Boxwood: Identify and Manage Common Problems (2024)

• Proper site selection and plant care are essential for maintaining the health of boxwood. In general, boxwood:
  • Needs well-drained soil and will not tolerate sites that are constantly moist. Work in some organic matter into the soil where the boxwood will be planted (not just in the planting hole).
  • Boxwood prefers a soil pH of 6.5- 7.2 and a location with some afternoon shade.Sites exposed to full winter sun can cause foliage to “burn” and turn orange. Boxwood planted with a south or southwest exposure suffer winter burn more than plants with an east or north exposure due to increased sun exposure.

To prevent boxwood blight, plant disease-resistant cultivars (e.g. ‘New Gen’™, ‘Green Beauty’, ‘Nana’) from reputable nurseries. Ask if they receive plants from producers that participate in the Boxwood Blight Cleanliness Program.

(PDF)Cornell Cooperative ExtensionDisease and Insect Resistant Plants Boxwood

• Winter injurymay be confused with the early stages of the fungal diseasesPhytophthoraroot rot or Volutella blight.
• Leaves turn from bronze to reddish-brown as a result of exposure to cold, dry winter winds.
• Tissue death is caused by the removal of water in the leaves faster than the plant can replace it through root uptake from frozen water in the soil.
• Bark splitting can be caused by a rapid temperature drop caused by a mid-winter thaw.
• Dead twigs and branches in the spring may be the result of ice and snow damage from the winter.

Management

• Winter damage can be reduced by locating plants in partially shaded areas protected from winter winds.
• Physical barriers made from materials such as burlap or plastic, placed about 18 inches from the plants on the windward side, can also lessen winter wind damage by reducing wind velocity.
• Maintain adequate soil moisture in the fall to prevent winter desiccation.
• To avoid damage from falling snow and ice do not plant boxwoods under roof eaves.
• For established boxwoods, tie a string or twine at the base of the plant and spiral the twine up and down the plant to hold it together and gently brush snow off plants as soon as possible. This will help prevent damage from falling ice and snow. Inspect plants for winter damage in the spring and prune out affected areas.

Often the term "decline" is used as a catch-all phrase forpoor boxwood growth, which is caused by a combination of factors. Most often, this occurs on older, well-established shrubs.

Symptoms and causes

Symptoms include poor, off-colored growth, dieback, small leaf size, yellowing of interior foliage, and premature leaf drop. These symptoms commonly occur without any single underlying cause evident and can mimic common boxwood problems.

Stresses from drought or excess water, excessive mulch, soil compaction, deep planting, the addition of soil over the root zone, and root injury from construction all can lead to poor growth of boxwoods.

Multiple insects (mites, leafminers, scales,psyllids) and diseases (Volutella,Macrophoma leaf spot) can also contribute to the overall decline of plants.

• Older boxwood plantings that are neglected and overgrown with weeds are prime candidates for vole damage. Voles damage boxwoods by girdling the base of the plant, feeding on roots, and tunneling through the root system. They cause plant damage primarily in fall and winter.
• Voles or meadow miceare found throughout Maryland. A vole is the same size as a house mouse, with small eyes and ears and a short tail. The color may vary between gray and brown.
• Voles are often confused with moles, but they are very different in their feeding habits and are not related to them. Moles live underground and feed on soil insects and earthworms. Voles are plant feeders and usually live on the surface but may travel in mole tunnels.

Management

• Voles can be controlled by habitat modification and trapping. Use no more than one inch of mulch around boxwoods. Deep mulch provides a good habitat for voles. Keep boxwood plantings free of weeds which provide protection for the voles.
• To reduce vole populations, mouse traps baited with apple slices or a peanut butter-oatmeal mixture should be placed across surface runways. Many predators prey on voles, including black rat snakes, owls, cats, etc.

• Boxwood blightis caused by a fungus calledCylindrocladium pseudonaviculatum(synonym:Cylindrocladium buxicola). The first symptoms begin as leaf spots followed by rapid browning and leaf drop starting on the lower branches and moving upward in the canopy.
• A key symptom that differentiates boxwood blight from other boxwood diseases, such as volutella blight and macrophoma leaf spot, are numerous narrow black cankers (black streaks) that develop on the green stems. The pathogen does not attack the roots, so larger plants may produce new leaves during the growing season but may lose ornamental value as defoliation becomes severe. Repeated defoliation and dieback from stem cankers will kill entire plants. The fungus can remain alive in fallen leaves which can then serve as the source of infection for subsequent years.
  

