PRUNING
Slow growing, broad-leaved evergreen shrubs, like boxwood, have the majority of their growth from buds at or near the ends of the branches. Some develop a dense outer shell of foliage with only a few leaves in the center. Most require little pruning except when an occasional branch outgrows the general habit of the plant. Boxwood have more of a main branch framework than do other faster growing shrubs (e.g., Forsythia) that tend to sprout vigorously from the base.
Some boxwood have central leaders with tall, straight trunks such as B. sempervirens ‘Fastigiata’, ‘Pyramidalis’, and ‘Graham Blandy’. Others have several main branches with spreading crowns such as B. sempervirens >Suffruticosa= and B. microphylla var. japonica. Then there is the low mounding habit, which would include B. microphylla ‘Compacta’, ‘Green Pillow’, and B. microphylla var. japonica ‘Morris Midget’. Between these extremes, many intermediate forms occur. Proper pruning can exploit the natural characteristics of a specific cultivar as well as enhance its appearance and maintain its health. There are three different methods, each of which may be appropriate depending on the desired results: thinning, shearing, and heading-back.
THINNING
Thinning is the most important and reliable method to avoid Macrophoma leaf spot and Volutella leaf and twig blight, as well as Cylindrocladium on B. sempervirens ‘Suffruticosa’. It is a type of pruning that reduces the density of the foliage by removing small branches about 6" to 8" (15 cm to 20 cm) in length. This should be done each fall, between October and December, when the temperature is above freezing. Boxwood branches are very brittle and can break easily in freezing weather. The cuttings can be retained and used for holiday decorations. If not cut, the overly-thick foliage at the end of the branches blocks both sunlight and air from reaching the interior portions of the shrub.
Annual thinning of the foliage in fall is the single most important method to prevent disease on English boxwood.
Without adequate sunlight, the interior leaf shoots of boxwood die, leaving very little green foliage on the majority of the branches. This causes the branches to become thin and weak, making them more susceptible to breakage by snow and ice. If the foliage is properly thinned, the branches will have leaves growing along the entire length, which gives the branch great strength and allows good interior branching.
Without adequate air circulation, the interior is chronically overly moist. The effects of rain and dew, along with stagnant air, create the perfect environment for fungal diseases that can destroy English boxwood. The primary cause of nearly all fungal diseases on English boxwood is overly thick foliage which reduces or blocks essential air circulation. In most of the remaining cases, boxwood growing too close to other plants or too close to a wall or barrier is generally responsible. These situations cause poor air circulation, not from within the plant, but from the restrictive nature of the surrounding environment. See Macrophoma leaf spot, Volutella leaf and twig blight, as well as Cylindrocladium pp. 000-000 for additional information.
While the slow growing, compact boxwood such as B. microphylla ‘Compacta’, ‘Green Pillow’, B. microphylla var. japonica ‘Morris Dwarf’ and ‘Morris Midget’, B. sempervirens ‘Memorial’ and ‘West Ridgeway’ have very dense foliage, they are not prone to fungal diseases. However, thinning will encourage important interior foliage.
SHEARING
In some North American and virtually all European gardens, boxwood are regularly sheared to control their size or shape. Probably the most common example of this is growing boxwood as a hedge. The hedge is sheared to contain its size and to obtain a geometrically straight top and sides. It is also possible to control the size by repeated shearing of new growth. Shearing is a critical requirement in both the creation and maintenance of topiary specimens. While there are many personal preferences and site-specific considerations that call for other tools, electric shears are the most effective ones to cut straight edges for a hedge. Hand shears excel for work on topiary specimens.
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