As noted in part 1, we eat only the leaf stems of rhubarb, not leaves, flowers, fruit or roots.
Rhubarb leaves are quite poisonous. They contain dangerous concentrations of oxalic acid compounds. In low concentrations, oxalic acid tastes good to people, but the concentration of oxalic acid compounds in rhubarb leaf blades is much higher than that. People have died after eating cooked rhubarb leaves.
Culinary History of Rhubarb
In China, the first written record of rhubarb, used as a medicine, was at least 5000 years ago. They used rhubarb medicinally under the name dà huáng, literally big yellow, referring to the root’s size and the fact that the powder of the yellowish-red dried root stained the mouth yellow. Primarily it was used as a tonic for the blood, the digestive tract and for imbalances specific to women.
Using rhubarb for anything other than a medicine is quite recent. The earliest report I found was from the Bynums (reference below), who wrote that rhubarb leaf stems were sold in London as a novelty food at Covent Garden, the theater district, in the early 19th century, but that it took the arrival of cheap sugar from the Indies to make rhubarb popular, since they are very tart. Grieve in her herbal reported people eating rhubarb leaf stems in England the 1830s and 40s. The food caught on and today, for many people “pie plant” is the usual name of rhubarb, in recognition of its main use.
Whether you want to call rhubarb a vegetable or a fruit depends on your use of the two words. In common language, vegetables are not sweet and are eaten in salads, soups, stews etc., while fruits are sweet and are eaten as dessert or as a sweet treat. Botanically, however, the fruit is the part of a plant that contains the seeds. Botanists apply the word vegetable to plant material generally, as in vegetable kingdom, so fruits are a particular vegetable structure. As you see: rhubarb comes out a fruit in common usage and a vegetable in botanical terminology. You have to like that whether you say fruit or vegetable, you will be both right and wrong at once.
Finally, consider this English riddle:
What is long and thin and covered in skin
Red in parts, stuck in tarts?
Ready for pie
Rhubarb, of course.