Boxwoods offer the gardener a rich variety from which to choose. There are nearly 100 naturally occurring species of this evergreen landscape plant. Most are native to the Caribbean Islands, East Asia, and central Europe. There are also about 300 different boxwood cultivars that grow in the northern Temperate Zone. The National Boxwood collection at the U.S. National Arboretum contains nearly 140 different species and cultivars of boxwood. It is one of the most comprehensive living collections of boxwood in the world.
Boxwood (Buxus sp.) is an ornamental, broad-leaved, evergreen shrub that has enriched gardens for centuries. The name derives from the elegant boxes made of boxwood that ladies in ancient Rome and Greece used to store jewelry. Because of its density, strength, and uniformity, the wood of boxwood was used by the ancient Greeks and Romans to make writing tablets, musical instruments, spinning tops, combs, jewelry cases, carved ornaments, inlays and veneers.
Today there are many landscape uses for boxwood. Buxus microphylla ‘Compacta’ and B. microphylla var. japonica ‘Morris Dwarf’ are used in bonsai. B. sempervirens ‘Suffruticosa’ is often referred to as edging boxwood because of its extensive use as a border in parterre gardens. Boxwood has long been used as topiary in North America. Europe (France in particular) has great enthusiasm for boxwood topiary.
Highly Recommended Boxwood Cultivars
The highly recommended boxwood cultivars are:
1. Buxus microphylla ‘Compacta’ – Grown since 1912, it is occasionally misnamed “Kingsville Dwarf from the Kingsville Nursery where this plant was first distributed. Annual growth averages ¼” to ½” which makes it the slowest growing boxwood. The small leaves average ½” long and less than ¼” wide. The plant has a tight low mounding habit and grows best in full shade. Twenty-five-vear-old plants average 10″ in height and 18″ in width. Hardy to Zone 5.
2. Buxus microphylla ‘Grace Hendrick Phillips’ – A handsome, broadly conical, dwarf growing plant with small dark green leaves throughout the year. At 21 years of age it will grow to 23″ tall and 35″ wide. Hardy to Zone 5.
3. Buxus microphylla ‘Green Pillow’ – This plant is similar to ‘Compacta’ except the leaves are about twice as large. The dense and compact habit makes it a good border or edging plant. At 30 years of age this plant can be 30″ high and 40″ wide. Hardy to Zone 5.
4. Buxus microphylla var. japonica ‘Morris Midget’ – A unique, dense boxwood, low mound with a smooth outline. It is very slow growing and at 40 years of age will be only 1 ½’ tall and 3 to 4′ wide. Hardy to Zone 6.
5. Buxus sempervirens ‘Arborescens’ – A very common landscape plant, its large size is best suited as a large hedge or in screen plantings. Specimens can live 175 years. Growth of 20′ tall and 15′ wide is rather typical for a mature 40-year-old plant. Hardy to Zone 5 or 6.
6. Buxus sempervirens ‘Elegantissima’ – The best of the variegated boxwoods, its leaves have a bright, irregular creamy-white margin with a green center. Both the leaves and plant are small in size. The mature size is 7′ tall and 7′ wide. Hardy to Zone 6.
7. Buxus sempervirens ‘Graham Blandy’ – This striking plant has straight sides that form a very narrow, upright plant. Spring growth is occasionally pulled down by spring rains and should be pruned to maintain the parallel sides. A 20-year-old plant will be about 9′ tall and only 1′ – 1 ½’ wide. Mature height is 15′-18′. Probably hardy to Zone 5.
8. Buxus sempervirens ‘Pendula’ – It has a unique J, L, or even K shape. A small plant at 30 years of age will be only 5 ½’ tall and averages about 5′ wide if not permitted to layer. Hardy to Zone 5 or 6.
9. Buxus sempervirens ‘Suffruticosa’ – The most popular and widely grown boxwood, it is usually referred to as English boxwood or true dwarf boxwood. A rounded plant with tufts of growth resembling a cloud, it has small, rounded leaves giving the plant a dense habit. Noted for its slow growth rate, averaging ¾” to 1 ¼” per year. Hardy to Zone 5.
10. Buxus sempervirens ‘Vardar Valley’ – Originated in the Vardar River Valley of Macedonia and was selected for its cold-hardy characteristic. It retains its dark green color throughout the winter. Spring growth has a prominent bluish cast that slowly weathers off by late summer or fall. Hardy to Zone 4.
11. Buxus sinica var. insularis ‘Justin Brouwers’ – A very superior, coldhardy Korean boxwood. Growing in a handsome conical shape, the pointy, dark green leaves make this a dependable plant. Hardy to Zone 4.
12. Buxus ‘Green Mountain’ – The carefree foliage remains dark green throughout the winter. Has a good, dense pyramidal habit. A 10-year-old is 3′ 6″ high and 18″ wide. The mature size is unknown. Hardy to Zone 4.
English Boxwood Culture
The single most important maintenance activity for keeping English boxwood healthy is thinning. Ibis improves the air and sun circulation through the interior of the plant, reducing the chance for infection by diseases such as Macrophoma and Volutella. First, thin the plant to reduce the dense foliage and prune out dead twigs. A vigorous shaking of the branches will then force the debris to fall to the ground. Finally, a leaf rake can be used to collect the debris, which should then be removed from the site.
The soil pH should be between 6.5 and 7.2 for English boxwood. Boxwood should therefore not be planted near acid-loving plants such as azaleas or hollies. If the pH is below the recommended range, add dolomitic lime.
Early fall is the best time to plant or transplant. This allows the boxwood to produce new roots before the new foliage appears. The root ball ought to be at least as wide as the drip line. The depth of the ball is usually determined by the height of the plant. A 3 to 1 ratio provides a general guideline. For example, a 6′ tall boxwood should have a root ball 1 ½’ to 2′ deep. When planting consider how large it will be at maturity. Future problems of overcrowding can be avoided by properly spacing the plant to account for its ultimate size.
English boxwood does best with partial sun during the growing season. The site should offer protection from sunshine and wind during the winter. Plants exposed to continual, direct sun in winter will have reddish-brown or yellow leaves due to rapid temperature changes. Boxwoods planted with exposure to the south or west sun in the winter often experience winter bronzing.
Apply and maintain mulch to a depth of one inch. Avoid mounding mulch under branches, which encourages adventitious rooting, or next to the trunk, which may attract moles.
Tying string around English boxwood will protect it from snow and ice damage. First, tie the string securely to the main trunk at the base of the shrub. Then wrap the string in an upward spiral pressing the branches upwards and inwards. Work up to the top of the plant then back down and tie the string onto the trunk again. The rows of string should be about 8″-10″ apart to provide the best support.
Propagation by stem cuttings can be successfully accomplished from July to December. Cuttings are taken from one or two-year-old branchlets. Remove the leaves from the bottom 1″ of the cutting. Treat this bottom portion with a rooting hormone, then place the cutting in a container with a media mix. Successful mixes have an equal portion by volume of pine bark and perlite, or coarse, sharp builder’s sand and perlite. Rooting usually occurs in two to three months. During this time, the cuttings benefit from high humidity. The plants can be planted outside in a protected area the following spring.
This article is derived from materials written by Lynn R. Batdorf, curator of boxwood, perennials, and crabapples at the National Arboretum. He also serves as the International Boxwood Registrar and is the author of Boxwood Handbook: A Practical Guide to Knowing and Growing Boxwood, the second edition of which was recently published.