The Emerald Green Arborvitae (Thuja occidentalis ‘Smaragd’) is a favored landscape plant prized for its dense, narrow, and consistent green foliage. When this evergreen turns brown, it causes concern for homeowners who rely on it for privacy screening and year-round color. Understanding the precise cause of the browning is the first step toward recovery. The solution for a watering issue is entirely different from the remedy for a pest infestation, making accurate diagnosis essential. This guide will help diagnose the underlying problem and provide actionable steps to restore your arborvitae’s health.
Non-Infectious Causes: Water Stress and Seasonal Damage
Environmental factors, primarily moisture extremes, are the most frequent reasons for browning. A lack of water, known as drought stress, causes foliage to yellow and then brown as the plant conserves resources. This browning typically starts at the tips and progresses inward. Conversely, overwatering or planting in poorly draining soil is equally damaging because saturated conditions suffocate the roots. This leads to root death and an inability to absorb water and nutrients, often causing browning that appears suddenly and affects the entire plant.
Seasonal changes cause predictable damage, particularly through winter desiccation. This occurs when the frozen ground prevents roots from absorbing water while cold winds or bright winter sun cause the foliage to lose moisture through transpiration. This moisture loss, known as winter burn, usually affects the most exposed sides of the plant, such as south or west-facing surfaces. The browning starts at the branch tips and is confined to the outer canopy.
A natural process called flagging or interior browning is often mistaken for sickness. As the dense outer canopy grows, it blocks sunlight from reaching the older, interior needles, causing them to turn brown and shed. This interior browning is concentrated deep within the plant. The outer foliage remains vibrant green, confirming the plant is simply self-pruning its oldest growth to improve air circulation.
Pests and Pathogens That Turn Foliage Brown
Insects and fungal diseases are biological threats that can cause widespread browning and require specific identification techniques. The Spider Mite is a common pest that feeds by piercing the foliage and sucking out cell contents, causing stippling or bronzing before the needles turn brown. These tiny pests thrive in hot, dry conditions. To check for mites, shake a branch over white paper and look for tiny, moving specks. Fine webbing may also be present near the branch tips, confirming their presence.
The Bagworm is an easily identifiable pest that creates distinctive, spindle-shaped casings made of silk and foliage pieces. As the larvae feed, they defoliate the plant, leading to large patches of brown, dead foliage, especially near the top of the tree.
Fungal pathogens, such as root rot, typically result from prolonged soggy soil conditions. Root rot causes the entire plant to decline rapidly as the roots turn black and mushy, failing to support above-ground growth. This condition is often fatal if not addressed quickly.
Tip blight, caused by Phomopsis or Pestalotiopsis fungi, results in brown or gray tips on new, young growth, often starting at the branch terminals. This disease is common on stressed or overcrowded plants where poor air circulation allows moisture to linger. Identifying these pathogens involves looking for small, dark fungal fruiting bodies, which appear as tiny black dots on the affected brown needles or twigs.
Immediate Care for Stressed Arborvitae
When browning is observed, the first step is proper triage, beginning with pruning out all dead or severely damaged foliage. Use clean, sharp shears to remove brown branches, cutting back to healthy green wood to prevent disease spread. It is important to note that arborvitae does not regenerate new growth from old wood stripped of green needles. Therefore, any branch that is completely brown will remain permanently bare.
The next action is to adjust the watering regimen based on the initial diagnosis. If the foliage is dry and brittle from drought, provide a deep, slow watering to thoroughly soak the root zone. If root rot is suspected due to soft, saturated soil, stop all irrigation immediately to allow the soil to dry out. A light application of a balanced, slow-release fertilizer may be considered in the spring to support recovery, but only after the primary environmental stressor has been corrected.
Long-Term Maintenance for Year-Round Health
Establishing a consistent, preventative care schedule is the most effective way to ensure the long-term health of your Emerald Green Arborvitae.
Watering and Mulching
The plant requires deep, infrequent watering, aiming for about one inch of water per week during the growing season. This deep soaking encourages a resilient root system that can better withstand drought. Proper mulching helps regulate soil temperature, retain moisture, and reduce weed competition. Apply a two-to-three-inch layer of organic mulch around the base of the tree, keeping it pulled back several inches from the main trunk to prevent moisture buildup and disease.
Winter Protection and Inspection
Winter protection is necessary in colder climates to prevent winter burn and desiccation damage. In late autumn, before the ground freezes, give the arborvitae a thorough, deep watering to hydrate the foliage for the dormant season.
Other preventative measures include:
- Installing a burlap screen or wind barrier on the side facing prevailing winds or bright winter sun to reduce moisture loss.
- Consistent annual inspections for pests, especially in early summer.
- Handpicking any bagworm casings upon detection.
- Early detection of spider mite activity before they cause extensive damage.