Boxwood (Buxus species) is an evergreen shrub prized for its dense, vibrant green foliage that serves as a landscape foundation. When this green shifts to orange, bronze, or reddish-brown, it signals that the plant is under environmental or biological stress. This discoloration is not natural aging. Understanding the specific cause is the first step toward correcting the issue and restoring the shrub’s deep green appearance.
Environmental Stress and Winter Bronzing
The most common reason for widespread orange discoloration is winter bronzing, a protective stress response. This color change occurs when evergreen leaves are exposed to intense winter sunlight and desiccating winds. The foliage produces reddish-orange pigments called anthocyanins to protect chlorophyll from damage, slowing photosynthesis during harsh conditions.
The color change is often more pronounced on the side facing the sun, particularly the south or southwest exposure, or on cultivars that are naturally less cold-hardy. This bronzing is generally not harmful, and the foliage typically reverts to its original green color in the spring as temperatures rise and the plant becomes more active. However, this discoloration is frequently exacerbated by a lack of moisture, known as winter desiccation.
Evergreen leaves continue to lose water through transpiration throughout the winter, even when the ground is frozen and roots cannot take up moisture. This drought stress causes the leaves to dry out, intensifying the orange to brown coloring. To prevent this, thoroughly water boxwoods in the late fall before the ground freezes. Applying a two- to three-inch layer of mulch around the base helps retain soil moisture and insulate the shallow root system, reducing desiccation effects.
For high-value plants in exposed locations, a physical barrier like a burlap screen placed on the windward side minimizes wind damage and sun scald. Anti-desiccant sprays, which coat the leaves to reduce water loss, can also be applied in late fall. These preventative measures help reduce the severity of bronzing.
Identifying Pest Infestations
If discoloration is patchy or concentrated on specific leaves throughout the year, the cause is likely a pest infestation. The boxwood leafminer (Monarthropalpus flavus) is one of the most damaging insects that causes foliage to turn yellowish-orange. The midge larvae feed within the leaf tissue, creating mines that look like raised blisters or irregular blotches on the surface.
To confirm leafminer larvae, carefully peel apart a discolored or blistered leaf to reveal the small, yellowish-orange maggots feeding inside. Severe damage can lead to defoliation and a decline in plant health over several years. Control typically involves applying a systemic insecticide, such as imidacloprid, as a soil drench in the spring, which the roots absorb to kill the larvae.
Another common culprit is the boxwood mite (Eurytetranychus buxi), a spider mite that thrives in hot, dry conditions. These tiny pests feed on the undersides of leaves, sucking out cell contents and causing stippling. This damage appears as numerous minute yellow or bronze dots across the leaf surface, which can progress to a faded or silvery cast on the foliage.
A simple diagnostic test involves holding a sheet of white paper beneath a branch and shaking it vigorously; the mites, which look like tiny specks, will fall onto the paper. For minor infestations, a strong jet of water can physically dislodge the mites. A more aggressive approach involves applying horticultural oils or insecticidal soaps in the early spring to target newly hatched mites. A repeat application may be necessary since mites have multiple generations per season.
Soil Conditions and Nutritional Issues
When discoloration is not caused by weather or pests, it may indicate an underlying problem with soil chemistry and nutrient availability. Boxwoods grow best in a slightly acidic to neutral soil pH, typically between 6.5 and 7.5. If the soil becomes too alkaline (above 7.5), it can chemically lock up certain nutrients, making them unavailable for the plant to absorb and leading to deficiencies.
Magnesium Deficiency
One specific nutritional issue presenting with orange or bronze coloring is magnesium deficiency. Magnesium is a mobile nutrient, meaning the plant will move it from older leaves to support new growth; thus, symptoms first appear on mature, lower foliage. The deficiency causes interveinal chlorosis, where tissue between the veins yellows, often accompanied by reddish or bronze tints along the leaf margins. Magnesium sulfate (Epsom salts) can be applied to the soil for correction or as a foliar spray for a quicker, temporary fix.
Iron Deficiency and pH
Iron deficiency, or iron chlorosis, is another concern, especially in alkaline soils where iron is abundant but chemically inaccessible. This deficiency usually results in a bright yellowing, or chlorosis, of the newest growth, while the veins remain dark green. In severe cases, this yellowing can take on an orange undertone before the leaf tissue dies. The most effective way to address both pH and nutrient issues is to start with a professional soil test, which will precisely determine the levels of magnesium, iron, and the current pH, guiding the application of soil amendments like chelated iron or elemental sulfur to correct the imbalance.