  Management
  

• Plant disease-resistant cultivars (e.g. ‘New Gen’™, ‘Green Beauty’, ‘Nana’) from reputable nurseries. Ask if they receive plants from producers that participate in the Boxwood Blight Cleanliness Program.
• Use landscapers and lawn care professionals who are educated about this disease and best management practices for preventing its spread.
• Send photos of suspicious boxwood symptoms to the Home & Garden Information Center’sAsk Extensionservice.
• If disease symptoms are diagnosed, immediately bag and remove infected plants along with fallen leaves. Mulch the area to bury the remaining debris.
• Do not compost infected boxwood material. Launder all clothing, gloves, and shoes, and sanitize gardening tools.
• Removal will not guarantee eradication of the boxwood blight pathogen since it can survive in fungal resting structures in the soil for many years.
• Fungicide sprays have shown some disease suppression in limited situations. However, these treatments do not eradicate boxwood blight and must be repeated throughout the growing season.
• Consider replacement of boxwoods with non-susceptible plants such as hollies and conifers.

• SeveralPhytophthoraspecies cause root rots in boxwoods. Symptoms include poor growth, loss of healthy foliage color (leaves eventually turn from green to yellow-green to purplish-brown or straw color), upward turning and inward rolling of leaf margins, dark brown discolored wood at the base of the stem for 2 or 3 inches above the soil line, and loosening and separation of the dead lower bark. As a result of the fungal infection, the root system is reduced and turns dark brown. Root diseases on older established plants can result from changes in water drainage patterns.

Management

• Although there are no chemical cures for these diseases they can be prevented by proper planting. Avoid planting boxwoods in poorly drained compacted soils or in low areas where water collects.
• Avoid placing boxwoods near downspouts.
• Construction of raised beds or grade changes may be needed to ensure proper drainage.

Volutella

• Many boxwoods are susceptible to this disease caused by the fungus,P. buxi.Before new growth appears in the spring, leaves on the tips of infected branches lose their green color and then fade to a light straw color. However, the infected branches retain most of their leaves for many months.
• Examination of affected branches reveals loose bark and girdling at varying distances from the tips and discoloration of the wood. In moist weather, the fungus produces salmon pink fruiting bodies on leaves and stems.

Management

• Diseased branches should be pruned out when the foliage is dry.
• Plants should be thinnedto improve air circulation and light penetration.
• Old fallen leaves and diseased leaves that have accumulated in the crotches of branches in the interior of the plant should be shaken out and removed.
• Improve growing conditions, especially to alleviate drought stress.

Macrophoma

• Many boxwood plants are susceptible to infection by the weakly parasitic fungus,Dothiorella candollei.The most obvious symptoms are the many tiny black raised fruiting bodies found on dying or dead straw-colored leaves.

Management

Pruning infected branches is sufficient management for this fungus. Thinning pruning is recommended to increase air circulation helping to reduce moisture.

• Boxwood leafminer (Monarthropalpus flavus) is the most significant insect pest of boxwood in Maryland.The larvae of this fly feed on the tissue between the outer surfaces of the leaves. This feeding results in blotch-shaped mines in the boxwood leaves. The infested leaves appear blistered from late summer through the following spring.
• New leaves do not show signs of mining until late summer when the larvae are larger.
• By fall, or in early spring, premature leaf drop may result from heavy infestation.

• Lifecycle:
  • In Maryland, adult leafminers emerge in late April or early May, depending on the weather.
  • The adults are small (3mm), orange, gnat-like flies.
  • The adult flies emerge over a period of 10-14 days, but individual flies only live about 24 hours.
  • After mating, each female inserts about 30 eggs into the surface of new boxwood leaves.
  • The larvae hatch in about 3 weeks and feed within the leaves from June through early fall. Feeding may slow or pause during hot summer periods.
  • They spend the winter in the leaves and pupate the following April.
  • There is one generation each year.

Management

• Prevention:
  • Many cultivars of Buxus sempervirens and Buxus microphylla var. japonica, are relatively resistant to this pest.
    
• Mechanical control:
  • Pruning boxwood back by about one-third to remove the stems with infested leaves will help reduce this pest. Dispose of the clippings. This should eliminate the need to use an insecticide. [Watch our boxwood pruning demonstration video.]
    
