Rhubarb is a vegetable derived from cultivated plants in the genus Rheum in the family Polygonaceae. The whole plant - a herbaceous perennial growing from short, thick rhizomes - is also called rhubarb. Historically, different plants have been called "rhubarb" in English. The fleshy, edible stalks (petioles) of other species and hybrids (culinary rhubarb) were cooked and used for food. The large, triangular leaves contain high levels of oxalic acid and anthrone glycosides, making them inedible. The small flowers are grouped in large compound leafy greenish-white to rose-red inflorescences.
The precise origin of culinary rhubarb is unknown. The species Rheum rhabarbarum (syn. R. undulatum) and R. rhaponticum were grown in Europe before the 18th century and used for medicinal purposes. By the early 18th century, these two species and a possible hybrid of unknown origin, R. × hybridum, were grown as vegetable crops in England and Scandinavia. They readily hybridize, and culinary rhubarb was developed by selecting open-pollinated seed, so that its precise origin is almost impossible to determine. In appearance, samples of culinary rhubarb vary on a continuum between R. rhaponticum and R. rhabarbarum. However, modern rhubarb cultivars are tetraploids with 2n = 44, in contrast to 2n = 22 for the wild species.
Although rhubarb is a vegetable, it is often put to the same culinary uses as fruits. The leaf stalks can be used raw, when they have a crisp texture (similar to celery, although it is in a different family), but are most commonly cooked with sugar and used in pies, crumbles and other desserts. They have a strong, tart taste. Many cultivars have been developed for human consumption, most of which are recognised as Rheum × hybridum by the Royal Horticultural Society.
Rhubarb is grown widely, and with greenhouse production it is available throughout much of the year. Rhubarb grown in hothouses (heated greenhouses) is called "hothouse rhubarb", and is typically made available at consumer markets in early spring, before outdoor cultivated rhubarb is available. Hothouse rhubarb is usually brighter red, tenderer and sweeter-tasting than outdoor rhubarb. In temperate climates, rhubarb is one of the first food plants harvested, usually in mid- to late spring (April or May in the Northern Hemisphere, October or November in the Southern Hemisphere), and the season for field-grown plants lasts until the end of summer.
In the United Kingdom, the first rhubarb of the year is harvested by candlelight in forcing sheds where all other light is excluded, a practice that produces a sweeter, more tender stalk. These sheds are dotted around the "Rhubarb Triangle" between Wakefield, Leeds, and Morley.
In the northwestern US states of Oregon and Washington, there are typically two harvests, from late April to May and from late June into July; half of all US production is in Pierce County, Washington. Rhubarb is ready to consume as soon as harvested, and freshly cut stalks are firm and glossy.
The colour of rhubarb stalks can vary from the commonly associated crimson red, through speckled light pink, to simply light green. Rhubarb stalks are poetically described as "crimson stalks". The colour results from the presence of anthocyanins, and varies according to both rhubarb variety and production technique. The colour is not related to its suitability for cooking:
The Chinese call rhubarb "the great yellow" (dà huáng ), and have used rhubarb root for medicinal purposes for thousands of years. It appears in The Divine Farmer's Herb-Root Classic which is thought to have been compiled about 1,800 years ago. Though Dioscurides' description of ? or indicates that a medicinal root brought to Greece from beyond the Bosphorus may have been rhubarb, commerce in the drug did not become securely established until Islamic times. During Islamic times, it was imported along the Silk Road, reaching Europe in the 14th century through the ports of Aleppo and Smyrna, where it became known as "Turkish rhubarb". Later, it also started arriving via the new maritime routes, or overland through Russia. The "Russian rhubarb" was the most valued, probably because of the rhubarb-specific quality control system maintained by the Russian Empire.
The cost of transportation across Asia made rhubarb expensive in medieval Europe. It was several times the price of other valuable herbs and spices such as cinnamon, opium, and saffron. The merchant explorer Marco Polo therefore searched for the place where the plant was grown and harvested, discovering that it was cultivated in the mountains of Tangut province. The value of rhubarb can be seen in Ruy Gonzáles de Clavijo's report of his embassy in 1403-05 to Timur in Samarkand: "The best of all merchandise coming to Samarkand was from China: especially silks, satins, musk, rubies, diamonds, pearls, and rhubarb..."
The high price as well as the increasing demand from apothecaries stimulated efforts to cultivate the different species of rhubarb on European soil. Certain species came to be grown in England to produce the roots. The local availability of the plants grown for medicinal purposes, together with the increasing abundance and decreasing price of sugar in the 18th century, galvanised its culinary adoption. Grieve claims a date of 1820 in England. Rhubarb was grown in Scotland from at least 1786, having been introduced to the Botanical Garden in Edinburgh by the traveller Bruce of Kinnaird.
