Deep green orbs of English box
Boxwood has been a garden staple for centuries, instantly infusing a landscape with structure and elegance. Long-living and versatile, its dense, evergreen foliage can be sheared into almost any shape imaginable. Recently, increased deer browsing in residential areas have caused deer-resistant boxwood to be in high demand. There’s just one problem: it is often plagued by a slew of pests and diseases.
One big family
The boxwood family is a big one. It includes five genera of trees, shrubs and herbs and around 247 flowering species. The species most familiar to Westerners, Buxus sempervirens (Common box), is a genus of about 70 plants.
Buxus is known as boxwood in America.
Boxwood fleur-de-lys at George Washington’s Mount Vernon
And in England it is known as box.
Formal English garden with box
A common foundation planting, boxwood has served as hedging and edging in gardens since the 16th century. Boxwood topiary, a practice that began in Roman times, is still popular today in great gardens across the world, including the lovely Jardin d’Eyrignac, in Dordogne, France. (Click on the link to see photos of my recent visit.)
Boxwood edging at Paris’ Luxembourg Gardens
Boxwood grows well in full sun or partial shade and once established, is relatively drought tolerant. Like all plants, it prefers well drained soil. Heavy or compacted soils that retain water can easily cause the roots to decay. If you want your boxwood to really flourish, make sure your soil is on the alkaline side, or a pH of 7.1 or higher.
To encourage healthy growth, feed your boxwood with a balanced fertilizer high in nitrogen in the early spring. Keep the soil cool and protect the roots by spreading a 1- to 2-inch layer of mulch around the plant base, making sure to keep a 6-inch distance from the trunk. This will prevent any moisture build-up that could lead to root rot.
An ancient plant with modern-day diseases
Although generally viewed as low maintenance (some specimens have lived for over 400 years), boxwood is nonetheless saddled with its own set of pests. Most recently, boxwood blight has become a significant problem, resulting in defoliation and eventual decline of susceptible plants. First seen in England in the 1990s, blight has traveled overseas and is now affecting plants in the United States.
What is blight? Blight is caused by the fungal pathogen, Calonectria buxicola, which produces leaf spots, stem cankers, defoliation and eventual death of vulnerable plants. The disease starts with an initial leaf spot, usually a black circle with red halo, and spreads outwards until entire leaf surfaces are infected. Eventually stems become infected, too, forming black lesions or cankers. Once the cankers girdle the stem, they cut off the water supply and the branch dies.
Boxwood blight symptoms. Photo credit/umd.extension.edu
Boxwood blight and other diseases have caused many people to switch from the disease-prone English box to more resistant species such as Japanese and Korean box. Many of these new varieties are drought-tolerant, have good disease resistance and keep their deep green color all winter. For cultivars and hybrids suitable to your area, check out the list at Dave’s Garden.
Treatment: If you suspect blight, get a diagnosis from a trained professional. Then, prune out diseased stems, rake up and dispose of all foliage and apply fresh mulch to prevent reinfection from the soil. Last resort would be to apply homeowner fungicides such as chlorothalonil or chlorothalonil mixed with thiophanate methyl, which have shown some promise in controlling blight.
Another common disorder of both American and English box is boxwood decline, which is usually caused by a combination of diseases brought on by poor-drainage, poor air circulation and occasionally weather. The fungal disease Volutella appears on leaves on infected branches before new growth appears in the spring. Leaves turn red, then yellow and infected branches die back. In most cases the fungus produces small pink-colored fruiting bodies on leaves and stems.
The parasitic fungus Macrophoma produces black fruiting bodies on infected limbs. The dying branches are usually straw-colored.
Fungal damage. Photo credit: University of Maryland
Treatment. The best way to control for fungal diseases is to thin your boxwood regularly to reduce dense branching and increase air-circulation within the bush. Do this by reaching down into the plant and pruning out handfuls of stems until light can penetrate into the center of the shrub. Prune only when the foliage is dry to prevent the spread of mold spores, and rake up and dispose of all infected leaves and branches.
You might have noticed that your boxwood leaves have little red spots and appear puffy. This is the work of Monarthropalpus flavus, or Boxwood Leafminer. The tiny orange mosquito-like insect buzzes around plants in spring when the first flush of new growth appears, inserting its eggs inside the tender leaf membrane. The larvae don’t hatch until autumn, when they begin mining inside the leaf, causing leaves to turn orange and eventually drop off.
Leaf Miner damage. Photo credit/Cornell University
Treatment: If you observe insects swarming around your shrubs, treat the plants with a systemic insecticide applied to the foliage in April or May or apply granular systemic insecticide to the soil around the trunks in early spring.
Boxwood psyllids are small insects that cause new leaves to cup as the nymphs extract sap from the tender foliage. Damage is especially noticeable on American boxwood. Psyllids may affect the looks of the plant, but unlike leaf miners, they are seldom a threat to the overall health of the shrub.
Treatment: Psyllid damage is more a question of aesthetics than anything else and will produce only scattered cupping. Prune and dispose of infected limbs in mid-May before nymphs become adults and have a chance to lay eggs.
Psyllid damage. Photo credit: extension.umd.edu
A final note: The name boxwood is single, never plural. There is no such word as ‘boxwoods.’