Q. How do I grow boxwood, and which varieties are good for the Northeast?
Boxwood (Buxus) is a very versatile and useful evergreen shrub that can be used as edging, hedges, screens and specimen plants. It will grow in full sun but prefers a slightly shaded location. As long as the ground is not too compacted, boxwood will thrive in many soil types. In areas with acid soil, lime should be added to raise the pH to 6.8-7.5. Soil should be well drained, as boxwood won't tolerate waterlogged conditions. A 2-3-inch layer of mulch helps keep the plant's shallow roots cool and moist. Amending the soil with compost will condition it as well, which enables the plant take up the nutrients it requires.
The most common problem encountered with boxwood is browning (bronzing) of the leaves in winter. This can often be prevented or reduced by planting the shrubs in a partly shaded site protected from winter winds. An alternative is to provide protection with a screen (e.g. burlap) or, if the shrubs are small, cover them with evergreen branches. Bronzing does not harm the plants; they will grow back green in the spring. Keeping the shrubs healthy will also help prevent bronzing.
A number of insect pests, particularly boxwood leafminer, can be a problem and should be treated. English boxwood (Buxus sempervirens ‘Suffruticosa’) is susceptible to a fungal disease known as “boxwood decline." Again, healthy plants in a proper growing environment are less susceptible.
If you need to raise the pH of your soil, dolomite lime (calcium carbonate or calcium magnesium carbonate), with its low oxide content, will maintain an elevated soil pH for longer period of time than quicklime (calcium and magnesium oxides) or slaked lime (calcium and magnesium hydroxides). Some shade, up to about 20 percent, reduces both summer scald and winter injury. Shade also reduces mite damage, as does summer irrigation. Do not overfertilize. Do not prune when the temperature is below 40°F for several weeks.
The preferred time to prune boxwood (Buxus) is late winter or early spring in the northeast. Boxwood takes to shearing and shaping quite well. Propagation can be done in late summer or early fall.
Preventing foliage diseases of English Boxwood
Mature English boxwood is prone to developing overly dense foliage, often with fatal results. The dense growth reduces sunlight into the center of the shrub, which causes premature foliage drop, weakening the plant. The resulting leaf debris accumulates in the lower branches, causing abundant and vigorous aerial roots to grow in this moist and dark environment. Eventually, the exposed aerial roots will die, shocking the weakened plant.
When planting, give each plant room for sunlight and good air circulation. Thin English boxwood in late fall if the foliage completely hides the view of the interior branches. To thin, using sharp bypass pruners, reach inside about 6 inches, and remove a twig. Continue to prune until the small interior twigs become intermittently visible, along with the older, light green interior leaves. When pruning is completed, the shape and size of the boxwood should look unchanged. Use clippings for propagation if desired.
Species and varieties to grow
The most commonly grown boxwoods are the common, or American, boxwood (Buxus sempervirens) and the English or dwarf variety. However, there are about 70 species of boxwood growing worldwide, and many cultivated varieties have been developed.
Common boxwood grows to a large shrub or small tree (to about 25 feet) but it can be easily pruned to any height or shape. The variety ‘Vardar Valley’ is among the hardiest of B. sempervirens cultivars available and retains its green color well in winter. It has a broad, spreading habit, and reaches a height of 6-7 feet.
The English or dwarf variety ‘Suffruticosa’ is also hardy to about Zone 5. It is a low-growing, compact shrub, rarely growing over 3 feet. It is often grown as an edging plant and in parterres. It is, however, very disease-susceptible.
B. sempervirens 'Graham Blandy' is fastigiate variety that grows to 6-8 feet but only 15 inches wide. It will make a fine vertical accent in the garden.
The Korean boxwood (B. microphylla var. koreana) is a hardy variety that grows slowly to a height of 1.5-2 feet.
B. sinica var. insularis ‘Wintergreen’ is also known as Korean boxwood. It is hardy to Zone 4 and grows to 2-4 feet in a spreading shape.
‘Green Mountain’ boxwood is a hybrid between B. sinica var. insularis ‘Wintergreen’ and B. sempervirens ‘Suffruticosa’. It's a dense, upright variety that can eventually grow to 5-7 feet tall.
It is worth noting that boxwoods are relatively deer resistant. They have a characteristic scent that some people find objectionable.
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- Courtesy of NYBG Plant Information Service