Arborvitae, a popular evergreen (Thuja), is a common choice for hedges and privacy screens due to its dense, fast-growing foliage. While generally robust, these trees frequently develop brown patches or experience dieback, causing concern for homeowners. This decline is usually a symptom of an underlying problem that has been developing over time. Successfully reversing this process depends entirely on accurately identifying the root cause of the discoloration and stress before applying treatment.
Diagnosing Common Arborvitae Issues
Determining the source of arborvitae browning requires distinguishing between environmental stress, insect activity, and fungal disease, as the visual symptoms can often overlap. Environmental stress often appears as browning starting from the base of the plant and moving upward, or on the side exposed to harsh conditions. This pattern can be a sign of root rot from excessive moisture, where the roots are unable to function, or it might be related to winter injury, where foliage loses moisture faster than the frozen roots can replenish it.
Fungal infections often exhibit specific patterns upon closer examination. Tip blights, caused by pathogens like Pestalotiopsis or Phomopsis, typically begin at the ends of the branchlets, turning the tips tan or brown before moving inward. A distinguishing feature is the presence of tiny, black, pimple-like fruiting structures (pycnidia) that appear on the dead tissue. In contrast, Cercospora blight usually attacks the older, inner foliage first, leaving the new growth at the tips temporarily green.
Pest damage presents differently, particularly with insects like bagworms or spider mites. Bagworms create characteristic, silk-spun bags made of foliage that hang from the branches, causing localized defoliation where they feed. Spider mites cause a stippling or bronzing of the foliage, often accompanied by fine webbing on the inner branches. Proper identification is important, as treating a fungal blight with an insecticide, or vice versa, will not resolve the issue and may further stress the tree.
Correcting Environmental Stressors
Cultural practices are frequently the cause of arborvitae decline, and correcting these conditions is often the first non-chemical treatment step. Improper watering is a common culprit, as these plants thrive in consistently moist but well-draining soil, suffering from both drought and excessive wetness. To alleviate this, adopt a deep, infrequent watering schedule that encourages deep root growth. It is also beneficial to avoid overhead irrigation, which keeps the foliage wet and creates an ideal environment for fungal spores to germinate.
Soil quality and drainage should be addressed if waterlogging is suspected, particularly where browning starts from the bottom up. Arborvitae planted in heavy clay or low-lying areas may require improved drainage through the addition of organic matter or by slightly elevating the planting area. When applying mulch, maintain a layer approximately three inches deep, but ensure it is pulled back several inches from the main trunk flare to prevent moisture from encouraging decay at the base of the tree.
Mitigating physical and environmental injuries is important for plant health. Winter burn, caused by desiccating winds and sun exposure while the ground is frozen, can be reduced by ensuring the tree is thoroughly watered late in the fall before the first hard freeze. In areas exposed to road salt, physical barriers or switching to a less toxic de-icing agent can prevent chemical burn. Addressing these underlying stressors significantly increases the arborvitae’s natural defense against opportunistic diseases.
Targeted Treatment for Fungal Diseases
Active fungal infections require a two-part approach involving physical removal of diseased material and protective chemical application. For blights such as Pestalotiopsis or Kabatina, the immediate step is sanitary pruning to remove all visible infected tissue. When pruning, cuts should be made several inches into the healthy wood below the visible point of infection to ensure the entire fungal colony is removed.
It is necessary to sterilize pruning tools between each cut, or at least between plants, using a solution of bleach or a commercial disinfectant. This prevents the accidental transfer of fungal spores to healthy parts of the plant or to nearby arborvitae. All pruned material must be promptly removed from the site and discarded, as leaving it allows fungal spores to overwinter and reinfect the tree.
Chemical treatments are generally preventative, focusing on protecting new, susceptible growth from infection, rather than curing damaged tissue. Fungicides containing active ingredients such as copper hydroxide, mancozeb, or chlorothalonil are recommended for controlling blights. These protectants should be applied in the spring when new growth emerges and conditions are warm and wet, which is when fungal spores are most active. Multiple applications, following the product label’s specified intervals, may be necessary throughout the spring and early summer to maintain coverage.
For root rot diseases, often caused by Phytophthora species, chemical applications are rarely effective for the homeowner. The only successful approach is to immediately correct the drainage and soil moisture issues detailed previously. If the root system is severely compromised, the plant may not be salvageable, even with cultural adjustments.
Preventing Future Disease
Long-term management focuses on creating an environment that minimizes the opportunity for fungal pathogens to establish. Proper spacing at planting is important, especially when establishing a privacy hedge, to ensure adequate air circulation between plants. Dense foliage and poor airflow allow moisture to linger, fostering the humid conditions that fungal spores need to germinate and spread.
Sanitation of the area around the arborvitae is an ongoing maintenance practice that reduces the fungal inoculum for the following season. Raking up and disposing of fallen needles and debris, particularly in the fall, removes fungal spores that would otherwise overwinter and initiate new infections. Monitoring the plants regularly for the first signs of browning allows for early, localized pruning to remove small infections before they become widespread.
Maintaining overall plant vigor through proper cultural practices, including appropriate fertilization if a nutrient deficiency is confirmed, helps the arborvitae resist disease naturally. A healthy, non-stressed tree is less susceptible to the opportunistic fungal pathogens that thrive on plants weakened by environmental factors. By consistently applying these preventative measures, the arborvitae can maintain its dense, green appearance for many years.