Boxwood shrubs are valued for their dense, evergreen foliage, but they can suffer from issues leading to discoloration, dieback, and decline. The initial inspection must distinguish between environmental damage, pest infestation, and disease infection. Uniform yellowing, especially if the foliage is scorched or bronzed, often indicates environmental stress like improper watering or winter burn. Winter burn happens when leaves lose moisture on sunny, windy winter days while the roots cannot absorb water from frozen soil.
Pest damage presents with specific, localized symptoms. If leaves appear stippled with tiny white or yellow marks, the boxwood mite is the likely culprit, feeding on the leaf surface. Conversely, boxwood leafminer larvae feed inside the leaf tissue, creating visible blisters or splotchy yellow and brown discoloration. Tearing open an infested leaf may reveal small orange larvae or pupae inside.
Diseases display distinct physical signs, often involving cankers or rapid defoliation. Boxwood Blight, an aggressive fungal disease, is characterized by dark leaf spots that quickly lead to defoliation, often starting on lower branches. A diagnostic sign for Boxwood Blight is the presence of narrow black lesions or streaks on green stems. Volutella Canker typically affects stressed plants, causing leaves on individual branches to turn bronze or straw-colored. The dead leaves often remain attached to the branch for an extended period.
Correcting Environmental and Cultural Stress
Cultural adjustments are essential for revival, as boxwoods require well-drained soil and decline quickly in constantly wet conditions, leading to root rot. Water management should focus on deep, infrequent irrigation, moistening the root zone to a depth of 12 to 18 inches during dry spells.
Boxwoods thrive best in a slightly acidic to neutral pH range (6.5 to 7.2). Testing the soil determines if amendments, such as lime, are necessary for nutrient availability. Fertilization should be light and balanced, using a slow-release product in late fall or early spring. Over-fertilization stresses the roots and attracts pests.
Site correction mitigates environmental stress, particularly in winter. Apply a two-inch layer of mulch to regulate soil temperature and moisture, keeping it several inches away from the trunk to prevent bark rot. Temporary barriers, such as burlap screens, protect the evergreen foliage from winter burn in areas prone to harsh sun and wind.
Targeted Treatment for Boxwood Pests and Diseases
For the boxwood leafminer, which is protected inside the leaf, systemic insecticides are effective. Ingredients like imidacloprid or dinotefuran should be applied as a soil drench in early spring. The plant absorbs the chemical, poisoning the larvae as they feed.
Boxwood mites are controlled with horticultural oils or insecticidal soaps, applied thoroughly to the undersides of the leaves where the mites reside. A dormant oil application in late winter targets overwintering eggs. Summer applications of oil or miticides control active populations, and this contact treatment avoids killing natural predators.
Fungal diseases like Boxwood Blight and Volutella Canker require immediate sanitation and chemical management. Infected material, including fallen leaves and diseased branches, must be removed and destroyed, as spores survive in debris. For Volutella, pruning out cankered wood and improving air circulation is paramount, sometimes followed by a protective fungicide. Boxwood Blight has no cure, so aggressive removal of infected plants and leaf litter is recommended, followed by a preventative fungicide program using products like chlorothalonil to protect healthy shrubs.
Pruning for Revival and Future Health
Pruning is a restorative technique performed after the primary cause of decline has been addressed. Sanitation pruning involves removing all dead, damaged, or visibly diseased wood back to healthy tissue, eliminating the source of fungal spores or pest eggs. Disinfect pruning tools between cuts, especially when dealing with fungal infections, using a 1:9 bleach-to-water solution or isopropyl alcohol to sterilize the blades.
Recovery pruning focuses on thinning the interior of the shrub by selectively removing entire stems back to a main branch. This practice increases light penetration and air circulation within the dense canopy, discouraging humid conditions favored by fungal diseases. Thinning encourages new, healthy interior growth and is more beneficial than simply shearing the outer layer.