Boxwood (Buxus) shrubs are popular evergreens, but their glossy green foliage can sometimes develop a yellow discoloration known as chlorosis. This change, often appearing first on the leaf tips and edges, signals a problem with the plant’s ability to produce chlorophyll, the pigment responsible for the green color. While various issues can cause this symptom, the most frequent causes are non-pathogenic, relating instead to soil composition, water availability, or external climate conditions. Understanding these factors is the first step toward restoring the plant’s vibrant green color.
Nutritional Causes of Tip Yellowing
The yellowing of leaf tips and margins frequently points to a nutritional imbalance within the soil, preventing the boxwood from fully absorbing the elements required for healthy growth. Boxwoods thrive best in slightly acidic soil, ideally with a pH between 6.5 and 7.2. Soil that is too alkaline can chemically lock up certain nutrients, making them unavailable to the roots. A soil test is the most accurate way to diagnose these deficiencies, providing specific details on pH and nutrient levels before any corrective action is taken.
Nitrogen (N) deficiency is a common cause of overall leaf paleness or yellowing, often showing up first on older, inner leaves before progressing to the tips of newer growth. Since Nitrogen is a mobile nutrient, the plant moves it from old leaves to new growth when supplies are low, causing the older foliage to suffer first. Conversely, a lack of Magnesium (Mg) or Potassium (K) also presents as marginal or tip chlorosis on older leaves, as these elements are also mobile within the plant.
Magnesium is a central component of the chlorophyll molecule; deficiency leads to interveinal chlorosis, where leaf tissue turns yellow while veins remain green. To address a confirmed deficiency, a slow-release, balanced fertilizer can be applied. Specific supplements like Epsom salts (magnesium sulfate) can quickly address a magnesium shortage. Applying fertilizer without a soil test, however, risks over-fertilizing, which can cause nutrient burn and further damage.
Water Stress and Root Health
Improper water management is a frequent cause of boxwood leaf tip yellowing, as it directly impacts the root system’s ability to function. Both insufficient and excessive moisture produce similar symptoms of yellowing and eventual browning at the leaf tips. This signals that the plant is stressed and unable to move water and nutrients efficiently. The fine feeder roots of the boxwood are sensitive to these extremes.
Overwatering is often the more insidious problem, leading to root rot caused by fungal pathogens like Phytophthora that thrive in soggy conditions. When roots are constantly waterlogged, they drown due to a lack of oxygen. Compromised roots cannot absorb water and nutrients, which mimics the symptoms of drought stress. This root damage results in yellowing leaves that may curl upward, and the plant’s overall growth becomes stunted.
To prevent this issue, boxwoods must be planted in well-draining soil, and watering should only occur when the top inch of soil is dry to the touch. Applying mulch helps regulate soil temperature and conserve moisture, but it should be kept several inches away from the trunk. Ensuring the soil drains freely is the main defense against fungal root diseases that prevent nutrient uptake, even when nutrients are present.
Environmental Damage and Winter Burn
External climate factors, particularly during colder months, can cause physical damage to the foliage, appearing as yellowing or bronzing at the leaf tips and margins. The most common form of this damage is winter burn, a desiccation injury caused by a mismatch between moisture loss and uptake. During the winter, evergreen leaves continue to lose water through transpiration, especially on sunny, windy days.
When the ground is frozen, boxwood roots cannot replace this lost moisture, leading to severe dehydration of the foliage. The resulting damage manifests as yellow, coppery, or bronze discoloration on exposed leaf tips, often facing the prevailing winter wind or intense sun. Symptoms may not become fully apparent until spring, when the weather warms but the damaged tissues are unable to recover.
Preventative measures are the most effective treatment for winter burn, beginning with consistent deep watering late into the fall until the ground freezes. Applying an anti-desiccant spray in late fall and again in mid-winter helps reduce moisture loss from the leaves. Protecting the shrubs from direct sun and wind with burlap barriers can also limit the environmental stress that causes the leaf tips to yellow.