Ume, the Japanese Plum Tree, Prunus mume

The Japanese plum tree, also known as ume, Prunus mume, is in fact an apricot, and originates not from Japan, but rather the mountains of southwestern China. Regardless, this is another iconic tree of Japan, famous not only for its early flowering, often while the snow still falls, but also for its extremely sour fruits which are used liberally in Japanese cuisine – from pickles to sauces, and also as a spice.

This flowering tree is a member of the Armeniaca section of the rather large genus Prunus, which includes cherries, peaches, plums, almonds, and apricots. This last group of fruit trees, the apricots, are best known by the species Prunus armeniaca, the source of the common edible apricot that has been in cultivation for thousands of years. The seven other members of this group are rather obscure with the exception of P. mume, which has been in cultivation in China for over a thousand years, and probably a least a thousand in Japan as well.

Prunus mume is a relatively small tree, ranging from 4 to 10 meters in height if left unpruned. It’s flaking bark is typically gray when mature, and with a green tint and smooth when young. Branching is complex and fine. It is a deciduous tree, bearing lanceolate to obovate leaves, typically finely haired, with toothed margins and usually not more than 8 cm in length. Trees are not terribly long lived, perhaps up to a hundred years. The flowers are born singly or at most doubly per fascicle, and open before the leaves by several weeks. Since they flower during cold weather, they can last quite long, up to two weeks or more, before falling.

In simple flowers, the petals are 5 in number, ovate, and about 1 cm long. Multi-petaled types can have many petals and be double that size. Flower color originally was typically white with a pink blush, but nowadays ranges from deep crimson to various shades of pink, pure white, and even green blushed flowers. Depending on latitude and elevation, flowering typically commences from late January into early April, but always well before most trees have begun to break dormancy. The fleshy fruits, known as drupes, range from 2 to 5 cm in diameter, maturing a deep yellow with red-purple blush, and are fully ripe by middle June in Japan.

The precise origins of this species are somewhat unclear given its long history of cultivation, however the likely center of its natural range was where the Yangtze River breaks into its three main constituent sources – the Min, Quingyi, and Dadu Rivers in western Sichuan, near the city of Leshan. It is said to be native to both northern Yunnan and western Sichuan Provinces in forested mountains ranging from 1700 to 3100 meters elevation. That said, it is widely cultivated throughout China, as well as northern Laos, northern Vietnam, Taiwan, Korea as well as Japan. Increasingly, it is is becoming more widely cultivated in Europe and North America in recent decades.

Prunus mume has been selected and cultivated over hundreds of years, and in some cases crossed with similar species, to produce an astounding array of named varieties. In Japan alone, over 500 types have been created, making classification complex and often obscure. Regardless, in Japan most fall within three main groups. They are as follows:

Yabai (野梅系) – These types are the closest to the wild type. Branching tends to be thin and dense, with the smaller branches being somewhat thorny. Flower color typically is white, with pink blushing, particularly on the tips of the buds and inner petals. The leaves are more lanceolate in shape, and are covered in soft, fine hairs.

Hibai (緋梅) – This group includes the forms known as Koubai (紅梅), characterized by their deeply red colored flowers. While these forms tend to produce many smaller flowers that are red in color, some can be white or pink. They are said to be derived from the Yabai group. The pith of the inner wood as well is red in color, as are the the leaf petioles. The leaves are small in size, and similar to the Yabai types. These are commonly grown as garden trees and as bonsai, and less so for their fruit.

Bungo (豊後) – This group was developed in the historical province of Bungo in present day Kyushu in Oita Prefecture. They are apparently crosses with the closely related P. armeniaca, known as anzu in Japan. Being hybrids, many are not self compatible and have to be out-crossed to create viable offspring. They are said to have low disease resistance, but ironically are also more cold hardy, presumably due to the influence of the colder growing P. armeniaca. For that reason they can be grown even into the Tohoku Region of northern Japan. Flower color and form is variable, from red to white, boasting both single to multi-petaled types, and tend to be later flowering. Branching is relatively thick and sparse. The leaves are usually more ovate than pure P. mume, and hairless. Unfortunately, the fruits tend to have rather fibrous pulp, and for that reason they are not favored for use in cuisine.

With those distinctions aside, there are other characteristics that define the many varieties of this tree. These include: type of branching, flower color, flower size, flower complexity, fruit size and texture, wood color, bud color, flower stance, and so on. Needless to say, differentiations are complex, and as with many things in Japan, subtle. What follows are a few broader examples of this complexity.

