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

Boxwood balls in the landscape

Deep green spheres of English box

Boxwood has been a garden staple for centuries, instantly infusing a landscape with structure and elegance. Its dense, evergreen foliage can be sheared into almost any shape imaginable. For those of us on the East Coast, the fact that deer won’t eat it only adds to its appeal. There’s just one problem: it’s plagued by a bunch of pests and diseases.

ONE BIG FAMILY

Most of us know boxwood as a shrub, but in fact the family includes trees, shrubs, herbs and around 247 flowering species. The species most familiar to Westerners is Buxus sempervirens, also known as common box. 

In America, Buxus is called boxwood.

Boxwood parterre in fleur-de-lys pattern at Mount Vernon

Boxwood fleur-de-lys at George Washington’s Mount Vernon

And in England it is called box.

Formal English garden with boxwood topiary

Formal English garden with box

Boxwood has served as hedging in gardens since the 16th century. Today, it is also a common foundation planting. And boxwood topiary, a practice that began in Roman times, is still practiced in gardens across the world, including Prague’s lovely Vrtba Garden.

Low boxwood edging at Paris' Luxembourg Gardens

Boxwood edging at Paris’ Luxembourg Gardens

HOW TO GROW BOXWOOD

Most boxwood cultivars will grow in full sun to part shade and once established, are relatively drought tolerant. Like all plants, they prefer well drained soil. Heavy or compacted soils that retain water can cause boxwood roots to darken and rot.

If you want your shrubs to really flourish, however, make sure your soil is on the alkaline side, or a pH of 7.1 or higher.

In fact, due to this preference for alkaline soil, experts generally recommend that you avoid planting boxwood close to azaleas and rhododendrons. That’s because these shrubs prefer acid soil, which has a lower pH of between 4.5 to 6.0.

Boxwood and azaleas have different soil requirements

Boxwood and azaleas have different pH requirements.

BE MINDFUL OF THE ROOTS

According to Lynn Batdorf, former curator of the U.S. Arboretum’s National Boxwood Collection (and internationally-recognized expert on everything boxwood), a boxwood rootball looks a lot like a pancake. Even on the largest shrubs, the roots typically extend down no deeper than a foot. On the other hand, the surface roots can travel many feet behind the drip line. As a result, it’s important to give the shrubs ample room to grow.

This includes avoiding planting anything directly underneath or around them. And although it’s a common practice, it’s best not to site shrubs adjacent to impermeable materials like walls or pathways that inhibit their growth. If you must plant in a boxwood’s vicinity, Batdorf suggests planting bulbs so that you only disturb the roots once.

Boxwood hedge next to a cobblestone path

This looks nice, but is actually not good for your boxwood.

FEED BOXWOOD IN EARLY SPRING

To encourage healthy growth, feed your boxwood in early spring with a balanced fertilizer high in nitrogen. Keep the soil cool and protect the roots by spreading a 1 to 2-inch layer of pine bark mulch around the plant base. (Batdorf recommends pine bark because it’s lighter than hardwood, which can suffocate the roots.)

And make sure to leave a 6-inch distance from the trunk to prevent any moisture build-up that might encourage root rot.

Formal garden with boxwood spheres, hedges and a white bench

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. First seen in England in the 1990s, blight has traveled overseas and is now decimating landscapes across America.

BOXWOOD BLIGHT

Boxwood blight is caused by the plant pathogen, Calonectria buxicola, which produces leaf spots, stem cankers, defoliation and eventual death of vulnerable plants. The disease first presents as dark leaf spots surrounded by black circles. Batdorf refers to these as ‘frog’s eyes.’

boxwood leaf spots

As the disease progresses, it spreads to cover both the tops and undersides of leaves, leading to repeated defoliation. Next the stems become infected, too, forming black lesions or cankers. Once the cankers girdle the stems, they weaken the plant and kill the branches. Eventually, deprived of its sources of energy, the boxwood succumbs from exhaustion.

BoxwoodBlightStemCankerswithinset

Blight symptoms. Photo credit/umd.extension.edu

Due to the prevalence of blight, many people are switching from susceptible English box, Buxus sempervirens ‘Suffruticosa’ to Japanese and Korean boxwood species. Many of these varieties are drought-tolerant and have good disease resistance. Moreover, they retain their dark green color all winter. 

Treatment: If you suspect blight, first get a diagnosis from a trained professional. Start by pruning out diseased stems, making sure to sterilize all clothing and equipment. Then rake up and dispose of the foliage. (Some people even vacuum.) Last resort would be to apply fungicides such as chlorothalonil or chlorothalonil mixed with thiophanate methyl. These have shown some promise in controlling this disease, although they must be applied every few weeks throughout the growing season.

For a more in-depth article on boxwood blight and ways to deal with it, click here for tips from Lynn Batdorf.

Updated 7/28/2018 Many professionals are now recommending the removal of the entire plant, as well as the soil around it to prevent the spread of the pathogen.

BOXWOOD DECLINE

Boxwood decline is common to both American and English box. It is usually caused by a combination of diseases brought on by poor drainage, excessive mulch, soil compaction, and occasionally weather.

Boxwood decline is often caused by the fungal disease Volutella. Its effects appear ahead of new growth in the spring, when leaves on the tips of infected branches turn red, then bronze and finally yellow. As infected branches die back, a dried-brown patch takes shape in the shrub. In moist areas, the disease will also produce pink fruiting bodies along infected leaves and stems.

Another fungal disease, Macrophoma, also causes leaf spot and straw-colored leaves. However, it is easily distinguished from Volutella by its many tiny black fruiting bodies. 

Fungal damage on boxwood

Fungal damage. Photo credit: University of Maryland

Treatment. The best way to control for fungal diseases is to thin your shrubs regularly. This will 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. Afterwards, 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 insect can be found swarming around plants in the spring. As soon as new growth appears, adult female leafminers insert their eggs into the underside of the leaves.

Although the adult fly dies soon after, the eggs hatch in 2 to 3 weeks into maggots that grow and feed for the rest of the summer. Feasting on the tissues between the outer surfaces of the leaves, the larvae eventually create blotch-shaped mines. Heavy infestations will result in premature leaf drop.

Unfortunately the larvae also spend the winter in the leaves before pupating the next spring. Leafminers are considered the most destructive insect pests known to boxwood.

 

Close up of leaf miner damage on boxwood leaves

Leaf Miner damage. Photo credit/Cornell University

Treatment: If you observe insects swarming around your shrubs, treat them 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 box. 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 on boxwood leaves

Psyllid damage. Photo credit: extension.umd.edu

SOUND LIKE A PRO

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

This post was updated April 2019. 

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