Rhubarb

Rhubarb is a perennial plant that grows from thick short rhizomes, comprising the genus Rheum.

The large, somewhat triangular leaf blades are elevated on long, fleshy petioles. The flowers are

small, greenish-white, and borne in large compound leafy inflorescences.

The plant is indigenous to Asia, and many suggest that it was often used by the Mongolians;

particularly, the Tatars tribes of the Gobi. Varieties of rhubarb have a long history as medicinal

plants in Traditional Chinese Medicine, but the use of rhubarb as food is a relatively recent

innovation, first recorded in 17th century England, after affordable sugar became available to

common people. Rhubarb is now grown in many areas, primarily for its fleshy petioles, commonly

known as rhubarb sticks. In temperate climate rhubarb is one of the first food plants to be ready

for harvest, usually in April/May. The petioles can be cooked in a variety of ways. Stewed, they

yield a tart sauce that can be eaten with sugar or used as filling for pies, tarts, and crumbles.

This common use led to the German slang term for rhubarb, piestengel or "pie plant." Rhubarb is

also used to make wine. In former days, a common and affordable sweet for children in Yorkshire was

a tender stick of rhubarb, dipped in copious amounts of sugar. In the UK the first rhubarb of the

year is grown by candlelight in dark sheds dotted around the famous ‘Rhubarb Triangle’ of

Wakefield, Leeds and Morley.

In warm climates, rhubarb will grow all year round, but in colder climates the parts of the

plant above the ground disappear completely during winter, and begin to grow again from the root in

early spring. It can be forced, that is, encouraged to grow early, by raising the local temperature.

This is commonly done by placing an upturned bucket over the shoots as they come up.

The plant occurs in at least four species. Those most commonly used in cooking are the

Garden Rhubarb (R. rhabarbarum) and R. rhaponticum, which though a true rhubarb bears the common

name False Rhubarb. The drug rheum is prepared from the rhizomes and roots of another species, R.

officinale or Medicinal Rhubarb. This species is also native to Asia, as is the Chinese Rhubarb (R.

palmatum). Rhubarb is used as a strong laxative and for its astringent effect on the mucous

membranes of the mouth and the nasal cavity.

Rheum species have been recorded as larval food plants for some Lepidoptera species

including Brown-tail, Buff Ermine, Cabbage Moth, Large Yellow Underwing, The Nutmeg, Setaceous

Hebrew Character and Turnip Moth.

Rhubarb leaves contain poisonous substances. Rhubarb leaf poisoning is most often caused by

oxalic acid, a corrosive and nephrotoxic acid that is abundantly present in a lot of plants. The

LD50 for pure oxalic acid is predicted to be about 375 mg/kg body weight, or about 25 g for a 65 kg

human. While the oxalic acid content of rhubarb leaves can vary, it averages about 0.5%, so a rather

unlikely five kilograms of the extremely sour leaves would have to be consumed to reach an LD50

dose. In the petioles, the amount of oxalic acid is much lower, especially when harvested before

mid-June so the phloem will explode (on the northern hemisphere), but it is still enough to cause

slightly rough teeth.