Though it is often asserted that rhubarb first came to the United States in the 1820s,John Bartram was growing medicinal and culinary rhubarbs in Philadelphia from the 1730s, planting seeds sent to him by Peter Collinson. From the first, the familiar garden rhubarb was not the only Rheum in American gardens: Thomas Jefferson planted R. undulatum at Monticello in 1809 and 1811, observing that it was "Esculent rhubarb, the leaves excellent as Spinach."
The advocate of organic gardening Lawrence D. Hills listed his favourite rhubarb varieties for flavour as 'Hawke's Champagne', 'Victoria', 'Timperley Early', and 'Early Albert', also recommending 'Gaskin's Perpetual' for having the lowest level of oxalic acid, allowing it to be harvested over a much longer period of the growing season without developing excessive sourness.
The word rhubarb is likely to have derived in the 14th century from the Old French rubarbe, which came from the Latin rheubarbarum and Greek rha barbaron, meaning 'foreign rhubarb'. The Greek physician Dioscorides used the Greek word (rha), whereas Galen later used ? (rh?on), Latin rheum. These in turn derive from a Persian name for species of Rheum. The specific epithet rhaponticum, applying to one of the presumed parents of the cultivated plant, means 'rha from the region of the Black Sea'.
Rhubarb is grown primarily for its fleshy leafstalks, technically known as petioles. The use of rhubarb stalks as food is a relatively recent innovation. This usage was first recorded in 18th to 19th-century England after affordable sugar became more widely available.
Commonly, it is stewed with sugar or used in pies and desserts, but it can also be put into savoury dishes or pickled. Rhubarb can be dehydrated and infused with fruit juice. In the United States, it is usually infused with strawberry juice to mimic the popular strawberry rhubarb pie.
In Northern Europe and North America the stalks are commonly cut into pieces and stewed with added sugar until soft. The resulting compote, sometimes thickened with corn starch, can then be used in pies, tarts and crumbles. Alternatively, greater quantities of sugar can be added with pectin to make jams. The most popular accompanying spice to use is ginger, although cinnamon and nutmeg are also popular additions.
In the United Kingdom, as well as being used in the typical pies, tarts and crumbles, rhubarb compote is also combined with whipped cream and/or custard to make respectively rhubarb fool and rhubarb and custard.
In the United States the common usage of rhubarb in pies has led to it being nicknamed 'pie plant', by which it is referred to in many 19th century US cookbooks. Rhubarb in the US is also often paired with strawberries to make strawberry-rhubarb pie, though some rhubarb purists jokingly consider this "a rather unhappy marriage".
In former days, a common and affordable sweet for children in parts of the United Kingdom and Sweden was a tender stick of rhubarb, dipped in sugar. It is still eaten this way in western Finland, Norway, Canada, Iceland, Latvia, Lithuania, the Faroe Islands and Sweden, and also some other parts of the world.
The roots and stems contain anthraquinones, such as emodin and rhein. The anthraquinones have been separated from powdered rhubarb root for purposes in traditional medicine, although long-term consumption of the plant has been associated with acute kidney failure.
Rhubarb leaves contain poisonous substances, including oxalic acid, a nephrotoxin. The long term consumption of oxalic acid leads to kidney stone formation in humans. Humans have been poisoned after ingesting the leaves, a particular problem during World War I when the leaves were mistakenly recommended as a food source in Britain. The toxic rhubarb leaves have been used in flavouring extracts, after the oxalic acid is removed by treatment with precipitated chalk (i.e., calcium carbonate).
The LD50 (median lethal dose) for pure oxalic acid in rats is about 375 mg/kg body weight, or about 25 grams for a 65-kilogram (143 lb) human. Other sources give a much higher oral LDLo (lowest published lethal dose) of 600 mg/kg. While the oxalic acid content of rhubarb leaves can vary, a typical value is about 0.5%, meaning a 65 kg adult would need to eat 4 to 8 kg (9 to 18 lbs) to obtain a lethal dose, depending on which lethal dose is assumed. Cooking the leaves with baking soda can make them more poisonous by producing soluble oxalates. The leaves are believed to also contain an additional, unidentified toxin, which might be an anthraquinone glycoside (also known as senna glycosides).
In the petioles (leaf stalks), the proportion of oxalic acid is about 10% of the total 2-2.5% acidity, which derives mainly from malic acid. Serious cases of rhubarb poisoning are not well documented. Both fatal and non-fatal cases of rhubarb poisoning may be caused not by oxalates, but rather by toxic anthraquinone glycosides.
Hungry wildlife may dig up and eat rhubarb roots in the spring, as stored starches are turned to sugars for new foliage growth.