Kuwai (Japanese:クワイ) is a Japanese plant whose underground stem (rhizome) is edible. It is a cultivar ofSagittaria trifolia. It has different common names in different regions of Japan.[1] The underground stems of this neighboring species are also eaten in China.
Kuwai and Chinesecígū (慈姑) are different cultivars, but both are treated askuwai in Japan andcígū in China. The etymology of the Japanese namekuwai is uncertain.[2]
Kuwai is a cultivar ofSagittaria trifolia. Its scientific name isSagittaria trifolia L. 'Caerulea.'[3] However, it is sometimes treated the same as another cultivar or as the same asSagittaria trifolia. On the contrary, some scholars have assumed it to beSagittaria sagittifolia without acknowledgingSagittaria trifolia.[4]
The cultivarSagittaria trifolia was created inChina.[1] For this reason, the origin ofkuwai is considered to be China.[5] It is cultivated as a vegetable only in China and Japan.[5] In Japan, it has been actively cultivated since theEdo period.[6]
Chinesekuwai is white, and its underground stem is larger than the Japanese one. This underground stem is also edible in China. It is cultivated only in Japan, but it is calledshiro(white)-kuwai to distinguish it from Japanesekuwai. In contrast, Japanesekuwai is sometimes calledao kuwai to distinguish it from Chinesekuwai. In Japan, the scientific name of the white kuwai is often given asSagittaria trifolia L. 'Sinensis', while the same species is known asSagittaria sagittifolia subsp.leucopetala in China.[4]
In Japan, three varieties ofkuwai are grown: the blue-violetao kuwai, the pale blueshiro kuwai, and the smallSuita kuwai.[7] All are grown inpaddy fields.
As mentioned above, the distribution ofkuwai varies greatly depending on how it is classified. If we consider itSagittaria sagittifolia, it is widely distributed inAsia,Europe, theAmericas, and other regions of the world, fromtemperate totropical across the globe. The wild speciesSagittaria trifolia is believed to be native toSoutheast Asia. The cultivated varietykuwai was created inChina.[1] For this reason, the origin ofkuwai is China.[5] The areas where it is cultivated as a vegetable are limited to China and Japan.[5] In Japan, it has been actively cultivated since theEdo period.[6]
Ao kuwai, a typical Japanesekuwai, is amonocotyledonous perennialaquatic plant, growing to about 110 - 125 centimeters (cm) in height.[5] The leaves are 30 cm long, arrowhead-shaped without notches, and the leaf blade has a spongy interior.[5]Stolon arise from each node of thestem and grow up to 60 – 80 cm long.[5] Nodes are also found on the creeping stem, from which two or three secondary creeping stems arise.[12]Tubers are attached to the tips of therhizome, which are larger than those of the originalomodaka species. The tuber is bluish with horizontal nodules and is surrounded by thin scales with a 5 – 6 cm long apical bud at the tip. The flowers are dioecious. It produces conical inflorescences with white-petaled flowers. Fruiting is almost non-existent after flowering.[12]
Kuwai is cultivated inpaddy fields. Germination ofkuwai occurs around early July, when planting takes place, at a germination temperature of 13 °C to 15 °C or higher.[12] The growth ofkuwai can be divided into two phases: the vegetative growth phase (July–August), from germination to leaf number increase, and the reproductive growth phase (late August to mid-November), from creeping stem development to tuber enlargement.[13] The optimum temperature for growth during the vegetative growth phase is 20 °C to 30 °C, but the rate of increase in leaf number is temperature-dependent and is accelerated by high temperature conditions.[13] Creeping during the vegetative growth period occurs after 14 to 15 true leaves have developed, followed by the development of one creeping stem for every leaf blade.[13] After the creeping stems have elongated, they form tubers encased in two bracts, which begin to enlarge when daylight hours are shortened and the plant is exposed to low temperatures of around 15 °C.[13] Although the stolons die when exposed to frost, tuber enlargement takes place below the water surface until late fall.[13]
It is essential that the field be flooded during the growing season, and good water availability is a necessary condition for cultivation.[14] Semi-wet fields withpeat or fine-grainedgleysol are suitable, whileandosol or sandy soils will reduce yields.[14] Weather conditions should be warm throughout the entire period, and the more significant the daily temperature difference during the tuber growth period, the better and fuller tubers can be produced.[14] In Japan, the best areas for cultivation are south of the southernKantō region.[8]
Kuwai is planted using tubers that have been harvested and refrigerated the previous year but are removed from the refrigerator one week before planting and acclimated to the open air so that they are not exposed to direct sunlight and do not dry out.[15]Kuwai fields are plowed in April, and two weeks before planting in late June or July, the fields are watered, and the tuber roots are planted after the fields have been padded by raking.[16] Two weeks later, leaves similar toomodaka appear, and the stem and leaves grow vigorously from around late July through September.[16] During this growth period, the rhizomes become fuller and of uniform size through fertilization, which involves moderately thinning the stems and leaves, and cutting some of the underground stems.[16] Water management is also essential, and water is kept at a shallow depth of 5 cm immediately after planting and in the fall, and at a slightly shallower depth of 6 to 9 cm in the summer during the growing season, so as not to run out of water.[14] When temperatures drop in late fall and the leaves become frostbitten, tuberous root enlargement ceases and it is time to harvest.[16]
