A dying boxwood—whether it is growing out in a hedgerow or planted alongside the foundation of your house—doesn’t always mean you will need a replacement shrub. There are many different reasons why your plant could be dying, and depending upon the cause, you may be able to revive it before it perishes completely.
Determine the Reason It Is Dying
Before you try to revive your struggling shrub, you must take the time to figure out why the boxwood is dying and solve the underlying problem. Common reasons for death include environmental conditions, biotic stressors, and abiotic stressors. You may need to reach out for professional help if you can’t determine the problem independently.
Solving Common Boxwood Problems
Problems with boxwoods can be broken down into two main categories: abiotic and biotic stressors. Abiotic stresses are problems that are non-living in their origins, such as drought, cold temperatures, fertilizer damage, and soil pH. Biotic stresses are caused by living organisms, including bacteria, insects, viruses, and weeds.
Boxwood blight is a relatively new pathogenic disease caused by the fungus, Calonectria pseudonaviculata. The disease is common in areas with high humidity and is particularly aggressive when shrubs are planted too close together. Once a shrub is infected, the disease causes rapid leaf drop and other telltale symptoms on all plant portions above the soil line.
When blight is present, you see:
- Circular, tan leaf spots with a brown or dark purple border.
- Blackening of the stems or blackish stem lesions.
- Affected leaves turn tan in color.
- Defoliation results in bare branches.
Unfortunately, boxwood blight isn’t treatable, so it’s crucial to prevent it from infecting your plants. If caught early, prune off diseased branches and remove any leaf litter on the soil surface.
Winter burn occurs when freezing temperatures and cold winter winds remove more water from the leaves than the plant can pull in from the frozen soil. The resulting desiccation leads to browning, yellowing, or bronzing of the foliage. Cold injury damage occurs more often in full sun locations and places with no shelter from the wind.
There is no cure for winter burn once it occurs, so prevention is critical. To prevent winter damage:
- Keep shrubs well-watered during the winter.
- Protect plants with wind barriers.
- Insulate the soil with a couple of inches of mulch or organic matter.
English Boxwood Decline
Boxwood decline is a complex condition that appears as weak growth, discoloration of leaves, and dieback of branches. It commonly occurs in the English boxwood, Buxus sempervirens ‘suffruticosa,’ and then spreads to neighboring boxwoods of the same variety. The disease also harbors in the soil, so a boxwood planted in the same spot will also become infected.
The best way to prevent boxwood decline is to keep your plants thriving and healthy. Make sure they are getting enough water and fertilizer and are pruned regularly.
Phytophthora Root Rot
Phytophthora root rot is caused by the soil-borne fungus Phytophthora parasitica. It affects boxwood shrubs’ roots, stems, and leaves. Infection commonly occurs in the spring and fall when the soil is wet and temperatures are between 57°F to 70°F. After infection, the roots rot, and the plant cannot take in water and nutrients, making the shrub appear to be wilting.
Once Phytophthora fungi infect a plant, you must remove it from the ground along with the soil, and both disposed of properly. You can treat the remaining soil and neighboring plants with a prophylactic fungicide to prevent infection.
There are specific nutrients needed for plant growth. These essential plant nutrients have different roles within the plant and are necessary in varying amounts. Nutrient deficiencies, especially nitrogen, phosphorus, or potassium, result in impaired photosynthesis, decreased growth, and other symptoms. In severe cases, the plant can’t perform its metabolic functions, and cellular death occurs.
To treat nutrient deficiency, apply a quick-release, water-soluble fertilizer to the soil or via foliar application at recommended dosage rates listed on the product label. A formulation with micronutrients is preferred if available.
The main problem with fertilizer damage is the chemical salts found in the products. Fertilizer toxicity leads to salt buildup in the soil, which can “burn” the roots and create an imbalance in water potential, making it hard for the plant to take in water. High levels of salts in the tissue also cause toxicity and cellular death.
