Boxwood: Common Health Issues in the Landscape
A go-to for the evergreen foundation in landscapes, boxwoods bring to mind formal gardens and sheered shapes. Although the hardiness of the species is stretched in southern New England, many cultivars and hybrids offer improved cold hardiness, better suited for northern climates. Unfortunately, boxwood blight has become a serious threat to boxwoods in managed landscapes throughout the region. In some landscapes, it may no longer be feasible to use boxwoods in the wake of boxwood blight occurrence.
Species, Varieties and Hybrids
Littleleaf boxwood (Buxus microphylla) is a densely-branched, rounded to mounded shrub growing 3-4’ tall and wide. Native to Japan and Korea, it is hardy from zones 6 to 8. Leaves are medium green leaves in summer turning yellow, brown, or bronze in winter.
Japanese boxwood (B. microphylla var. japonica) has darker green leaves and less winter color change compared to little leaf boxwood. This variety grows 3-6’ tall and wide and is adaptable in zone 6.
Common or English boxwood(Buxus sempervirens) is the traditional boxwood of formal, European gardens. The species is quite large, growing 15-20’ tall and wide. Native to Europe, northern Africa, and western Asia, B. sempervirens is hardy is zones (5)6 to 8. Leaves are dark green with a yellow-green underside. Leaves can discolor in winter but generally better color than B. microphylla.
Korean boxwood (Buxus sinica var. insularis, formerly B. microphylla var. koreana) exhibits superior cold-hardiness compared to B. microphylla or B. sempervirens, surviving temperatures as low as -25° F. Hardy in zones 4-5, Korean boxwood are more common in trade than B. microphylla. The leaves are smaller than B. microphylla but otherwise similar in appearance.
Sheridan hybrids (Buxus sinica var. insularis × B. sempervirens) are very common in the nursery trade and include the cultivars ‘Green Gem’, ‘Green Mound’, ‘Green Mountain’, and ‘Green Velvet’. These hybrid cultivars combine the cold hardiness of B. sinica var. insularis and the darker green leaf color of B. sempervirens.
Buxus spp. need a well-drained soil and benefit from mulching to keep the roots cool and moist. Plants do best is full sun to light shade but newly planted shrubs benefit from shade during establishment. Plants should be protected from winter winds, harsh sun and cold. Cultivation after planting should be avoided to not disrupt the shallow roots. Plants are slow growing and deer resistant.
Boxwood lacks heat and cold stress tolerance. They are especially susceptible to foliar desiccation from drying winter winds. Symptoms of winter burn on boxwood typically appear as a reddish-orange to brown discoloration along the midrib or entire interior portion of the leaf. The leaf margins, however, often retain some green color. Boxwoods, like many woody plants in the landscape, are more susceptible to winter burn when they are drought-stressed in autumn. New spring leaves are also sensitive to frost damage. Appropriate cultivar selection for winter hardiness and a protected planting location are key for plant success.
Boxwood Blight, caused by Calonectria pseudonaviculata, is a destructive disease that affects all Buxus species and cultivars. The disease was first reported in the U.S. from Connecticut and North Carolina in the fall of 2011. Since its introduction, the disease has spread throughout the United States. Boxwood blight was first described in the U.K. in the mid-1990s and is present throughout Europe. Common boxwood (B. sempervirens) and edging boxwood (B. sempervirens ’Suffruticosa’) are especially susceptible to infection and death by C. pseudonaviculata. Other members of Buxaceae (Pachysandra and Sarcococca) also serve as hosts, but may experience minor to moderate symptoms of the disease. Additionally, numerous, non-woody groundcovers can also harbor the pathogen, serving as “reservoir hosts” that may lead to outbreaks on nearby boxwoods. Symptoms first appear as scattered leaf and branch dieback followed by a rapid collapse of the entire plant. Black, circular to zonate leaf spots and black stem lesions are the most conspicuous symptoms. Less often, a grayish, fuzzy growth of fungal mycelium may be observed on leaf and stem tissue. The fungus is able to attack all plant parts, but only rarely does it infect the roots. The fungus has the ability to produce resting structures (known as microsclerotia) that allow it to survive in dead plant parts (leaves, stems and roots) and persist at sites where infestations establish. Rapid spread through susceptible boxwood plantings has been well documented. Therefore, if the infected plants are removed, so must all organic matter under and near the plants.
Boxwood Blight Management: Infected plants that were recently transplanted should be removed immediately and destroyed. In addition to the plant, remove all organic matter around the plant so the pathogen cannot persist at the site. Regularly scout all other boxwoods on the property for symptoms of the disease. For large and mature plants, prune and destroy all blighted stems and remove all downed foliage around the plant. Chemical management may also be required to save the plant. Sanitize all pruning equipment prior to and after working with infected boxwoods.