The harvesting method is similar to that oflotus root, using water pressure from a power pump to dig up the rhizomes in the mud below the surface of the water and separate the rhizomes that come to the surface from the stems to avoid damaging the shoots.[7] Alternatively, the water is dropped. The above-ground parts are harvested, leaving the field flooded once until harvesting, and the rhizomes are dug up after the water is wholly dropped when harvesting.[17] Seed balls, which are reserved for planting the following year, are stored in vaults in underground pits or refrigerator storage.[18]
Diseases and pests are known to affect significantly yield, including red blight byFusarium species, leaf blight byMarssonina species in the mid-season, andaphids from September onward.[18] Immediately after planting, the crop may be subjected to feeding damage by theduckweed.[18] Although there are few problems with continuous cropping, continuous cropping in fields where red blight has occurred is discouraged.[8]
The two main production areas in Japan areHiroshima andSaitama prefectures, which account for more than 80% of the market.[16] The most significant production volume in Japan is inFukuyama City, Hiroshima Prefecture, where it spread in the earlyShowa period as a post-crop ofigusa, and became a local specialty around 1955.[16] Because it is grown in time for the New Year when demand is high, shipments peak from late November to December.[16] There has been little differentiation in crop types, with some cultivating earlier planting (September–October shipments.)[8]
In Japan, since "signs of growth" (目が出る) and "seed sprouts" (芽が出る) are pronounced the same (me-ga-deru,) it is considered a good-luck item.[20] Therefore, it is customary to makenimono and eat it atOsechi (New Year's dishes), etc.[1][6]
Theseason as a foodstuff is from November to April. Good quality sprouts are considered high market value if they grow into a beautiful shape and are shiny all over.[6] The buds are cut diagonally at the tip and left. The bottom of the tuber is thinly sliced and trimmed, then peeled, exposed to water, and cooked after thelye is removed.[6] The tuber has a characteristic bitter taste due to itsoxalic acid content, so it is best to boil it once inrice water to remove it.[21] It has a slightly sweet and bitter taste similar to that of chestnuts and lily bulb.[7] It is usually boiled in a broth to enjoy its tender texture.[6] It is also used indeep-fried andnabemono.[21] Processed products includekuwai chips andkuwaishōchū (shochu made fromkuwai.)[5]
In China, there is a record of it in the Mingyi bielu (名醫別錄) written byTao Hongjing in the 5th century, and its Chinese name iscí gū (慈姑, The pronunciation on the left is modern and differs from that of the 5th century.)[9]
The date of arrival ofkuwai in Japan is unknown, but it is believed to have existed in Japan in theNara period of the 8th century.[5] TheMan'yōshū, Japan's oldest poetry collection composed around 780, contains a poem
Egu is thought by some to bekuwai. Some say they areJapanese parsley.[9]
In old Japanese literature,kuwai is often written as a translation of the Chinese wordcí gū (慈姑) or written as today's kuroguwaiEleocharis kuroguwai. It may also refer to the original species,omodaka (Sagittaria trifolia.) Scholars are therefore divided in their interpretations of whether the description in these books is thekuwai of today.[22] The following is an interpretation often presented as a standard theory but may not be a description ofkuwai.
In theHonzo Wamyo (本草和名, Japanese name for medicinal herbs) compiled in 918, there is a reference toomodaka (於毛多加.)[23] In another place, it is described asKurokuwai (久呂久和為.)[24]
On the other hand,Wamyō Ruijushō, compiled around 938, mentionskuwai (久和井.)[25][26]
Production and use flourished during theEdo period (1603–1867), and the main production areas are thought to beKyoto,Osaka, and theEdo area, but little is known about the actual situation.[5]
Yamato Honzo (1709, 大和本草) has a section on "Raven potato(烏芋) -kurokuwai(クロクワイ)."[27] Thekurokuwai in this book appears to refer tokuwai, not today'skurokuwai, based on the characteristics of the leaf shape.
InSetsuyo Gundan (摂陽群談, 1701), there is a description of "sukitakuwai, it is grown in the paddy fields of Suita Village and sold in the market. It is small in size but tastes delicious."[9][28]
Kaibara Ekken clearly distinguishes between慈姑=kuwai and烏芋kurokuwai, in his bookNabu (菜譜, 1704.) From the description, they seem to be the same as today'skuwai andkurokuwai. There is a description thatSuita kuwai is smaller and tastier than commonkuwai.[29]
It served as a relief crop during theGreat Tenmei famine (1782–1788.)[5]
Goki-nai sanbutsu zue (五畿内産物図絵, 1813) also depictSuitakuwai as an illustration.[30]
Ōta Nanpo (太田南畝, 1749–1823,) who traveled to Osaka during the Edo period, read the following poem;
During theMeiji era,Kyoto,Osaka,Saitama,Tokyo,Ibaraki, andChiba were the main production areas in Japan.[5]
During thePacific War,kuwai cultivation became one of the controlled items during the war and was suppressed.[5]
Although cultivation was revived after the war, the area cultivated shrank from the prewar level as urbanization progressed and the number of rice paddies in urban areas decreased.[5] Since then, it has been gradually reducing.[5]
