Boxwood plants are popular for their evergreen foliage and dense growth. Leaf loss can be concerning, but understanding its cause is key to restoring plant health. Various factors, including environmental conditions, pests, and diseases, contribute to foliage decline. Identifying the exact cause is important for effective intervention.
Environmental Factors Causing Leaf Loss
Environmental stressors often cause boxwood leaf loss. Both insufficient and excessive watering lead to symptoms. Drought stress causes leaves to turn brown and crispy, as the plant cannot absorb enough moisture. Conversely, overwatering, especially in poorly draining soils, causes root rot, leading to yellowing and dropping leaves due to oxygen deprivation. Checking soil moisture deeply helps determine if water levels are appropriate.
Temperature extremes also affect leaf retention. Winter burn, a common issue, results in browning on sides exposed to cold winds and winter sun. This occurs when moisture evaporates from leaves faster than the plant replenishes it from frozen soil. Sudden temperature drops can cause frost damage, leading to rapid blackening or browning of leaves.
Soil conditions and nutrient imbalances also contribute to leaf loss. Compacted soil restricts root growth and nutrient uptake, leading to overall decline. Incorrect soil pH (ideally 6.5-7.2) prevents nutrient absorption, even if nutrients are present. Deficiencies in essential nutrients like nitrogen cause light-colored or yellowish foliage, while a lack of magnesium contributes to discoloration and thinning.
Pest Infestations Leading to Leaf Drop
Insect pests significantly impact boxwood health, often causing leaf drop and disfigurement. The boxwood leafminer, a small fly, causes damage when its larvae feed inside leaves. This feeding creates characteristic blisters or mines on foliage, which may appear yellow-spotted before turning brown and leading to premature leaf drop. The larvae weaken the plant, making foliage appear weak.
The boxwood psyllid primarily affects new growth. Its feeding causes tender new leaves to cup or curl, often appearing distorted. Small, light green nymphs, sometimes covered in a white, waxy residue, may be found inside these cupped leaves. While damage is mainly aesthetic, severe infestations weaken leaves, leading to eventual drop.
Spider mites, tiny arachnids, also infest boxwoods. Their feeding results in a speckled or stippled appearance on leaves, which can then turn yellow or bronze. In severe cases, fine webbing may be visible, and significant leaf drop can occur. These mites are active in warm, dry conditions and cause rapid decline if not addressed.
Diseases Affecting Boxwood Foliage
Several fungal diseases cause significant leaf loss and decline. Boxwood blight, caused by Calonectria pseudonaviculata, is a severe fungal disease. It leads to dark spots on leaves, often with darker borders, which rapidly expand and cause leaves to turn brown or yellow and drop. A distinguishing symptom is narrow black streaks or cankers on green stems. This disease spreads quickly through splash dispersal by rain or irrigation, or by contaminated tools or infected plants.
Volutella blight, also known as Pseudonectria canker, typically affects stressed or damaged plants. Symptoms often appear in early spring, with leaves on affected branches turning light green to yellow, then red or bronze, and eventually straw-colored. A key characteristic differentiating it from boxwood blight is that dead leaves often cup upward and remain attached to branches. Salmon-colored fungal spores are visible on the undersides of affected leaves and stems during high humidity.
Macrophoma leaf spot, caused by Dothiorella candollei, is often associated with stressed or winter-injured boxwoods. This fungus produces tiny black, raised dots (pycnidia) on the upper surface of dead or dying straw-colored leaves. While a weaker pathogen that typically attacks already weakened plants, it contributes to discoloration and premature shedding of infected foliage. These small black dots are a diagnostic feature.
Preventative Care and Recovery Strategies
Maintaining boxwood health begins with proper planting and cultural practices. Ensure boxwoods are planted in well-drained soil, as they do not tolerate consistently wet conditions. Avoid planting them too deeply. Deep and infrequent watering is beneficial, encouraging deeper root growth while allowing soil to dry slightly between waterings. Watering at the base of the plant rather than overhead minimizes leaf wetness, which reduces the risk of fungal diseases.
Apply a light layer of organic mulch around the base of the plant, keeping it a few inches from the main stem, to regulate soil temperature and moisture. Balanced fertilization supports robust growth, but avoid over-fertilization due to boxwood’s shallow root system. Pruning by thinning branches improves air circulation within the canopy, deterring fungal issues, unlike shearing which creates a dense outer layer.
Sanitation practices are important for preventing disease and pest spread. Promptly remove and dispose of fallen leaves and infected plant debris to reduce fungal inoculum. If replanting, consider selecting disease-resistant boxwood varieties to minimize future problems. Regular inspection allows for early detection of pests or diseases; washing off minor pest infestations with a strong stream of water or pruning out small infected branches often mitigates issues.