To prevent further fertilizer damage, flush the soil with clean water to push the excess salts deeper into the ground, out of the root zone. Always follow label recommendations for correct application rates to prevent over-fertilization.
Inadequate soil moisture is one of the most concerning abiotic stressors of boxwoods. When soil moisture is lacking, nutrient uptake into the roots is impeded, leading to deficiencies. Since water is needed for photosynthesis, a lack of water reduces the energy a plant can produce, and metabolic processes slow or halt altogether.
To fix drought stress, keep the soil around the plant consistently moist, without overwatering to the point there is standing water.
A severe insect pest of boxwoods, the boxwood leafminer (Monoarthropalpus flavus), can defoliate and kill weakened shrubs if the population is high. After hatching, larvae feed on the tissue between the outer and lower leaf surfaces, creating tunnels that look like large blisters. The best way to confirm an insect infestation is to dissect a leaf or hold it up to the light.
To control boxwood leafminer, spray infected plants with insecticide as the adult flies emerge from leaves.
Volutella Stem Canker
An opportunistic pathogen, Volutella buxi predominantly infects and causes problems on weakened or stressed boxwoods. This stem canker is more prevalent during periods of high humidity and results in poor growth, leaves browning and turning upward, and black streaks on petioles and stems. Salmon-colored fruiting structures develop underneath the leaves and stems.
Volutella can be treated and controlled using copper-based and other approved fungicides. When spraying plants, make sure to coat the branches and foliage thoroughly.
Boxwoods prefer to be planted in a spot with slightly alkaline soil, meaning the soil pH is above neutral. When the soil is too acidic, macronutrients like phosphorus, calcium, and magnesium become unavailable, and deficiencies may occur. Aluminum and other essential micronutrients may also become readily available and, when taken into the plant in large amounts, cause toxicity.
To treat problems related to soil pH, check the soil pH with a home test kit—or send a sample to a professional testing lab—and add dolomitic lime to raise the soil pH, so it is just above neutral.
Salt damage is typically seen in shrubs grown close to driveways, roadways, or sidewalks in areas with harsh winter conditions. During the winter, ice melt or salt is applied to these surfaces for safety. But once the sodium gets into the plant, it causes toxicity and perhaps cellular death. High sodium also keeps impedes water uptake.
To correct salt damage flush the soil with plenty of fresh, clean water at the end of winter or the beginning of spring. This fresh water helps to push any residual sodium out of the root zone of the shrub. Be careful when throwing ice melt on surfaces to keep it away from plants.
If boxwood shrubs are pruned late in the season, it can cause dieback to occur during the winter once temperatures drop below freezing. Pruning triggers new growth, regardless of the time of year. When improper pruning occurs in the fall, this tender foliage isn’t hardened enough to survive winter conditions and can die.
To prevent dieback from hard pruning, only prune shrubs in the spring, so the new leaves have plenty of time to mature and harden off before winter.
Bringing a Boxwood Back to Life
As mentioned above, before trying to revive your boxwood, you need to address and treat the underlying problem, if possible. Once you know the cause has been fixed, turn your attention to removing the damaged plant parts and encouraging new growth. Unfortunately, once the boxwood leaves or branches die, you can do nothing to repair those tissues.
- In mild cases, use disinfected shears to prune back the branches, removing dead foliage until you reach healthy wood. Be mindful not to prune more than necessary and always make angled pruning cuts at a leaf node.
- In more severe situations where an entire branch has died, use disinfected lopers or a hand saw to remove the branch at the junction with the trunk. Do not use a pruning sealer to cover the wound.
After removing dead or damaged leaves and branches, you need to nurse the shrub back to health by taking proper care of the plant.
- Keep shrubs well-watered all year round, especially in between rainstorms. The soil around the bushes should stay moist but not soggy.
- Prune the center of the shrub to promote good air movement through the plant.
- Apply a slow-release, balanced fertilizer to the soil in the late fall.