Thuja koraiensis / Korean arborvitae

Thuja koraiensis, as described in 1919 by Takenoshin Nakai (1882-1952), in Botanical Magazine, Tokyo, vol. 33, is commonly known in English as Korean arborvitae in the English language, as will as 朝鲜崖柏 (chao xian ya bai) in Chinese and as 눈측백 (nun cheugbaeg) in Korean. The epithet refers to its (partial) distribution in Korea.

Description. Korean arborvitae is an evergreen, coniferous species of broad shrub or small tree that grows to mature heights of 20 to 33 feet (6 - 10 m) tall with a trunk up to 32 inches (80 cm) in diameter, measured at breast height.

• When young, the bark is reddish-brown and smooth. With age, it becomes grayish-brown, fissured, and flaking.
• Branches may be ascending or spreading, forming a pyramidal crown, young branchlets relatively glaucous, becoming green.
• The flat sprays of foliage consist of scale-like leaves 0.08 to 0.16 inches (2 - 4 mm) long, though up to 0.6 inches (15 mm) long on strong-growing shoots. The facial leaves are triangular-ovate, with the tip short and spreading, glandular, bright green with conspicuous white stomatal bands beneath. Lateral leaves are as long as or shorter than the facial leaves, with apex incurved.
• Pollen cones are purplish in color and measure 0.08 to 0.12 inches (2 - 3 mm) long.
• Seed cones are oval, yellowish green, ripening to reddish-brown, measuring 0.28 to 0.44 inches (7 - 11 mm) long and 0.16 to 0.2 inches (4 - 5 mm) wide. Pollination occurs in May, with seed maturity in September.

Distribution. Thuja koraiensis is native to both Koreas, North and South, as well as the southern portion of the adjacent Jilin province in China (where its small polulation is protected in the Changbaishan Nature Reserve). The small South Korean plant population is protected in the Soraksan Nature Reserve. Unfortunately, most of the species' range is in North Korea, where its habitat is unprotected and threatened by habitat loss. In the wild, it grows in mountainous locales at elevations of 2,300 to 5,900 feet (700 - 1,800 m) above sea level. In exposed settings, where it becomes dense and shrubby.

Though of restricted access in the wild, in a garden setting its ornamental characteristics become quite evident: the striking contrast between the green upper side and whitish underside of the foliage. In literature, it is considered winter hardy to USDA Zones 5 or 6.

Attribution from: Fu Liguo, Yu Yongfu, and Robert R. Mill; Taxodiaceae. In Wu Zheng-yi and Peter H. Raven (editors). Flora of China, Volume 4; ©1999, Science Press. Beijing.