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Boxwood: Types of Boxwood

Distinctive narrow, blunt leaf and square stem of Buxus sempervirens (common boxwood); photo courtesy of Flickr/ cc Botanischer Garten TU Darmstadt
The distinctive narrow, blunt leaf and square stem of Buxus sempervirens (common boxwood); photo courtesy of Flickr/ cc Botanischer Garten TU Darmstadt
 

Boxwood (Buxus) has been grown in the United States since the mid-17th century, when the first plants were brought to New York from Europe to plant in the gardens of well-to-do and well-established colonists.  There are now approximately 150 boxwood cultivars and species that are available to home gardeners.

The most commonly grown boxwoods are the common (or American) boxwood, Buxus sempervirens, and the English variety Buxus sempervirens 'Suffruticosa'. While many landscapers refer to boxwood as English or American, that is not really a true difference as the “American” species actually originated in England as well.

These two, beautiful, boxwood types are slowly being replaced in gardeners' affections by varieties that are less susceptible to disease due to their looser, less dense structures.

Buxus sempervirens (common or American boxwood)

grows to a large shrub or small tree (to about 25 feet) but it can be easily pruned to any height or shape. There are many common boxwood cultivars and a form, leaf size and coloration is available to fit almost any purpose. Hardy in zones 6 to 8, common boxwood has long been considered the backbone of a formal garden but it can suffer in prolonged cold weather and late spring chill. Do not prune after August 1st or branches may die back in cold weather.

Typically, B. sempervirens cultivars grow more tall than wide. New foliage is blue-green and darkens to glossy, dark green and convex, with yellow-green leaf reverse.  Narrow and bluntly pointed leaves, ½ to 1½" long, grow opposite, on square stems. Foliage is dense. The smell that many consider an unpleasant attribute of boxwood is most associated with common boxwood.

Common (American) boxwood is prone to developing fungal disease, as the dense leaf architecture retains moisture.

Buxus sempervirens ‘Suffruticosa’ (English or dwarf boxwood)

is also hardy to about Zone 6. It is a slow-growing, compact shrub, rarely growing over 3 feet, with leaves less than 1" in length. It is often grown as an edging plant and in parterres. Trials have shown B. sempervirens 'Suffruticosa' to be somewhat more resistant to leafminer problems than common boxwood; it is, however, very disease-susceptible, particularly to boxwood blight, and prone to be troubled by mites and psyllids

Korean boxwoods 'Winter Gem' and 'Wintergreen' grown with yew at Chicago Botanic Garden; photo courtesy of Flickr cc/ Cultivar413

Korean boxwoods 'Winter Gem' and 'Wintergreen' grown with yew at Chicago Botanic Garden; photo courtesy of Flickr cc/ Cultivar413

Buxus sinicia var. insularis (Korean boxwood)

is a low boxwood that grows slowly to a height of about 1½ - 3 feet and is prized in the northeast for its resistance to cold temperatures. A series of botanical name changes over a relatively short time period, along with the variability of natural characteristics, have confused gardeners and garden professionals. Most typically, Korean boxwood has a less dense growth habit than common boxwood, and rounder, shorter (½ to ¾"), thicker, medium-green leaves with rolled edges. Young branches and the undersides of leaves are covered in fine hairs. Some cultivars can grow successfully in zones as low as USDA Zone 4.

The loose plant form and tendency to discolor to a red-brown in winter detract from the reputation of Korean boxwood as a worthy replacement for  common and English boxwoods. While Korean boxwood lacks the density that gives versatility to other species, the more open habit allows greater air circulation which reduces the shrub's susceptibility to disease. Many of the cultivars considered most disease resistant are Korean boxwoods and varieties less likely to brown in winter are available.

Synonyms: B. microphylla var. koreana, B. koreana f. insularis.

Littleleaf boxwood (Buxus microphylla) typically has a more rounded leaf shape; photo courtesy of Flickr cc/ harum.koh
Littleleaf boxwood (Buxus microphylla) typically has a more rounded leaf shape; photo courtesy of Flickr cc/ harum.koh

Buxus microphylla (littleleaf boxwood)

is generally hardy in zones 6 to 9 but subject to late frost damage in the New York area. These plants grow quickly in the three to four foot range, though plant forms and sizes are extremely variable. If left to grow in their natural habit, shrubs are up to twice as wide as they are tall.

The mid-green leaf color changes to a strong orange after the first fall frost, particularly if not protected by some shade, though some cultivars retain greener coloring all year. Individual leaves tend to be more rounded than common box and grow from square, grooved stems. The leaves are similar to Korean boxwood, 1/3 to one inch in size, and waxy. The growing habit is even more loose than Korean boxwood and can be ungainly, though regular pruning can create an attractive shape.

Without the cold tolerance of Korean boxwood and tending to become strongly discolored in winter, littleleaf boxwood is less desirable in the northeast than it is in warmer climates.

