An Introduction to Boxwood
There’s always more to learn! Here are some important considerations to improve the culture of boxwood.
Boxwood has a long, proud, even romantic history in the landscape. From a small pot on a window sill to a large formal parterre, boxwood options exist for all gardeners who have a passion for growing and displaying this versatile shrub. It’s all about having boxwood nearby, an omnipresent old friend offering beauty and reassurance.
Even after centuries of cultivation, there is still something boxwood can teach us. Boxwood is tenacious, so a small cultural deficiency for a few years is of little concern. As the years slip by and become a decade, however, this chronic “small cultural deficiency” begins to weaken boxwood. Perhaps the leaves aren’t quite as large or as green as they once were. Or, the leaves on a branch suddenly and mysteriously become brown-colored. When these and/or other maladies appear, they are often overlooked or misunderstood and incorrect treatment is provided. So, the small problem from many years ago has now become serious. How did this happen? What is the correct care?
Some pests or diseases are merely a nuisance, while others can destroy a boxwood in just a matter of a few weeks. As custodians of any majestic plants, we need to identify and correctly diagnose these concerns in a timely manner. Perhaps one of the greatest joys in growing boxwood is learning what they need to thrive, not simply survive.
Non-living conditions are often overlooked but are always an important consideration for healthy boxwood. These include: air quality, maintenance practices, soil compaction, limited area for the roots to grow, salt, and excessive sunlight.
There is no regular schedule to guide the fertilization of boxwood. Fertilization is unnecessary unless the soil has a proven deficiency; therefore, the most reliable way to determine your boxwood’s needs is by testing the soil. Soil samples analyzed by a professional laboratory will provide appropriate fertilizer recommendations for a specific site. Applying fertilizer when unnecessary, or applying the wrong type or at the incorrect rate, may result in the death of the foliage, a branch, or even the entire shrub.
Site selection and soils
One of the easiest and most important practices to ensure the health of boxwood is to maintain a proper soil pH. When the soil pH is in the correct range (between 6.8 and 7.5), the nutrients in the soil are then most available to the plant. If the pH is below this recommended range, it is too acidic and requires application of dolomitic lime. This type of lime, with its low oxide content, can persist in the soil for three to seven years depending on application rates, existing soil pH, and soil type. Because agricultural lime, burned lime, and dehydrated lime buffer the soil pH for generally less than one year, they are not recommended for use on boxwood.
Mulch greatly inhibits weed growth. Weeds will consume water and nutrients in the soil that would otherwise be available to boxwood. Mulch will reduce weed vines growing up the shrub, shading the leaves and reducing plant vigor. Soil fertility is enhanced as nutrients leach from the mulch, and the organic matter content of the soil is increased as the mulch decomposes and enters the soil. The secret is installing the right type of mulch and applying it correctly.
The boxwood leafminer, Monarthropalpus buxi, is actually a gall midge and not a leafminer. It is a serious insect pest of boxwood, as a high population can defoliate and kill weak boxwood. Injury is caused by the larvae (maggots) feeding inside the leaf, resulting in premature leaf drop. Most cultivars of B. sempervirens and B. microphylla are susceptible to boxwood leafminer. There are no effective natural predators or parasites known for boxwood leafminer.
The mite is not an insect, but a member of the Arachnida class (to which spiders belong). The boxwood mite, Eurytetranychus buxi, is a rather common and widespread pest. Mites are very inconspicuous. Early damage is typically not obvious, and they are often overlooked until high populations and extensive damage have occurred. The mite is a serious problem on most B. sempervirens cultivars, particularly those grown in sunny locations. It also attacks cultivars of B. microphylla var. japonica.
The boxwood psyllid, Psylla buxi, attacks B. sempervirens and its cultivars, as well as some B. sinica var. insularis cultivars. While probably the most common boxwood pest, it is generally not as damaging as other pests.
There are a variety of armored or soft scale insects that attack boxwood, such as the wax scale (Ceroplastes ceriferus), which has common names that include Japanese, Indian and ceriferus scale; oystershell scale (Lepidosaphes ulmi); cottony cushion scale (Icerya purchasi); and others.
For the majority of boxwood, these scales are only occasional problems and are not generally destructive. However, in isolated cases, scale damage can have fatal results, particularly if the boxwood is already weak. Perhaps most notable, the wax scale (with its wide distribution) can attack boxwood almost everywhere it is grown.
Phytophthora is a Greek word meaning “plant destroyer,” which accurately defines its effects. There are about 40 species, but only one is important to boxwood: Phytophthora parasitica. This soil-borne fungus affects all cultivars of Buxus sempervirens at any age or size. It damages leaves, stems, and roots. Infection begins when the soil is wet and cool (about 57°F to 70°F), which is common in both spring and fall. The disease will progress and cause injury only under higher soil temperatures. Damage occurs at about 24°C, with the greatest effects occurring at 29°C. Boxwood infected with Phytophthora seldom survive. Phytophthora produces spores that move in a water film in the soil. As a result, Phytophthora is most damaging to boxwood growing in poorly-drained soils.
Calonectria pseudonviculatum (synonym Cylindrocladium buxicola), commonly known as boxwood blight, is a relatively new fungal disease of boxwood, first discovered in Great Britain in 1998. By 2001, it had spread to Ireland, Belgium, Italy, France, Holland, and surprisingly, New Zealand. This disease was identified in the mid-Atlantic region of the United States in 2004 and has since rapidly spread to other regions.
As with other boxwood fungal diseases, Calonectria pseudonviculatum prefers high humidity, moderate temperatures of 77°F, and poor air circulation. This disease has proved particularly aggressive where boxwood is tightly sheared or heavily fertilized or where plants are placed too closely together. All of these conditions result in over-dense foliage, which leads to poor air circulation.
The spores are spread by surface water, rain drops, or overhead irrigation. Also, the sticky spores can attach themselves to animals (including birds) and humans (clothing, equipment, etc.), potentially carrying them long distances. Because the earliest signs and symptoms of boxwood blight may not be apparent by visual inspection (infected, but asymptomatic), this scenario offers the greatest opportunity for rapid widespread distribution.
Macrophoma Leaf Spot
Macrophoma candolleri, an imperfect fungus, attacks weakened or decaying branches of many cultivars of Buxus sempervirens, especially ‘Suffruticosa’. Fortunately, Macrophoma is only of minor concern and is a secondary invader or weak parasite. Usually the fungus infects plants that have been weakened by poor culture, winter burn or overly thick foliage.
Volutella, or canker
This imperfect fungus, Volutella buxi, is stem blight or canker. It primarily affects Buxus sempervirens and its many cultivars. An opportunistic pathogen, Volutella most often causes problems on weakened or stressed boxwood. A few common examples would include a mature B. sempervirens ‘Suffruticosa’ with overly thick foliage, boxwood growing in a low soil pH, and frequently sheared shrubs. Volutella is more common during periods of high humidity. A heavy infestation results in defoliation and the death of entire stems.