Simple flower types – these can be any color, from red to white, but are consistently 5 petaled, and are closer in form to ancestral types.

Multi-petaled types (called yayae in Japan) – these have thicker looking flowers, with varying numbers of petals depending on the variety. Again, flowers can be various colors, and tend to be rather large in size, commonly twice the size of many simple flower varieties.

Large vs. smaller flower types – flower size can vary considerably regardless of flower complexity. Some, particularly the deep red varieties, can have almost tiny flowers (~1 cm across) while some of the larger ones can approach 6 cm in diameter.

Weeping branched types (called shidare in Japan) – like the weeping cherry trees, one can see lovely examples of weeping ume too. Most commonly the flowers are pink and also multi-petaled. White flowered forms are more uncommon.

Green Flowered types (midorihana) – yes, breeders even have managed to produce flowers with distinctly green tinted blossoms.

Early and late flowering types – some varieties have been selected and bred to flower as early as December, others as late as early April (these are Japan flowering times).

The subtle variations between size, flower form, color and so on seem almost limitless – enough to intrigue you for a life time, or drive you stark raving mad (or perhaps bored). Trying to keep track of the names of each variety, its characteristics and province is literally a field of study within itself.

The scope of influence this tree has had on Japanese culture is equally impressive. During the Nara Period the plum flower was adopted as a crest (umemon) by several prominent Japanese families, including samurai. Famously, this tree and flower was beloved by Michizane Sugawara of Dazaifu Tenmangu shrine in Kyushu – a politician, scholar, and poet of the Heian Period. To this day his beloved trees adorn the grounds of the shrine, and thousands of students visit annually to get his blessing, since he is to this day worshiped as a god of learning.

The fruits of this tree remain a significant part of the Japanese diet. They are commonly consumed in a salted, pickled form known as umeboshi. These pickles are somewhat of a surprise to the uninitiated, being that they are at once sour in the extreme, as well as very salty. I have watched more than one foreigner choke one up. They are best eaten as a topping for white rice or in a rice ball (onigiri). Eating them as is takes some practice. Pickled ume fruit can have various levels of sweetness, from quite sweet to purely sour, as well as in texture, from crunchy to sticky soft, and even as a slightly moist, candy-like snack.

Beyond pickling them, ume fruit are used to make ume plum vinegar (commonly taken as a health drink), umeshu (a strong liquor made by soaking the unripe fruits in white liquor for months or even years), and also as a flavoring for almost any dish including crackers, candies, dressings and even seaweed.

The driving force behind all this ume fruit consumption is not only the pleasant sour taste, but the presumed health benefits they are said to have on the human body. The sourness of the fruit belies the presence of a healthy dose of ascorbic acid, common known as vitamin C. They are so acidic that they are literally impossible to eat as is, even if fully ripe, soft and yellow. For that reason they must be processed in some way to mediate their extreme sourness.

Ironically, while eaten as a health food, the unripe fruit also contain toxins that if consumed in large quantities can kill. These poisons are glycosides of hydrocyanic acid, amygdalin and prunasin, commonly found in many Prunus tree leaves and seeds, including common fruit trees such as peaches, plums and apricots. Consumption of these glycosides and the subsequent interaction with gastric acid in the stomach can cause the production of hydrogen cyanide which ultimately leads to convulsions, shortness of breath, paralysis, and eventual death to those who eat too much.

But here’s the rub – the danger rests only with over-consumption of the seeds (which amazingly ARE still eaten by some stubborn people). Moreover, the processing of the fruits, whether unripe or ripe, through the use of alcohol, salt and sun drying, inactivates the enzymes necessary to produce cyanide, hence toxicity levels drop to a minimum. Believe me, eating potato chips is far more dangerous to your health than eating ume based products! All that said, care has to be taken since umeboshi can contain up to 10-20% salt content per weight.

Far from being a dangerous food, thousands of years of eating these fruits in the far east has confirmed that they are not only safe to eat, but also harbor medicinal effects. The known health benefits of vitamin C aside, P. mume fruit have been used in China medicinally for centuries to aid in stomach and intestinal ailments, in controlling intestinal parasites, preventing bleeding (hemostasis), and in suppressing both gram-positive and gram-negative bacteria, as well as pathogenic fungi in vitro. Research in Japan has also indicated that consumption of ume fruit reduces fatigue, increases HDL cholesterol significantly, decrease the chances of developing arteriosclerosis, decreases blood pressure, and stabilizes blood sugar levels.

