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Boxwood shrubs turn yellow for several common reasons, and understanding these causes is the first step toward fixing the problem.
Many gardeners notice their boxwood shrubs turning yellow and worry about disease, but often the issue is environmental stress or a simple nutrient imbalance.
When you know what causes boxwood shrubs to turn yellow, you can take the right action to bring them back to a healthy, vibrant green.
In this post, we’ll break down the main causes of yellowing boxwood, how to diagnose the problem, and what you can do about it.
Let’s dig into the details of why your boxwood shrubs turn yellow and how to fix it.
What Causes Boxwood Shrubs to Turn Yellow
Several factors behind what causes boxwood shrubs to turn yellow fall into four main categories.
Each one affects the shrub differently, so it helps to know the signs.
1. Environmental Stress
One of the biggest reasons boxwood shrubs turn yellow is environmental stress from improper watering or poor site conditions.
Overwatering can drown the roots, leading to root rot and yellow leaves that eventually drop off.
Underwatering, especially during dry spells, causes the leaves to turn yellow from dehydration.
Another environmental trigger is poor drainage; boxwood hates sitting in wet soil, and waterlogged roots will quickly make the shrub turn yellow.
Winter burn is also a common cause of yellowing boxwood shrubs in cold climates.
When the ground freezes, the roots can’t take up water, but the sun and wind still dry out the leaves, causing them to turn yellow or bronze.
2. Nutrient Deficiencies
What causes boxwood shrubs to turn yellow can also be linked to a lack of key nutrients in the soil.
Nitrogen deficiency is the most frequent culprit, because nitrogen is essential for leaf greenness and growth.
When boxwood shrubs turn yellow from low nitrogen, the older leaves usually fade first while new growth stays green.
Iron deficiency can also cause yellowing, but it shows up differently: the leaf veins remain green while the area between them turns yellow.
This condition, called interveinal chlorosis, is common in alkaline soils where iron becomes unavailable to the plant.
If your boxwood shrubs turn yellow despite regular watering, check the soil pH and nutrient levels.
3. Pests
Pests are another reason what causes boxwood shrubs to turn yellow, and they often leave telltale signs.
The boxwood leafminer is a tiny fly whose larvae tunnel inside the leaves, creating yellowish blisters or blotches.
Infested leaves may also look puckered or blistered, and heavy infestations cause widespread yellowing.
Boxwood mites are another pest that causes yellowing boxwood shrubs.
These spider mites suck sap from the undersides of leaves, giving them a stippled, yellow appearance.
Scale insects can also lead to yellow leaves by feeding on plant fluids and excreting honeydew that encourages sooty mold.
4. Diseases
Fungal diseases play a big role in what causes boxwood shrubs to turn yellow, especially when conditions are wet.
Boxwood blight is a serious disease that starts with dark spots on leaves, followed by rapid yellowing and leaf drop.
Volutella blight causes leaves to turn yellow or bronze and then develop pinkish spore masses on stems.
Root rot, often from Phytophthora or Rhizoctonia fungi, leads to yellow leaves, wilting, and overall decline.
If you notice yellowing boxwood shrubs that also have black streaks on stems or sunken cankers, a disease is likely the cause.
How to Diagnose Yellowing Boxwood Leaves
Once you know what causes boxwood shrubs to turn yellow, the next step is to pinpoint the specific problem.
Here’s how to diagnose the issue in your own garden.
Check Soil Moisture
Stick your finger a few inches into the soil around your boxwood shrubs to check for moisture.
If the soil feels soggy or waterlogged, overwatering or poor drainage is likely what causes boxwood shrubs to turn yellow in your yard.
If it’s bone dry, underwatering is the culprit, and a deep soak is needed.
Inspect Leaves and Stems
Look closely at the yellow leaves for patterns: uniform yellowing suggests nutrient issues or stress, while spots, blisters, or stippling point to pests or disease.
Check the undersides of leaves for mites, scales, or tiny larvae.
Also examine stems for cankers, cracks, or black discoloration that indicate fungal infections.
Test the Soil pH and Nutrients
Boxwood prefers a slightly acidic to neutral soil pH between 6.0 and 7.0.
If your soil is too alkaline (above 7.5), iron becomes locked up, leading to yellow leaves with green veins.
A simple soil test will tell you if a deficiency in nitrogen, iron, or another nutrient is what causes boxwood shrubs to turn yellow.
Treating and Preventing Yellow Boxwood Shrubs
After figuring out what causes boxwood shrubs to turn yellow in your garden, you can take steps to fix the issue.
Here are practical treatments and prevention tips.
Adjust Watering Practices
Water boxwood shrubs deeply but infrequently, allowing the top few inches of soil to dry between waterings.
Improve drainage by amending heavy clay soil with organic matter or planting in raised beds.
During winter, consider using an anti-desiccant spray to protect from winter burn if that’s what causes boxwood shrubs to turn yellow in colder months.
Fertilize Properly
If nutrient deficiency is what causes boxwood shrubs to turn yellow, apply a balanced slow-release fertilizer in early spring.
For iron chlorosis, use an iron chelate supplement according to package directions.
Avoid over-fertilizing, which can burn roots and worsen yellowing.
Prune and Improve Air Circulation
Thin out dense inner branches to improve airflow and reduce humidity that fosters disease.
Remove and dispose of any yellow leaves that have fallen to prevent fungal spores from spreading.
Pruning also helps rejuvenate the shrub and encourages new, healthy green growth.
Manage Pests and Diseases
For boxwood leafminer, apply a systemic insecticide in spring when new growth appears.
For mites, use horticultural oil or insecticidal soap, making sure to spray the undersides of leaves.
If boxwood blight is what causes boxwood shrubs to turn yellow, remove infected plants immediately and avoid replanting boxwood in the same spot for several years.
Fungicides can help prevent disease spread but are less effective once symptoms are visible.
So, What Causes Boxwood Shrubs to Turn Yellow?
What causes boxwood shrubs to turn yellow is typically a combination of environmental stress, nutrient deficiencies, pests, or diseases.
Identifying the exact reason requires checking soil moisture, inspecting leaves and stems, and testing soil pH and nutrients.
Once you know the cause, you can adjust watering, fertilize appropriately, prune for better airflow, and treat any pests or infections.
By addressing the root issue, your boxwood shrubs can recover their lush green color and thrive for years to come.
Now you’re ready to take action and restore your yellowing boxwood shrubs to health.