Boxwood has been a garden staple for centuries. Not only does it add structure to outdoor spaces, but 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 heightens its appeal. There’s just one problem: boxwood is often plagued by a host of pests and diseases.
ONE BIG FAMILY
Most of us know boxwood as a shrub, but it might surprise you to know that the broader family (Buxaceae) includes trees, herbs and around 247 flowering species. Even so, the species most familiar to Westerners is common box, or Buxus sempervirens.
In America, Buxus is called boxwood.
Boxwood fleur-de-lys at George Washington’s Mount Vernon
And in England it is called box.
Formal English garden with box
Boxwood has served as hedging since the 16th century. Today, it is also a common foundation planting. And boxwood topiary, a practice that began in Roman times, still plays a prominent role in gardens across the world, including France’s Jardin d’Eyrignac and Prague’s lovely Vrtba Garden.
Boxwood edging at Paris’ Luxembourg Gardens
There’s a reason why boxwood has survived for centuries. Typically low maintenance, it’s slow-growing and relatively drought-tolerant. Given well-drained soil, most varieties thrive in both sun and part shade. Heavy or compacted soils, however, can kill healthy plants by depriving them of oxygen, which can end up asphyxiating their roots.
That said, ff you want your shrubs to really flourish, your soil needs to be on the alkaline side, or a pH of 7.1 or higher. These are boxwood’s preferred conditions.
This is why experts generally recommend that you avoid planting boxwood close to acid-loving shrubs like azaleas and rhododendrons. These shrubs prefer a lower pH of between 4.5 to 6.0.
Boxwood and azaleas have different pH requirements.
MIND THE ROOTS
If you ask Lynn Batdorf, former curator of the U.S. Arboretum’s National Boxwood Collection, boxwood care is all about the roots. He likens the shrub’s rootball to a pancake. Even on the largest plants, roots typically extend down no more than a foot. However, on the surface, they can travel many feet. As a result, it’s important to give the shrubs lots of room to grow.
If you want your boxwood to thrive, avoid planting anything directly underneath. And though it’s a common practice, it’s best not to site shrubs adjacent to impermeable materials like walls or pathways, which inhibit their growth. If you must plant in a boxwood’s vicinity, Batdorf suggests planting bulbs, which cause only minimal root disturbance.
This looks nice, but is actually not good for your boxwood.
FEED BOXWOOD IN EARLY SPRING
To encourage healthy growth, Batdorf recommends feeding 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 base of the plant. (He recommends pine bark over hardwood because it is less likely to suffocate the roots.)
Most importantly, make sure to leave six inches between the trunk and mulch circle. This will prevent any moisture build-up that might encourage root rot.
AN OLD-LINE PLANT WITH MODERN DISEASES
Still, despite its reputation for low maintenance (some specimens have lived for over 400 years), boxwood is saddled with its own set of pests and diseases. 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.
Nowadays, just the word blight can send gardeners into a tizzy. That’s because boxwood blight produces leaf spots, stem cankers, defoliation and eventual death of vulnerable plants. Caused by the plant pathogen, Calonectria buxicola, it first appears as dark leaf spots surrounded by black circles. Batdorf refers to these as ‘frog’s eyes.’
As the disease progresses, it spreads to cover both the tops and undersides of leaves, causing repeated defoliation. Then the stems become infected, 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.
Blight symptoms. Photo credit/umd.extension.edu
Blight will devastate your shrubs, turning them into an ugly mass of defoliated stems with black blotches. Because of this, many people are switching from English box to Japanese and Korean boxwood species. Not only are these varieties drought-tolerant, but they also 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 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.
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, making things confusing. However, it is easily distinguished from Volutella by its many tiny black fruiting bodies.
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.
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 undersides of the shrub’s leaves.
Although the adult fly dies soon after, the eggs hatch in two to three 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 tunnels called mines. Often, 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.
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 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. Photo credit: extension.umd.edu
SOUND LIKE A PRO
The name boxwood is single, never plural. There is no such word as ‘boxwoods.’
To see photo of my garden designs, including plant lists, check out my Instagram at carole.herebydesign. I post seasonally from spring through fall.