Boxwood plants are a popular choice for many landscapes, offering year-round green foliage and a classic structure. However, gardeners often encounter the frustrating issue of boxwood leaves turning yellow. This discoloration can signal various underlying problems, from nutrient imbalances to pest infestations or environmental stress. Understanding the specific reasons behind yellowing leaves is the first step toward restoring the health and vibrant appearance of these cherished shrubs.
Why Boxwood Leaves Turn Yellow
Nutrient Deficiencies and Soil pH
Yellowing boxwood leaves can often indicate a lack of essential nutrients. Boxwoods generally prefer a soil pH between 6.5 and 7.2, and deviations from this range can hinder nutrient uptake, even if nutrients are present in the soil.
Iron deficiency, for instance, typically causes new leaves to turn yellow while the veins remain green, a pattern known as interveinal chlorosis. Nitrogen deficiency, in contrast, results in a more uniform yellowing across older leaves, as the plant moves this mobile nutrient to new growth. Magnesium deficiency might also cause yellowing, sometimes with reddish or purplish tints. Addressing these issues often begins with a soil test to determine the exact pH and nutrient levels, allowing for targeted fertilization or pH adjustments.
Watering Issues
Both too much and too little water can cause boxwood leaves to yellow. Overwatering is a common problem, especially in poorly drained or compacted soils, leading to root rot. Symptoms of root rot include yellowing leaves that may curl inward and poor overall growth, as the damaged roots struggle to absorb water and nutrients.
Conversely, underwatering or drought stress also causes yellowing, often accompanied by wilting or dry, brittle leaves. To maintain healthy boxwoods, it is important to water deeply but infrequently, allowing the soil to dry slightly between waterings. Checking the soil moisture a few inches deep before watering can help prevent both extremes.
Pest Infestations
Several common pests can lead to yellowing boxwood foliage. Boxwood leafminers are tiny flies whose larvae tunnel within the leaves, causing blister-like swellings and yellow or brownish discoloration. Infested leaves may also appear smaller or drop prematurely.
Spider mites, another frequent culprit, feed on leaf surfaces, resulting in fine yellow stippling or a grayish, dingy appearance, often accompanied by fine webbing on the undersides of leaves. Nematodes, microscopic worms that feed on boxwood roots, deprive the plant of nutrients and water, leading to general decline, stunted growth, and yellowing or browning leaves. Managing these pests often involves horticultural oils, insecticidal soaps, or in some cases, specific insecticides.
Common Diseases
Boxwoods are susceptible to several diseases that cause leaf yellowing and decline. Boxwood Blight, caused by the fungus Calonectria pseudonaviculata, presents with light or dark brown leaf spots, rapid yellowing, and quick leaf drop. A distinguishing feature is the presence of black streaking or cankers on the stems.
Volutella Blight, another fungal disease, typically appears in early spring, causing leaves to turn light green-yellow, then bronze or straw-colored, often remaining attached to the branches. Pink or salmon-colored fungal fruiting bodies may be visible on infected leaves and stems, especially in moist conditions. Root rot, caused by various fungal pathogens like Phytophthora, results in yellowing leaves, poor growth, and a mushy, discolored root system, often occurring in waterlogged soil.
Environmental Stress
External factors can significantly impact boxwood health and cause yellowing. Winter damage, also known as winter burn, occurs when cold temperatures, wind, and frozen soil prevent the plant from absorbing enough water, leading to yellowing, bronzing, or reddish-brown leaves, particularly in exposed locations. Sun scorch can also cause yellowing or browning, especially on plants suddenly exposed to intense sunlight.
Soil compaction restricts root growth and air circulation, contributing to overall plant stress and yellowing. Transplant shock, experienced by newly planted boxwoods, can also manifest as yellowing leaves due to the stress of relocation and root disturbance. Providing adequate protection from harsh elements, ensuring proper planting, and avoiding soil compaction can help mitigate these stresses.
General Care Practices to Prevent Yellowing
Proactive care is important for preventing boxwood leaves from turning yellow.
- Ensuring proper planting depth is crucial; boxwoods should not be planted too deeply, as this can hinder establishment and lead to decline.
- Consistent mulching with a thin layer, about one inch deep and kept clear of the main stem, helps retain soil moisture and regulate temperature without suffocating roots or encouraging pests.
- Adequate spacing between plants allows for good air circulation, which is important for preventing fungal diseases.
- Regular, balanced fertilization, based on soil test results, provides the necessary nutrients for healthy growth without over-applying, which can cause nutrient burn.
- Avoiding excessive pruning or shearing can also improve air circulation within the plant, reducing susceptibility to diseases.
These consistent maintenance practices promote strong, resilient boxwoods better equipped to resist stressors that lead to yellowing.
When to Seek Professional Help
Consulting a certified arborist, landscaper, or plant pathologist becomes advisable when the cause of yellowing remains unclear despite home remedies. If a problem is widespread, affecting multiple boxwoods, or if the plant’s health continues to decline significantly, professional expertise can provide a more accurate diagnosis. Specialized diagnostic labs can analyze soil and plant tissue to identify specific pathogens, nutrient deficiencies, or pest issues that might be difficult to pinpoint otherwise. Early intervention by a professional can often save affected plants and prevent the spread of problems to other landscape elements.