The boxwood shrub, a member of the Buxus species, is a staple of many landscapes due to its dense, evergreen foliage and versatility in formal garden design. When these shrubs begin to turn yellow, a condition known as chlorosis, it signals a deeper problem requiring investigation. Yellowing is a common symptom of distress, but it is not a diagnosis in itself, as the cause can range from simple environmental stress to serious biological threats. Successfully restoring the plant’s deep green color depends entirely on accurately identifying the specific underlying issue.
Environmental and Soil Stressors
The most common causes of boxwood yellowing stem from the immediate physical environment surrounding the root system. Boxwoods require well-drained soil and are sensitive to moisture extremes. Overwatering or planting in poorly draining, compacted soil often leads to root rot. This causes roots to suffocate and fail to absorb nutrients, resulting in limp, yellow, or drooping foliage.
Conversely, underwatering or drought stress causes leaves to become dry, crispy, and eventually brown or yellow. Because boxwoods have a shallow root system, they are susceptible to drying out during extended dry periods, especially when newly planted. Consistent moisture is necessary, but the soil must never remain waterlogged.
Soil chemistry also affects plant health, as boxwoods thrive in a slightly acidic to neutral pH, ideally between 6.0 and 7.2. Highly alkaline soil changes the chemical structure, which can “lock up” certain micronutrients, making them unavailable for absorption. This nutrient unavailability, caused by incorrect pH, is a frequent cause of chlorosis and can stunt growth.
Seasonal changes can also cause yellowing, often seen as “winter burn” in late winter or early spring. This occurs when cold winds and frozen ground cause the evergreen foliage to lose moisture faster than the roots can replace it. This results in the leaf tips or entire leaves turning a bronze or reddish-orange color. Damage is usually limited to the exposed parts of the plant and is remedied by pruning out the damaged shoots in the spring.
Specific Nutrient Deficiencies
Once environmental and soil conditions are ruled out, yellowing leaves may indicate a specific elemental shortage, which presents with distinct visual patterns. Nitrogen (N) deficiency typically causes uniform yellowing across the entire leaf surface, starting with the older, lower leaves. Since Nitrogen is a mobile nutrient, the plant redistributes it to new growth, leaving older foliage pale and discolored.
A shortage of Iron (Fe) or Magnesium (Mg) results in interveinal chlorosis, where tissue between the veins turns yellow while the veins remain green. Iron deficiency usually affects the newest growth first and is often a secondary symptom of high soil pH, not a true lack of iron. Correcting this requires lowering the soil pH or applying a chelated iron supplement, which is more readily absorbed.
Magnesium deficiency is often seen as yellowing or bronzing around the edges of older leaves. If a soil test confirms a deficiency, fertilization with a balanced product or specific supplement can help restore leaf color. However, testing the soil first is important, as applying fertilizer to an already stressed plant can cause further root damage.
Identifying Pests and Pathogens
Biological threats like insects and fungal diseases can cause yellowing, often accompanied by physical signs. The Boxwood Leafminer (Monarthropalpus flavus) is a common pest whose larvae feed inside the leaf tissue. This feeding causes characteristic blister-like swellings or bumps on the leaf surface. This internal damage leads to yellow or brown blotches and premature leaf drop.
Boxwood Mites (Eurytetranychus buxi) are tiny pests that feed on the undersides of leaves, resulting in minute, pale dots called stippling on the upper surface. A heavy infestation gives the foliage a pale, yellowed, or bronzed appearance. Fine webbing may sometimes be visible, particularly on the leaf undersides. These pests thrive in hot, dry conditions and produce multiple generations quickly.
Fungal diseases, such as Boxwood Blight (Calonectria pseudonaviculata) or Root Rot (Phytophthora spp.), present serious threats. Boxwood Blight causes dark leaf spots and black streaking on the stems, often resulting in rapid defoliation. Root rot, which is secondary to poor drainage, causes leaves to turn from green to a dull yellow-green or straw color, accompanied by a sudden decline of the entire plant.