What are the characteristics and uses of Hokkaido boxwood?

Hokkaido boxwood is a new boxwood variety introduced by the Chinese Academy of Forestry in the mid-1980s. It is an evergreen broadleaf tree of the genus Euonymus in the family Celastraceae, and a cultivated variety of large-leaf boxwood. It has the characteristics of a tall trunk, an upright posture, evergreen foliage in all four seasons, and ornamental red fruits in winter. As the construction of new rural areas is gradually placed on the agenda, greening and beautifying the environment will become an important part of new rural construction. In order to change the single pattern of deciduous trees in temperate regions and move toward high-grade greening and beautification, more and more attention has been paid to planting evergreen, pollution-free greening and beautification tree species. Practice has proven that Hokkaido boxwood is a rare and valuable greening and beautification tree species.

I. Morphological characteristics

Hokkaido boxwood is an evergreen tree, up to 8-10 m tall. Its leaves are leathery, dark green on the upper side and light green on the underside. In the severe cold of winter the leaves remain green, with no leaf drop. The leaves are ovate or long elliptical, 5-6 cm long and 4-5 cm wide, with slightly wavy leaf margins and petioles about 1 cm long. The flowers are light yellow, 0.1-1 cm in diameter. The capsule is nearly spherical, with 4 shallow grooves, 1-2 cm in diameter; when young the fruit is light green, with the sun-facing side brownish red. The seeds are nearly round, ripening in November. When ripe, the pericarp splits open automatically, exposing the seeds covered with orange-red arils. With red fruits all over the tree among green leaves, it is quite charming and scenic whether viewed from afar or up close.

II. Biological characteristics

Hokkaido boxwood likes light but is also fairly shade-tolerant. It prefers a warm, humid climate and is cold-tolerant and drought-tolerant, remaining green at a minimum temperature of -23.9°C. Its annual height growth can reach up to 170 cm, with an average growth of about 70 cm, 3-5 times faster than ordinary large-leaf boxwood. A 5-year-old seedling can reach more than 3 m in height; a 3-year-old seedling can reach 3.5 m with a ground diameter of more than 3 cm. The growth of seedlings in the first three to four years is particularly noticeable, with few lateral branches, a thick terminal shoot, obvious apical dominance, and a single trunk growing straight up, showing excellent tree-form traits; after 5 years it enters the initial fruiting stage, and at 6-8 years it enters the peak fruiting stage. It propagates easily, and branch cuttings have a high survival rate, allowing rapid mass propagation.

III. Strong stress resistance

Hokkaido boxwood has strong resistance to powdery mildew and black spot. Through years of acclimatization, breeding, and selection, it has become an excellent new variety suitable for the climatic conditions of northern China. Its drought tolerance is also superior to that of ordinary large-leaf boxwood; it has a strong ability to absorb harmful gases and strong resistance to harmful gases such as sulfur dioxide, hydrogen, and hydrogen fluoride.

IV. Suitable growing range

It is suitable for planting in areas of northern China with cold, dry winters. In urban courtyard greening it can be planted as a solitary specimen, in rows, or in groups. In the north it can flower and fruit normally, and its seeds have germinating power and can be used to propagate offspring.

V. New forms available

In the more than 20 years since Hokkaido boxwood was introduced to China, two new varieties of Hokkaido boxwood have been bred and are worth using in production.

1. Winter Red Hokkaido boxwood

It is an evergreen shrub or small tree. Its leaves are glossy and leathery, dark green in spring, summer, and autumn, while in winter both the branches and leaves turn bright red, and it bears twice as many fruits as the ordinary variety. It is highly ornamental and strongly cold-tolerant, suitable for urban greening in the north.

2. Variegated-leaf Hokkaido boxwood

It is an evergreen shrub or small tree. Its leaves remain interspersed with golden yellow and emerald green all year round. It has a strong upright habit and is resistant to severe cold. In winter the tree is full of red fruits, and it can be used for hedges or trimmed shapes.

VI. Current greening situation

The area north of the Huai River lies in the warm temperate zone, where winters and springs are dry and cold with strong winds, and after the leaves fall there are few cold-tolerant, drought-tolerant evergreen trees. At present, there are only evergreen coniferous and broadleaf tree species such as Chinese arborvitae, Chinese juniper, Oriental arborvitae, boxwood, privet, photinia, small-leaf boxwood, small-leaf privet, and in recent years introduced osmanthus, southern magnolia, camphor tree, Michelia maudiae, Elaeocarpus, palm, and Euonymus. Some of these tree species are subtropical species and have been planted north of the Huai River for only a short time; the fact that they have survived the winter for the past few years should not be taken as successful introduction. Species such as southern magnolia, camphor tree, Euonymus, and Michelia maudiae show varying degrees of yellowing, greatly marring the landscape. Most of the native evergreen trees are shrubs; the only tree species are the cypresses, but because they grow slowly and their winter foliage is dark and monotonous, they can hardly meet people's visual, landscape, and new rural construction needs.

VII. Wide range of uses

The crown of Hokkaido boxwood is a beautiful green. After autumn, the tree is covered with ripe fruits that expose red arils; strings of red fruits are set among clusters of green leaves. Even in the dry, cold depths of winter, the entire crown remains a beautiful green, with green leaves and red fruits, offering extremely high ornamental value. Hokkaido boxwood has an upright posture and is evergreen throughout the four seasons, and it tolerates pruning and shaping, making it an excellent variety for landscaping in the small towns and cities of northern China. Because it has a certain shade tolerance, it can be used to create mixed road forests.

VIII. Development prospects

In order to increase the adaptability and ornamental value of Hokkaido boxwood, horticulturists have recently succeeded in grafting it onto the native northern Euonymus bungeanus. Now the relatively abundant wild Euonymus bungeanus in rural areas can be used as rootstock to graft Hokkaido boxwood, Winter Red Hokkaido boxwood, Variegated-leaf Hokkaido boxwood, golden-leaf Euonymus japonicus, variegated-leaf Euonymus bungeanus, or golden-center and golden-edge boxwood and other colorful-leaf species of Celastraceae, creating a distinctive landscape. On the basis of continuously expanding seedling propagation, Hokkaido boxwood and its similar varieties can be brought out of cities, market towns, gardens, streets, parks, and courtyards, frequently appearing along roads, riverbanks, fields, and field edges, to build a colorful living environment.