The boxwood (Buxus species) is a staple in landscape design, valued for its dense, evergreen foliage and formal shape. The sight of its leaves turning yellow, a condition known as chlorosis, is the most common concern for boxwood owners. This discoloration is not a disease in itself but a visible symptom of underlying stress. Diagnosing the exact cause requires careful observation, as the issue can range from simple environmental factors to more serious pest infestations or systemic diseases.
Cultural and Environmental Stressors
Boxwoods are particularly sensitive to fluctuations in soil moisture, with both extremes leading to distress. Overwatering saturates the soil, suffocating the roots and preventing them from absorbing necessary oxygen, which often results in a uniform yellowing across the entire plant. Conversely, underwatering causes a general fading of the foliage, potentially progressing to brown tips as the plant attempts to conserve moisture.
Nutrient deficiencies also manifest as chlorosis, with the pattern of yellowing offering specific diagnostic clues. A lack of Nitrogen typically causes pale yellowing in older, lower leaves first, as the plant moves this mobile nutrient to new growth. Magnesium deficiency often appears as bronzing or yellowing specifically around the leaf margins, with the center remaining greener initially. Correcting these deficiencies often begins with professional soil testing to accurately determine which element is lacking.
Soil pH imbalance is a frequent culprit, as boxwoods prefer a slightly acidic to neutral range (ideally between 6.5 and 7.5). When the soil becomes too alkaline, certain essential micronutrients, particularly Iron, are chemically locked up and become unavailable to the plant roots. This results in interveinal chlorosis, where the tissue between the leaf veins turns yellow while the veins themselves remain dark green.
Seasonal changes can also induce yellowing, most notably through winter injury or sun scald. This occurs when harsh winter sun and wind cause water to evaporate from the leaves faster than the frozen roots can replace it. The resulting damage is often seen on the exposed side of the shrub as a pale yellow or bronzed discoloration that may not become fully apparent until spring.
Insect Infestations
The Boxwood Leafminer (Monarthropalpus flavus) is a common pest whose larvae feed internally between the upper and lower leaf surfaces. This internal feeding causes the leaf tissue to blister and eventually turn pale yellow or brown before dropping prematurely. If an infested leaf is held up to the light, the characteristic blotch mines or blistering on the underside are usually visible, confirming the leafminer’s presence.
The Boxwood Mite (Eurytetranychus buxi), a type of spider mite, is another significant pest causing chlorosis. These microscopic arachnids pierce the leaf surface to suck out the plant sap. Their damage is characterized by fine stippling—hundreds of tiny, pale yellow or white dots—that gives the foliage a dull, unhealthy, or grayish cast. Mites thrive in hot, dry conditions and can reproduce quickly, accelerating the visual decline of the shrub.
Fungal and Root Diseases
Phytophthora root rot, caused by several species of water molds, is a major threat that leads to severe chlorosis and dieback. This disease is almost always linked to poor soil drainage or chronic overwatering, as the pathogen thrives in saturated soil conditions. Infected roots become soft, dark, and water-soaked, preventing the plant from absorbing water and nutrients. This causes the foliage to turn a dull, uniform yellow before progressing to a purplish-brown or straw color.
Volutella blight (Pseudonectria buxi) is a fungal stem and leaf disease that can also cause leaf discoloration, particularly on stressed or winter-damaged shrubs. Initial symptoms on the lower branches may include yellowing and reddening of the leaves, which often turn upward and lie close to the stem. The definitive sign of this disease is the appearance of small, salmon-colored, waxy spore masses or fruiting bodies on the underside of the leaves and stems during moist periods.
Boxwood Blight (Calonectria pseudonaviculata) is generally less associated with slow, uniform yellowing. This aggressive disease typically causes rapid, dark leaf spots, black streaking on the stems, and extensive, fast defoliation, differentiating it from the slower decline caused by root rot or Volutella. A thorough examination of the plant’s roots and stems is often necessary to distinguish these systemic diseases from less severe environmental issues.
Remediation and Long-Term Health
Before applying any nutrient amendments, a professional soil test is the most accurate way to confirm a deficiency or pH imbalance. This prevents the misapplication of fertilizers, which can potentially worsen the plant’s condition by causing root burn or nutrient antagonism.
Proper water management is an immediate step that can correct or prevent the most common causes of decline. Boxwoods should be watered deeply but infrequently, allowing the top layer of soil to dry out between waterings to discourage the water molds that cause Phytophthora root rot. Improving the overall drainage of the planting site is paramount for long-term root health.
For confirmed fungal issues like Volutella blight, affected branches must be removed by pruning back to healthy wood, with pruning tools sterilized between each cut to prevent disease spread. Applying a two-to-three-inch layer of organic mulch helps regulate soil temperature and moisture, but it must be kept several inches away from the shrub’s trunk to prevent moisture buildup and potential crown rot. Avoiding overhead watering is also beneficial, as wet foliage creates an ideal environment for fungal spore germination.