Synonym: B. microphylla var. japonica

The Sheridan hybrid 'Green Velvet' at Chicago Botanic Garden;photo courtesy of Flickr cc/ cultivar413
The Sheridan hybrid 'Green Velvet' at Chicago Botanic Garden; photo courtesy of Flickr cc/ cultivar413

Buxus sinicia var. insularis x B. sempervirens (Sheridan hybrids)

are widely grown hybrids developed by Sheridan Nursery in Ontario, Canada to combine one parent's hardiness with the other's attractive form. The cultivars 'Green Gem',  'Green Mountain', and 'Green Velvet' all form taller, 4 foot+ plants and are considered outstanding varieties. All discolor in the winter and are prone to leaf miner damage and fungal disease.

Cultivar Choice 

Careful cultivar selection can reduce the incidence of disease, particularly boxwood blight. Many consider common boxwood and English boxwood  to be the most attractive varieties for plant form and leaf but they are also the most disease-prone.

The Boxwood Society of America lists ten cultivars that, after evaluation by Dr. Kelly Ivors of North Carolina State University, proved to be less blight prone than other common cultivars evaluated. No English, American or Sheridan Hybrid boxwoods warranted inclusion on this list.

It should be noted that plants that have shown greater disease resistance in one trial will not necessarily prove as resistant in another environment. Further research is underway in an attempt to establish consistent recommendations on cultivar choice to growers and gardeners. Many gardeners are simply replacing boxwood with similar plants in their gardens. (A good list or replacement plants for the New York area, and their growing characteristics, suggested by the Cornell University Cooperative Extension can be found through this link.) 

While more is learned, careful cultivation of plants improves strength and disease resistance.

The Ivors list of cultivars includes mostly Asiatic cultivars belonging to B. microphylla (littleleaf boxwood) or B. sinica var. insularis (Korean boxwood), including

Buxus microphylla ‘Green Beauty’ 

a medium sized, bush-type, littleleaf boxwood, fast-growing and dense; pollution tolerant and hardy in zones 6 to 9. Grows up to 5 feet tall and wide, tending to be wider than tall, and requiring shaping or it will become loose and leggy. Reasonably drought resilient in dry periods. Found to be highly susceptible to leafminers in trials.

Buxus sinica var. insularis 'Eseles' (Wedding Ring®)

a rounded, Korean boxwood with leaf variegation that changes from lime to gold over the growing season. It is hardy from zones 5 to 9 and grows, slowly, to 2 to 3 feet.

Buxus sinica var. insularis ‘Wintergreen’

a slow-growing, Korean boxwood cultivar that does especially well in the northeast as it is particularly hardy and less prone to winter bronzing. Grow in zones 4 to 9; it needs some pruning or may sprawl and is usually grown to a height of around 3 feet. Somewhat resistant to leafminers in trials.

Buxus microphylla ‘Peergold’ (GOLDEN DREAM):

has gold variegation that creates the illusion of lime green foliage. A smaller plant for zones 6 to 9, it is compact, dense and rounded, growing at a medium pace to about 3 feet tall. Found to be highly resistant to leafminers in trails.

Buxus sinica var. insularis 'Winter Gem'

a Korean boxwood that matures to a rounded 3 feet. It is very winter hardy, growing in zones 5 to 9, but appreciates some wind protection, and some shelter when grown at the cold end of its range. Slow growing and dense, the name signifies the cultivar's resistance to winter bronzing of leaves.

Buxus sinica var. insularis 'Nana'

a dwarf, Korean boxwood, hardy in zones 4 to 9 and grown for its small, wide stature, around 2 feet tall and relatively quick growth. Prune lightly to encourage a dense structure. Some growers find that it can brown in the first few winters if planted in sun and then acclimate and improve.

Buxus sinica var. insularis 'Franklin’s Gem'

a slow-growing and low spreading cultivar that reaches little more than 2 feet tall and 3 feet wide. Prune lightly to prevent opening of the plant over time. Hardy in zones 5 to 9.

Buxus sinica var. insularis 'Wee Willie®'

a dwarf, Korean boxwood that grows only 2 feet tall. Rounded and dense, it is hardy in zones 5 to 9 and reputed to resist winter bronzing.

Buxus harlandii 'Richard'

Harland boxwood is less well known and has larger leaves than more common types. It grows up to six feet tall, in a vase shape that requires little pruning, but only in zones 7 to 9. A protected location is necessary in the New York area. Strangely, plants sold as B. harlandii are sometimes, in fact, B. microphylla or B. sinicia var. insularis and more cold hardy.The cultivar 'Richard' has a nearly heart-shaped leaf and can reach six feet (but tends to stay smaller). Found to be highly resistant to leafminers in trails.

Ilex crenata (Japanese holly) is frequently considered as a replacement for boxwood planting in our area; photo courtesy of Flickr cc/ harum.koh
Ilex crenata (Japanese holly) is frequently considered as a replacement for boxwood planting in our area; photo courtesy of Flickr cc/ harum.koh

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