In Winter, ‘Kobai’ Flowering Plum Opens its Fragrant, Deep Pink Flowers

By Corinne Kennedy

‘Kobai’ flowering plum in the entry courtyard of the Seattle Japanese Garden. (photo: Chie Iida)

Prunus mume is an iconic flowering tree in the culture and gardens of Japan, where it is known as ume. Native to China (and known there as mei), it has long been cultivated in both countries, beloved for its delicate winter flowers and for the many culinary uses of its small, sour fruit. A tree with various common names—including ume, mei flower, Chinese plum, Japanese flowering plum, and Japanese apricot—it’s generally known as flowering plum in the West. We’re fortunate that three Prunus mume ‘Kobai’ trees were planted two years ago on the eastern edge of the Seattle Japanese Garden’s entry courtyard. Even before the Garden re-opens each year in March, they delight Arboretum visitors with their very early, deep pink blooms.

Prunus is a large genus that includes deciduous trees such as cherries and peaches, as well as evergreen trees and shrubs, such as English laurel. The deciduous plants have generally been bred either for their fruit or as ornamentals, but Prunus mume is cultivated for both beauty and fruit. As an ornamental, its main season of interest is mid to late winter, when small, dainty, fragrant flowers open on the bare branches, persisting in the cold weather for many weeks.

Prunus mume is native to China—growing in the wild inthe mountains of southwestern China, at elevations of 5,000 to 10,000 feet—and also Korea, according to some authorities. In cultivation for more than two millennia, it was first domesticated for its fruits, prized for their sour flavor, and later cultivated as an ornamental. Over time, its beauty and cultural importance came to be celebrated in poetry, paintings, woodblock prints, textiles, and ceramics. Once introduced into Japan, Korea, Taiwan, and Vietnam, itnaturalized on forested slopes and along streams. As in China, it was widely cultivated for its flowers and fruit, and celebrated in literature and the arts.

Woodblock triptych of a walkway through flowering plum trees, Kameido umeyashiki zenzu (A complete view of the Plum Estate, Kameido), 1836. (image: Utagawa Hiroshige, Library of Congress collection: Fine Prints: Japanese, pre-1915)

Prunus mume is more closely related to apricots than to plums, and is referred to as a flowering apricot in Japan. (The large-fruited, edible apricot tree is another species, Prunus armeniaca, native to northeastern China.) It was introduced from Japan to the West in about 1844, but even today it remains relatively uncommon in Europe and North America—as is the common name flowering apricot. Instead, it’s inaccurately referred to as a flowering plum or plum blossom, possibly in response to seeing images of Prunus mume in Chinese and Japanese paintings and the other arts.

This terminology is needlessly confusing: Westerners are more familiar with two other plums, one an ornamental and the other grown for its fruit. The flowering plum familiar to Westerners (though usually in its purple-leafed forms, such as ‘Thundercloud’) is a Western Asia native, Prunus cerasifera. And the fruiting plum widely available in the West (originally native to China, not Japan) is Prunus salicina (sumomo), known in English as Japanese plum. It would be less confusing if Prunus mume was consistently referred to in the West as a flowering apricot. But since that’s not the case, I’ve used the common name flowering plum as well as the Japanese name ume in this article.

A relatively small deciduous tree, Prunus mume grows to about 20-30 feet tall if left unpruned. Mature trees have fine and complex branching, a dense, rounded habit, and gray or brownish flaking bark. Young branches, though, are green and smooth. The leaves are dark green, simple, sharply toothed, 2-4” long, and broadly ovate with a wedge-shaped base and pointed tips. They open after flowering, and sometimes develop yellowish fall color.

Small, fragrant flowers—singly or in pairs, and on very short stalks—open in winter on the young green branches. Though typically less than one inch across, their fragrance is powerful, sweet and spicy. Colors range from white to deep pink, but trees in the wild typically have single, 5-petaled flowers, white or blushed pink. Cultivars include singles, semi-doubles, and doubles, with colors ranging from white to pink and deep red. Depending upon latitude, elevation and the weather, flowering begins from January to early April—and may begin even earlier in mild winters.

The flowers are pollinated by insects, including bees, but the 1-1½” fruits do not always develop. When formed, they are fuzzy-skinned, yellow in color (sometimes with a reddish blush), and ripen in summer. Botanically, they are drupes (fleshy fruits with thin skins and a central stone that contains a single seed). Sour or bitter in flavor, they are typically not eaten fresh, even in Asian countries, but are processed using alcohol, salt, or drying to create popular foods and beverages. Fortunately, processing also inactivates enzymes that produce poisonous chemicals in the fresh fruit.