Volutella blight, caused by Pseudonectria buxi and P. foliicola (previously Volutella buxi), is a common disease of ornamental boxwood. The fungus mostly behaves as an opportunistic pathogen, attacking the foliage, shoots and branches on stressed and weakened plants. Symptoms of infection include browning leaves and scattered branch dieback. In contrast to boxwood blight, foliage killed by Volutella blight is held in the canopy and not shed for many weeks or months. Salmon-colored masses of fungal spores can often be observed on the underside of infected leaves and on blighted shoots. While the disease primarily affects shrubs predisposed to stress, in certain cases the pathogen occurs on otherwise healthy plants. Wet, humid weather is conducive to disease development and whenever weather conditions are favorable the pathogen will grow and reproduce. Shearing wounds are a common source of establishment for Volutella. Along with aggressive pruning, drought stress, insect feeding, girdling/circling roots and soil compaction are common sources of stress that facilitate disease development.
Macrophoma blight, caused by Dothiorella candollei (also known as Macrophoma candollei), is a weak pathogen that typically attacks boxwoods under stress. This fungus is very common in the landscape and is often present at low levels on healthy boxwood. Macrophoma causes a leaf spot and blotch that can quickly consume the entire leaf when weakened plants are attacked. Infected leaves transition from yellow to brown before they are prematurely shed from the canopy. Numerous black-colored, dome-shaped pads of fungal tissue may be visible on both the upper and lower surface of infected leaves. From these pads, masses of clear-colored spores are produced and are spread by splashing water and wind. This allows the pathogen to spread to nearby foliage within the canopy. Macrophoma often occurs in conjunction with Volutella blight and on plants in decline from drought stress and excessive shearing.
Volutella and Macrophoma Blight Management: Overhead watering of boxwoods often facilitates disease development and spread. When the canopy is wetted by overhead sprinklers, free moisture is supplied to the pathogen, allowing spores to germinate and invade leaves and stems. Also, splashing water readily disperses spores to neighboring branches and plants. Regular sanitation pruning and removal of dead plant material is very important to control diseases of boxwood. Dead plant parts harbor fungal pathogens and allow them to both persist and disseminate spores in very close proximity to healthy tissues. Damage from insect pests (e.g. boxwood leafminer, boxwood spider mite and boxwood psyllid) and shearing is the primary means of entry for many boxwood pathogens. Avoid pruning during prolonged wet periods, as fungal pathogens may be sporulating at this time and can readily colonize wounds for entry. Fungicides may be effective in conjunction with good cultural practices. Preventative and therapeutic applications should take place in the spring when new, succulent tissues are developing. Irrigation of newly established boxwoods is also important to avoid the development of drought stress. Ensure plants are not established too deep in the soil and/or over-mulched.
Common Insect Pests
The boxwood leafminer, Monarthropalpus flavus, is a true fly (Diptera) that was first reported in the United States in 1910. It is thought that this insect arrived in the US with boxwoods brought from Europe in the 1800’s, or earlier. This insect has a single generation per year. Partly grown fly larvae overwinter in the leaves of susceptible boxwood. Yellowish mines may be noticeable on the undersides of leaves. This insect grows rapidly in the spring, transforming into an orange-colored pupa. After pupation, adults will develop and white colored pupal cases may hang down from the underside of leaves where adults have emerged. Adults may be observed swarming hosts between 300-650 GDD’s, base 50°F, or roughly the end of May through June. Adults are tiny (2-3 mm. in length), yellow-orange, gnat-like flies. Females will lay eggs in the upper side of the current season’s leaves. Each female lays on average 29 eggs which hatch in approximately 3 weeks. Larvae grow slowly in the summer and many larvae can live in the same mine. Larvae feed within the mine and leaves may blister, turn yellow or brown, and drop from the plant. Most cultivars of Buxus sempervirens, B. microphylla, and B. harlandii are thought to be susceptible. (Also note that B. harlandii is hardy to zones 7-9, and therefore would not be the best choice for most of Massachusetts.) If installing new boxwoods, resistant cultivars such as ‘Vardar Valley’ and ‘Winter Gem’ are good choices at sites where this insect has been a problem. Additional options are listed under Alternative Species below.
The boxwood mite (Eurytetranychus buxi) is not an insect, but rather a closely related mite pest of this genus of plants. It has been noted as most problematic on European, common, and English boxwood. Japanese boxwood are often noted as much less susceptible. This species overwinters as tiny eggs on the undersides of boxwood leaves that hatch mid-spring. These mites are tiny (about the size of a period) and difficult to detect. Feeding on the upper and lower surfaces of leaves by this mite may cause plants to appear off-color. If management is deemed necessary, the timing for treatment may be between 245-600 GDD’s, base 50°F, or roughly the beginning of May. Damage from this mite is often noted to be minor, however it may range from various degrees of stippling, yellow or bronze streaking, or in some limited cases, premature leaf drop.
