Arborvitae are popular evergreen shrubs and trees, often chosen for their dense foliage and versatility in landscaping. A common concern for many gardeners, however, is when their arborvitae leaves begin to turn black. This blackening is a symptom, not a disease itself, and can indicate various underlying issues affecting the plant’s health. Understanding the potential causes is the first step toward addressing the problem and restoring the plant’s vigor.
Why Arborvitae Leaves Turn Black
Blackening foliage on arborvitae can stem from several distinct problems, ranging from fungal infections to environmental stressors and pest activity. Each cause typically presents with unique characteristics in how the blackening appears and spreads across the plant. Identifying these specific patterns helps in pinpointing the exact issue.
Fungal diseases are a frequent culprit behind black or discolored arborvitae foliage. Phomopsis blight often causes the tips of new growth to turn brown, then black, eventually leading to dieback of entire branches. Cercospora blight, on the other hand, typically affects older, inner foliage first, causing it to turn bronze or dull green before darkening to a sooty black. Kabatinia canker can also manifest as blackening, often accompanied by sunken lesions or cankers on the stems, leading to branch death. These fungal issues are often more prevalent in wet or humid conditions, as moisture facilitates spore germination and spread.
Environmental stressors significantly impact arborvitae health, leading to various forms of foliage discoloration, including blackening. Insufficient water, or drought stress, causes foliage to dry out, turn brown, and sometimes appear scorched or black. Conversely, overwatering or poor soil drainage can lead to root rot, which starves the plant of oxygen and nutrients, resulting in black, decaying foliage. Winter burn, caused by cold winds and sun desiccating the needles when the ground is frozen, can also lead to browned or blackened tips. Furthermore, exposure to de-icing salts or coastal salt spray can cause foliage to turn brown or black due to chemical burn.
Pest infestations can also result in black-looking arborvitae, though often indirectly. Spider mites are tiny pests that feed on plant sap, causing stippling (tiny dots) on the foliage, which can eventually lead to a bronzed or blackened appearance as the plant declines. Fine webbing may also be visible on affected branches. Scale insects, another common pest, excrete a sticky substance called honeydew, which then becomes a substrate for sooty mold. This sooty mold grows as a black, powdery or velvety layer on the leaves and stems, giving the plant a distinctly black appearance.
Finally, arborvitae naturally shed older, inner foliage as part of their growth cycle, which can sometimes be mistaken for a disease. This natural inner shedding typically occurs in the fall and appears as a uniform browning or blackening of the needles deep within the plant, not affecting the outer, new growth or branch tips. This process is normal and helps the plant maintain its health by focusing resources on new growth. It is distinct from issues that cause irregular patterns or affect the outer parts of the plant.
Diagnosing the Specific Issue
Observing specific characteristics of the blackening can help pinpoint the underlying problem. Begin by examining where the blackening is occurring on the plant. Determine if it’s affecting the inner foliage, the tips of branches, or scattered patches across the plant.
Next, look for other accompanying symptoms. Check for the presence of cankers on stems, which might indicate a fungal disease like Kabatinia canker. Inspect the foliage closely for fine webbing, a sign of spider mites, or the presence of small, immobile bumps on stems and needles, characteristic of scale insects. Also, observe if there are any tiny, raised black spots or if the discoloration progresses from bronze to black.
Evaluate the plant’s environment and recent conditions. Consider recent weather patterns, such as prolonged dry spells or excessive rainfall, which could point to drought or overwatering. Assess if the plant is exposed to harsh winter winds or de-icing salts from nearby roads or walkways. Differentiating natural shedding from disease or pest damage is important; natural shedding is gradual, uniform, and affects only the inner foliage, unlike the often irregular or spreading patterns of disease or pest damage.
Treatment and Management Strategies
Once the specific cause of the blackening has been identified, appropriate treatment and management strategies can be implemented. For fungal diseases, promptly prune affected branches to prevent further spread, ensuring to sterilize pruning tools between cuts to avoid contaminating healthy parts of the plant. Improving air circulation around the plant by strategic pruning can also help. In some cases, fungicides may be considered, but their effectiveness often relies on proper timing and application, typically as a preventative measure.
Addressing environmental stress involves adjusting care practices. For drought-stressed plants, provide consistent, deep watering, especially during dry spells. If overwatering or poor drainage is suspected, improve soil drainage around the plant or consider relocating it. To mitigate winter burn, apply anti-desiccants in late fall or use burlap wraps to shield the plant from harsh winds. Protecting arborvitae from salt exposure, such as by building barriers or using salt-free de-icers, can prevent chemical burn.
Pest infestations require targeted approaches for effective control. A strong stream of water can dislodge spider mites, or insecticidal soaps and horticultural oils can be applied according to product instructions. For more severe scale infestations, horticultural oils or specific insecticides may be necessary. Always read and follow product labels carefully to ensure safe and effective application, focusing on thorough coverage of affected areas. Maintaining overall plant health through balanced care can also enhance the plant’s natural resilience.
Preventing Future Problems
Proactive measures are effective in maintaining arborvitae health and preventing the recurrence of black leaves. Proper planting is foundational; ensure arborvitae are planted at the correct depth and with adequate spacing to allow for good air circulation and root development. Consistent and appropriate watering is crucial, especially during dry periods and before winter, to prevent both drought stress and root rot.
Applying a layer of organic mulch around the base of the plant helps retain soil moisture, regulate soil temperature, and suppress weeds. Fertilize arborvitae according to soil test recommendations, providing necessary nutrients for healthy growth without over-fertilizing, which can stress the plant. Routine light pruning to remove any dead or diseased branches and maintain the plant’s natural shape can also contribute to overall vigor. Regularly inspecting your arborvitae for early signs of pests or diseases allows for prompt intervention before problems escalate. Providing winter protection, such as burlap screens, especially for young or exposed plants, can shield them from desiccation and salt damage.