, causing the branch tips to turn light green, then reddish-brown, progressing inward. This disease is favored by wet conditions and often starts at the branch tips, sometimes leaving a gray band or lesion where the healthy tissue meets the blighted tissue.
Immediate Treatment and Long-Term Recovery
Addressing the browning requires immediate action based on the identified cause, starting with removing the dead material. Pruning out all visibly brown foliage should be done by cutting back to healthy green wood. This helps improve air circulation and removes potential sources of fungal spores or pests. Tools should be sterilized with a bleach solution between cuts to prevent the accidental spread of pathogens.
For pest management, bagworms can be controlled manually by hand-picking and destroying the bags before the larvae hatch in early summer. Chemical control for bagworms and spider mites involves using targeted products. Mites are typically treated with miticides or horticultural oil, sometimes requiring multiple applications due to their life cycle. Fungal blights are managed by improving air circulation and may require a fungicide, which is most effective when applied preventatively to protect new growth.
Long-term recovery and prevention rely on cultural adjustments to minimize plant stress. Ensure the arborvitae receives consistent deep watering, aiming for soil that is moist but never waterlogged. Applying a layer of mulch around the base helps conserve soil moisture and moderate root temperatures. To prevent future winter burn, especially on exposed trees, apply a deep watering in late fall before the ground freezes. Consider using burlap screens to shield the foliage from harsh winter sun and wind.