My own experience with this tree started when I moved into my house in southern Japan 15 years ago. Next to the carport was a well aged, overgrown Prunus mume, though I didn’t know it was that species at the time. In time I found out what it was and realized it had to be pruned soon or it was going to overtake the yard the following season. So, with little knowledge, I hard pruned it soon after it flowered, sometime in mid April. Much to my surprise, it responded by shooting out hundreds of meter long whip-like shoots a month later. Not sure what to do, I in response pruned these back by half.

I learned the hard way that if you want to keep your ume tree to a contained size, it is important to not only know how, but when to prune. I learned that it is best to prune after the fruits have fully matured, that being around late June to early July in my area. If you prune at this time, the tree will not respond by sending up lots of vigorous shoots, but will rather harden off and heal the cut shoots, and then put resources into forming new flower buds on the remaining green wood. This last bit is important when pruning them since cutting back to the older, hardened wood leads not only to completely defoliating the tree (and it subsequently suckering like crazy), but it also prevents the tree from flowering the following season.

For trees that are being pruned to small size, new shoots will begin growing a few weeks after flowering is finished. These normally range from 20-40 cm in length, and start hardening off by the time the fruits are ripening. I usually leave 2-4 buds on each pruned shoot, thus allowing new flower buds to form for next year, as well as containing the overall growth of the tree. In addition, since these trees will produce many new shoots off old wood, it is necessary to routinely remove these to prevent dense branching from forming. P. mume can produce many fruits and this can lead to branches breaking in the wind if shoot length and density is not managed properly.

Here’s a video showing these trees in southern Japan:

For those interested in growing this tree primarily for fruit production, I suggest following methods used to prune other similar trees. In Japan fruit producing trees are typically grown rather flat by constantly cutting out the central lead growth, and encouraging lateral branching. This allows for easier picking since if left unpruned these trees can get quite tall. Again, branch density as well has to be controlled, since heavy fruiting will lead to lower quality and sized fruit.

Regardless, unless you are willing to allow these trees to grow to normal proportions, yearly pruning will be necessary to either keep them smaller, or to optimize fruit production. Luckily, this tree is very responsive to annual pruning, and as stated before, is commonly trained as bonsai.

Interestingly, there is one exception to the pruning technique outline above, that being how to prune the weeping forms. It is common to hard prune weeping ume trees directly after flowering to the old wood, thus causing the tree to sprout many long suckers which naturally hang downward in almost an inverted umbrella shape. This type of pruning leads to massive displays of weeping branches covered in thousands of flowers the following year.

Beyond pruning, cultivation is fairly straightforward. Trees should be sited in full sun, preferably in a well drained loam with a moderately acidic reaction. Established trees can endure some drought, but will suffer if not watered. I have found fertilizing unnecessary in the native soils of Japan, but in low nutrient soils, particularly deep sands, you may need to feed them. This is best done in the spring, and perhaps again in summer.

The greatest limitation of this tree is its cold tolerance, which is fairly low. It can be grown reliably in USDA cold hardiness zones 8, 7 and perhaps the warmest parts of zone 6. In colder winter regions it should be sited well, offering as much protection as possible from winter winds in particular. A south facing wall or courtyard may be the best choice in these places. Since flowering starts so early, the risk of it being affected by late cold snaps is another issue, though the flowers and flower buds are able to endure moderate frosts.

Disease issues are fairly minimal, and mostly involve the young tender leaves and shoots. Each year my tree gets infestations of aphids, and at times these can get quite intense, though short lived. If there is a severe outbreak I spray effected branches with strong jets of soapy water which usually seems to do the trick. Since aphids can also spread disease, mass infestations can lead to sooty molds growing on the effected branches as well. I have seen outbreaks of both simultaneously on trees in Japan, and though this rarely hurts them in the long-term, it can effect the growth and health of the developing fruits.

Another common issue with older trees is heart rot. This does not usually kill the tree outright, but it can weaken it significantly, causing the tree to become less stable. Assuming an effected tree is otherwise fairly healthy, it can live with heart rot for many years. The best way to avoid the development of this problem (which is a fungal rot of the inner wood) is maintaining good cultivation practices such as not over pruning trees, not injuring the bark, and otherwise providing good growing conditions – plenty of sun, not overcrowding the tree with other vegetation, and providing healthy levels of nutrients and moisture to the roots.

If you live in a warm temperate climate, or have a protected place in the garden, you might want to try growing Prunus mume yourself. It’s lovely little flowers blooming far in advance of most garden plants are a respite from winter’s cold, and serve as a welcoming harbinger of the spring to come.