The boxwood psyllid, Psylla buxi, is a piercing-sucking pest of boxwoods. Feeding by this insect can cause conspicuous cupping of susceptible boxwood leaves. Leaf symptoms/damage may remain on plants for up to two years. English boxwood may be less severely impacted by this pest, whereas American boxwood is more severely and frequently infested. Small, orange, spindle-shaped eggs overwinter, buried in budscales, and hatch around budbreak of boxwood. (Developed nymphs actually overwinter inside these egg shells.) Nymphs may emerge around 80 GDD’s, base 50°F, and begin feeding. This feeding causes the leaves to cup and curl around the insects, sometimes several of them in one pocket. Nymphs produce waxy filaments that cover their bodies and provide protection. By late May and June greenish adults develop and jump from leaves and then fly using wings. Foliar applications may be made between 290-440 GDD’s, base 50°F. Only one generation occurs per year. The damage caused by this insect is mostly aesthetic. Therefore, in most cases, no management may be necessary.
The oystershell scale (Lepidosaphes ulmi) is only an occasional pest of boxwoods. There are many similar, easy to confuse, species of scales in this genus. L. ulmi, according to the literature, has a very wide distribution of host plants. This insect is distributed throughout the world except in the artic and tropical regions. In 1918, 128 species of plants were recorded as hosts for this insect. Luckily, however, this insect is a serious pest on relatively few of those recorded host plants. Lilac, willow, ash, poplar, and maple are the most commonly infested. Boxwood is only occasionally infested by the oystershell scale. The oystershell scale overwinters in the egg stage hidden beneath the protective, armored covering of the female. In New England, one generation per year is recorded for this insect. Many parasites and predators of the oystershell scale are recorded in the literature. Predatory mites feed on their eggs and twicestabbed lady beetles are also important predators. Prune out heavily infested branches. Dormant oil applications may be made according to label instructions between 7-91 GDD’s, base 50°F.
· Buxus microphylla ‘Golden Dream’: dark green leaves with a yellow margin
· Buxus sinica var. insularis ‘Justin Brouwers’: 3–4′ tall and wide, deep green foliage maintained in winter
· Buxus sinica var. insularis ‘Wintergreen’: 2–3′, bright green, small leaves that don’t discolor in winter
· B. sempervirens ‘Suffruticosa’: dense, compact and old cultivar. Less susceptible to boxwood leaf miner
· B. sempervirens ‘Vardar Valley’: grows wider than tall (2–3′ tall × 4–5′ wide). Noted for its cold tolerance
· B. sempervirens ‘Inglis’, ‘Northern Beauty’, ‘Northern Find’, ‘Northland’, ‘Pullman’, and 'Welleri': cultivars known to be cold hardy
· B. sinica var. insularis × B. sempervirens hybrids:
a. ‘Green Gem’: 2′ × 2′
b. ‘Green Mountain’: 5′ × 3′
c. ‘Green Mound’: 3′ × 3′
d. ‘Green Velvet’: 3′ × 3′
Alternative Species & Options
· NewGen boxwoods:
1. Buxus NewGen Independence: substitute for‘Suffruticosa’, ‘Green Velvet’ and ‘Green Mound’ cultivars, high resistance to boxwood blight and boxwood leafminer
2. Buxus NewGen Freedom: substitute for ‘Wintergreen’, ‘Winter Gem’ and ‘Green Mountain’ cultivars, high resistance to boxwood blight and boxwood leafminer
· Similar foliage, growth, and landscape use:
1. Japanese holly (Ilex crenata): slow growing and prone to winter burn in zone 5
2. Inkberry (Ilex glabra): winter injury in exposed locations, can have a thin, leggy appearance but performs well in wet soils
· Similar in function but with a different texture:
1. Blue holly (Ilex × meserveae): Can be susceptible to winter burn, dioecious (need male and female plant for fruit set)
2. Japanese plum yew (Cepaholtaxus harringtonia): Very similar to yew (Taxus) but deer resistant and not as cold tolerant
3. Hinoki Falsecypress (Chamaecyparis obtusa): Countless dwarf and intermediate cultivars available, good heat and drought tolerant with scale-like foliage
4. Arborvitae (Thuja spp.): Numerous dwarf and intermediate cultivars available, but very susceptible to deer browsing.
5. Hybrid yews (Taxus × media): good drought resistance when established, dense and strong growth rate but very susceptible to deer browsing
6. Mountain Laurel (Kalmia latifolia): performs best in full sun but can experience burn, lacebug and leaf spot issues
7. Andromeda (Pieris japonica): prominent early spring flowers, various disease and pest issues must be controlled
8. Rhododendron (Rhododendron spp): a multitude of compact and dwarf cultivars available (e.g. ‘Percy Wiseman’ and ‘Yaku Princess’), high shade tolerance, various pest and pathogen issues