Prunus mume trees require winter dormancy, growing well in temperate regions with four-season climates. Hardy to Zone 6 (minimum temperature at least -5 degrees Fahrenheit), they perform best when grown in deep, rich, acidic, well-drained soil, and full sun. Well-grown and healthy trees are unlikely to develop significant problems, but potential pests and diseases include aphids, borers, tent caterpillars, bacterial canker, and brown rot.

Botanical drawing of Prunus mume flowers, leaves, branches, and fruit, from Flora Japonica, 1870. (image: Philipp Franz von Siebold and Joseph Gerhard Zuccarini, Wikimedia Commons)

Prunus mume has been an iconic tree throughout Japanese cultural history. It was planted in Japan’s early capitals, and its flowers, treasured by the aristocracy, were represented in literature and the arts. In haiku poetry, ume became a season word (kigo) for early spring. And during the Nara period (710-794 C.E.), several important families chose it for their crests (umemon). In both China and Japan, it was associated with good fortune, and included in the “three friends of winter,” along with pine and bamboo. However, though greatly beloved in the Nara period, ume lost this position in the Heian period that followed (794-1185 C.E), when flowering cherries came to be more highly regarded.

In Japan, Prunus mume has traditionally been planted as a specimen tree as well as in orchards, both ornamental and fruiting. It is used for bonsai, and its branches used in flower arrangement (ikebana). It takes well to annual pruning, a traditional practice there that’s used to create an attractive, picturesque habit and to promote flowering and fruiting. With gnarled trunks and branching, regularly pruned and very old trees are revered for their ancient appearance.

When processed, ume fruits have long been an important part of the traditional Japanese diet. Products include salted and pickled plums (umeboshi); umeboshi plum vinegar, considered a health drink; sauces; flavorings for many foods, including crackers and candies; and “plum wine" (umeshu), a popular sweet/tart liqueur. In Japan as well as China, medicinal uses have also been developed.

Hundreds of cultivars have been created—at least 250 in Japan alone, where complex systems of classification have been developed to account for subtle variations in flower form, color, and size. In the United States, beginning in the 1920s, the W. B. Clarke Nursery made many early introductions—both new selections and Japanese plants that were given English names. More recently, the late Dr. J.C. Raulston (of North Carolina State University’s J.C. Raulston Arboretum) was a learned and enthusiastic promoter of the species and its cultivars, contributing to their wider availability in recent years.

Unfortunately, both species and cultivars remain relatively uncommon in the U.S., including Seattle, and may be difficult to locate at retail nurseries. However, at least a few selections are readily available from online “mail order” nurseries. Some of the more well-known cultivars are ‘Fragrant Snow’ (a very early bloomer, with beautiful white, semi-double flowers), ‘Peggy Clarke’ (deep rose pink, double flowers), ‘Matsubara Red’ (dark red, double flowers), and our Garden’s ‘Kobai’ (deep pink, semi-double flowers, sometimes described as deep red and fully double). Weeping selections include ‘W.B. Clarke’ (pale pink, double flowers) and ‘Pendula’ (soft pink, single or semi-double flowers).

The delicate flowers of Prunus mume ‘Kobai’ in the Seattle Japanese Garden. (photo: Chie Iida)

Prunus mume ‘Kobai,’ the Seattle Japanese Garden’s cultivar, was developed in Japan but has been available in North America since at least 1991. A vigorous selection, it matures at about 25 feet tall and wide. Deep pink flowers, borne profusely on the bare branches, are semi-double, with white stamens and a sweet, spicy fragrance, sometimes described as “cinnamon-like.” In the Pacific Northwest, flowering may begin in mild years as early as December, but more typically in February. Flowers are hardy to about 20 degrees Fahrenheit, but unopened buds will tolerate even colder temperatures. ‘Kobai’ often sets fruit, and its fall color is pale yellow.

‘Kobai,’ however, is not the only Prunus mume planted in the Garden. In 1993, a pink-flowered tree, either the species or an unnamed cultivar, was planted in the orchard area, next to the path. Skillful pruning has created an elegant, wide-spreading habit. In most years, it will still be blooming in early March, when the Garden reopens.

Prunus mume is an excellent tree for small gardens and should be planted close to homes or near well-used walkways to ensure that its delicate winter blooms will be seen, even on cold and rainy days. I’m grateful that three specimens of the cultivar ‘Kobai’ are planted prominently outside the Seattle Japanese Garden, to be seen and appreciated by all who walk nearby in the dark months of late winter.

Corinne Kennedy is a Garden Guide, a frequent contributor to the Seattle Japanese Garden blog, and a retired garden designer.