Boxwood Care: How To Identify and Treat 4 Common Pests and Diseases

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Who doesn’t love boxwood? There’s something about the broadleaf shrub that turns every garden into a private haven. Boxwood functions well as a stand-alone shrub, but as a hedge it really shines; providing a sense of enclosure while defining separate areas of the garden. Perhaps best of all, boxwood is malleable. It can be simply left alone or carved into any shape to create ‘rooms,’ borders, edging and parterres, offering the gardener an infinite array of possibilities.

Ancient Roots

According to the European Boxwood and Topiary Society (EBTS), boxwood has been around for thousands of years and has survived in a wide variety of climatic conditions. Pollen records indicate that box plants were growing in ancient forests in England as far back as the end of the Ice Age (or more than 11,000 years ago).

With such a long history on earth, it comes as no surprise that the boxwood family, Buxaceae, has spawned lots of relatives. Today the extended family comprises five genera of trees, shrubs and herbs and around 247 flowering species. Buxus (commonly known as box in England and boxwood in North America) is a genus of about 70 plants. This includes the species most familiar to Westerners, Buxus sempervirens, which is adapted to the more temperate parts of the world.

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Boxwood in its natural state on the chalk downs of Chilterns, England

In its natural state, a mature box plant often breaks under its own weight. And, once the branches touch the ground, they grow their own roots, spawning new plants that can quickly fill the forest understory. Thousands of years ago it was natural to find boxwood growing under large trees in forests and today the remnants of ancient plants can still be found growing on the chalk downs of Chilterns, England.

In modern times, however, boxwood is associated more with hedges and topiary than with ancient forests. A common foundation planting, it has also served as a low edging in garden beds since the 16th century. Boxwood topiary, a practice that began in Roman times, continues to this day in great gardens across the world, including the lovely Jardin d’Eyrignac, in Dordogne, France.

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Photo Credit: Jardin d’Eyrignac

 

Growing boxwood

When growing boxwood, it’s important to remember the shrub originated in the forest, so it naturally performs best in dappled shade. It grows in a wide variety of soil types, but prefers soil on the alkaline side, or a pH of 7.1 or higher. In fact, most growers recommend you maintain a soil pH of 7.5 to 8.6 to enable your box shrubs to flourish.

Feed your boxwood with a balanced fertilizer high in nitrogen in the early spring, making sure to distribute it evenly and keep it off the leaves. To protect the roots, spread a 1- to 2-inch layer of organic mulch around the base of the plant, keeping a 6-inch distance from the trunk. If the soil surrounding your shrubs is acidic, you made need to add dolomitic lime to bring up the pH to provide the proper growing conditions.

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Boxwood does poorly in compacted or heavy soils. Dense soils like clay can cause major drainage problems. If you live in an area with heavy soil, try making it more porous by adding lots of organic matter. Add compost, finely-ground wood chips, well-rotted manure, or one of the many soil improvement products (like Leaf Gro) available at your local nursery.

 

An ancient plant with modern-day diseases

Although generally viewed as low maintenance (some specimens have lived 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 and elsewhere.

Boxwood Blight

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.

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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.

 

Boxwood Decline

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.Screen Shot 2016-03-14 at 6.31.31 PM

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.

 

Boxwood Leafminer

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.

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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 Psyllid

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.

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Psyllid damage. Photo credit: extension.umd.edu

A final note: The name boxwood is single, never plural. There is no such word as ‘boxwoods.’

 

This entry was posted in Gardening How-To, Plants and tagged by carole funger. Bookmark the permalink.

About carole funger

I'm a garden designer and Maryland Master Gardener living in the Washington, DC area. I blog about new trends in horticulture, inspiring gardens to visit and the latest tips and ideas for how to nurture your own beautiful garden. Every garden tells a story. What's yours?

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