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, more tender and sweeter-tasting than outdoors rhubarb. In temperate climates, rhubarb is one of the first food plants harvested, usually in mid- to late spring (April/May in the Northern Hemisphere, October/November in the Southern Hemisphere), and the season for field-grown plants lasts until September. 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. Rhubarb is ready to consume as soon as harvested, and freshly cut stalks are firm and glossy.
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 noted "Rhubarb Triangle" of Wakefield, Leeds, and Morley.
The advocate of organic gardening Lawrence D Hills lists his favorite rhubarb varieties for flavor 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.
Because rhubarb is a seasonal plant, obtaining fresh rhubarb out of season is difficult in colder climates, such as in the UK, Ireland, Russia, etc. Rhubarb thrives in areas of direct sunlight and can successfully be planted in containers if they are large enough to accommodate a season's growth.
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. A bundle of organic, cultivated rhubarb
The color 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 color results from the presence of anthocyanins, and varies according to both rhubarb variety and production technique. The color is not related to its suitability for cooking: The green-stalked rhubarb is more robust and has a higher yield, but the red-coloured stalks are much more popular with consumers.