Arborvitae (Thuja species) are highly valued evergreens, frequently planted as lush, fast-growing hedges and privacy screens. Despite their reputation for resilience, arborvitae browning is a common issue for homeowners. While the sight of brown foliage can be alarming, it is often a sign of environmental stress or pest activity, and the plant is salvageable if the underlying problem is identified and addressed promptly.
Non-Infectious Causes of Browning
The most frequent causes of browning in arborvitae are environmental or cultural stressors that interfere with the plant’s ability to manage water. A common occurrence is winter injury, often called desiccation or windburn, which appears when the foliage loses moisture through transpiration on sunny or windy winter days. This moisture cannot be replaced because the ground is frozen, locking water away from the roots. This lack of water causes the foliage tips to turn brown and die back.
Drought stress is a significant factor, particularly during hot, dry summer periods when the air pulls water from the leaves faster than the roots can absorb it. The resulting lack of hydration causes the foliage to turn brown as a survival mechanism. Conversely, overwatering or poor soil drainage can also lead to browning by suffocating the roots, preventing them from taking up necessary water and nutrients.
Cultural issues related to planting and soil chemistry can also cause foliage discoloration. Improper planting depth, especially planting too deep, leads to root stress and suffocation, inhibiting water uptake. Fertilizer burn occurs when excessive amounts of fertilizer, particularly those high in salts, are applied. This effectively draws moisture out of the roots and foliage, resulting in brown or scorched tips. The symptoms are typically distributed evenly across the canopy.
Recognizing Specific Pest Infestations
When browning is not caused by environmental factors, it often points to feeding damage from insect pests. The spider mite, a tiny arachnid, is a common culprit that uses needle-like mouthparts to remove the contents of foliage cells. This feeding causes a fine, yellowish speckling, known as stippling, which results in a bronze or dull discoloration across the leaves. In severe infestations, spider mites spin fine, silk-like webbing visible on the branchlets and foliage.
The bagworm is a recognizable pest, a caterpillar that constructs a spindle-shaped silk casing covered with small pieces of leaves. These protective bags hang from the branches and can be mistaken for pine cones until closer inspection reveals the feeding larvae inside. Bagworms cause localized defoliation, where entire sections of the plant turn brown and die, often appearing as patches of stripped branches.
Steps for Diagnosis and Recovery
Diagnosing the cause of arborvitae browning requires a systematic inspection. First, check the soil moisture by probing several inches below the surface; dry soil suggests drought stress, while soggy soil indicates drainage problems or overwatering. Next, check for pests, looking for the fine webbing of spider mites or the distinctive hanging bags of bagworms. A simple test for mites is to shake a suspected branch over a white piece of paper and watch for tiny specks that begin to crawl.
Recovery begins by addressing the identified cause. If the issue is drought, implement a deep, infrequent watering schedule, aiming to soak the soil to a depth of at least a foot. Proper pruning is necessary for foliage that has already turned completely brown, as this dead material will not return to green. Prune dead branches back to healthy green wood or to the main trunk, being careful not to cut into the old wood beyond the live foliage.
If pest activity is confirmed, apply appropriate treatments. Use a specialized miticide for spider mites, as general insecticides often fail to control them. Bagworms can be manually removed and destroyed, or a targeted insecticide can be applied when larvae are actively feeding in late spring or early summer. Consistent monitoring and adjusting cultural practices, like ensuring well-draining soil, provide the best chance for recovery.