Rhubarb is grown widely, and with
greenhouse production it is available throughout much of the year. It needs rainfall and an annual cold period of up to 7–9 weeks at 3 °C (37 °F), known as 'cold units', to grow well. The plant develops a substantial underground storage organ (rhubarb crowns) and this can be used for early production by transferring field-grown crowns to warm conditions. 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. After forcing for commercial production, the crowns are usually discarded. These sheds are dotted around the "
Rhubarb Triangle" in
Yorkshire between
Wakefield,
Leeds, and
Morley. In the United States, rhubarb is primarily produced in the states of
Oregon,
Washington, and
Wisconsin with approximately 1,200 acres in production, of which 175 are covered in hothouses. 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 commercial production is in
Pierce County, Washington. Rhubarb is ready to consume as soon as harvested, and freshly cut stalks are firm and glossy. Rhubarb damaged by severe cold should not be eaten, as it may be high in
oxalic acid, which migrates from the leaves and can cause illness. 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.
Historical cultivation The Chinese call rhubarb "the great yellow" ( ), and have used rhubarb root for medicinal purposes. 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 plant 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 began to arrive via new maritime routes and 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 2020 edition of
Pharmacopoeia of the People's Republic of China lists the following species as medicinally acceptable:
Rheum officinale,
Rheum palmatum, and
Rheum tanguticum. Grieve describes "Turkish rhubarb" as a mixture of
R. palmatum and
R. rhaponticum. The high price, as well as the increasing demand from apothecaries, stimulated efforts to cultivate the different species of rhubarb on European soil.
John Bartram was growing medicinal and culinary rhubarbs in
Philadelphia from the 1730s, planting seeds sent to him by
Peter Collinson.{{Cite web|editor=David H ==Cultivars==