• Chemical control:
  • Pesticides are hazardous and may harm organisms that are not the target pest. If you choose to use a pesticide, read and follow the directions and safety precautions on the label.
  • It is difficult to control adult boxwood leafminers because of their short adult lifespan. Beginning in late April, shake the branches of boxwoods to detect flying adults. When they are present, thoroughly spray the plants with a registered insecticide (active ingredient spinosad).
  • If developing mines are observed in the leaves, larvae can be controlled from late June through the summer by applying a registered systemic insecticide. It is best to control larvae in June before serious damage has occurred. Some systemic insecticides may only be applied by certified pesticide applicators, as per Maryland’s Pollinator Protection Act of 2016.

• The boxwood psyllid,Psylla buxi, causes cupping of the leaves on the terminal and lateral branches of boxwood. This insect can overwinter as an egg or as a first-instar nymph under the bud scales. As the buds develop in the spring, the eggs hatch and nymphs emerge to infest the leaves.

Boxwood mite
(Eurytetranychus buxi)

• Boxwood mites are yellowish-green or reddish and are 0.5mm long. The yellow eggs overwinter on the leaves and hatch in April. These spider mites breed rapidly and have 5 or 6 generations each summer. They are most active in hot, dry summers. Injury shows as a fine stippling of the leaves early in the season, followed by a general grayish, dingy, unhealthy appearance.
• This is a common pest wherever boxwoods are grown. SomeBuxus microphyllacultivarsappear to be more resistant.

Management

• Damage is primarily superficial and aesthetic. For light infestations, use a sprong spray of water from a hose to dislodge the mites.
• For large infestations, use a horticultural oil or insecticidal soap spray in the summer following label instructions.
• A biological control option for heavy mite infestations may be the release of predatory mites that can be purchased from mail-order sources.
• Some insecticides used to treat boxwood leafminers may exacerbate spider mite problems because they killnatural predatorsof mites.

Oystershell scale
(Lepidosaphes ulmi)

• Heavy infestations of this armored scale will cause yellowing and wilting of leaves and eventual dieback of branches. Infested plants have an unhealthy appearance overall. To monitor for this pest look for tiny (3mm), oyster shell-shaped, brown to gray scale covers on the bark of wilting or dead branches. There may be one or two generations each year.
• Crawlers, newly hatched scale insects, are about the size of a pinhead and light-colored. Look for crawlers near the old scale covers in May.

Management

• Prune out heavily infested branches. A dormant oil (3-4%) spray may be applied in late winter. Be sure to thoroughly cover all of the branches.
• A summer spray (2%) of horticultural oil may be applied in late May.

Box tree moth
(Cydalima perspectalis)

• Box tree moth is an invasive insect native to easternAsia. This moth was first detected in North America (in Toronto, Canada) in 2018. Though not currently present in Maryland (as of April 2024), the box tree moth is a potential new threat to boxwoods. In May 2021, USDA/APHIS confirmed that box tree moth was found in the continental United States (in New York), and it has since been found in Ohio, Michigan,and Massachusetts.
• The larvae (caterpillars) are green and yellow with white, yellow, and black stripes and black spots. Injury begins as chewing on the undersides of leaves. Older larvae can defoliate leaves either by consuming the entire leaf or by just leaving behind the leafedges and/or leaf midvein (creating a "curlicue" leaf appearance). Extensive feeding results in "see-through" brown boxwoods, eventually leading to plant death. So far, this insect has only been found feeding on boxwoods (Buxusspp.) in the United States.
• Caterpillars produce wispy silk within boxwood branches and leave behind small, light green-brown frass pellets (excrement) within leaves or on the ground under infested plants.
• Monitor for caterpillars, chewing damage, and the presence of webbing and frass from May through October.
• If you think you have seen this insect in Maryland, please send photos toAsk Extension. Refer to the USDA/APHIS website onbox tree mothfor more information and updates.

Management

• Confirm the identification of box tree moththrough Ask Extension before applying controls. Unnecessarytreatments can harm beneficial insects and cause secondary pest outbreaks. Do not apply preventative insecticide applications.
• With small infestations, hand-pick caterpillars off plants and place them in a bucket of soapy water.
• For large infestations, use a horticultural oil or insecticidal soap spray on small caterpillars, following all label instructions. Please note adequate coverage is needed for effective control, aiming for the underside of leaves (where the young caterpillars feed). A professionalpest control company can be contacted for further guidance.
• Heavily infested plants may need to be removed and destroyed to prevent further spread.
• Further management options are currently being researched.