By Larry Hodgson
Rhubarb is a common garden plant, usually harvested in mid-spring as the leaves expand. Only the petiole is consumed, often after cooking with plenty of sugar to reduce its tartness, although as a child, I used to like to munch on fresh rhubarb petioles simply dipped in sugar.
But while many of us grow rhubarb, what do we really know about it? Here are some interesting bits of information that you might not know about this plant, its history, its use and the details of its culture.
- Rhubarb was first imported from China as a medicinal plant. It has been cultivated there for over 2,000 years as a sort of panacea, good for whatever ails you, and as a laxative, even though today studies struggle to find any serious medicinal value in the plant. It was so valuable that one of the goals Marco Polo (1254–1324) had in traveling to China was to discover where the plant came from (he discovered it being grown in Tangut, now in northern China). Ruy González de Clavijo, Castillian ambassador to the court of Timur, reported in 1403–1405 that “the best of all merchandise coming to Samarkand was from China: especially silks, satins, musk, rubies, diamonds, pearls, and rhubarb.”
- It’s only quite recently that rhubarb has been used as an edible plant. Until the 18th century, it was considered strictly a medicinal plant. However, as sugar became more widely available in that century (and sugar is required to diminish the tartness of the petiole), it began to be used as a source of food in England, then elsewhere in Europe. By the 19th century, the medicinal use of rhubarb had largely been abandoned in Europe and North America and it was being grown for its delicious stalks alone.
- The name “rhubarb” means barbarian root. It comes from the Greek rha barbaron. The Greeks knew the plant we call rhubarb as an import, its roots having been brought from China at great expense. The new plant was considered foreign and of course, to the ancient Greeks, anything from outside their civilization would have been seen as barbarian.
- Putting the proper name on rhubarb is complicated. It was long identified taxonomically as Rheum rhabarbarum, but recent studies have shown most rhubarb plants grown today actually belong to a hybrid species, probably R. rhabarbarum crossed with R. rhaponticum, and should be called R. × hybridum. It’s a tetraploid, with 44 chromosomes, while most species rhubarbs have 22 chromosomes.
- Rhubarb belongs to the Buckwheat or Polygonaceae family and is thus a relative of plants such as sorrel, knotweed and buckwheat.
- Rhubarb is very cold hardy. It will grow in climates as cold as USDA zone 2 and positively thrives in Alaska and Siberia. It’s less tolerant of heat and will do better in cool-summer climates. It is often short-lived in hot-summer areas (where temperatures much of the summer are over 75˚F/24˚C) and may even be treated as an annual there.
- Rhubarb leaves are not as poisonous as often thought. It’s well known among gardeners that you should consume only the leaf stalk (petiole), not the leaf blade itself, as the latter is poisonous. However, they’re only moderately poisonous. A 145 lb (65 kg) adult would need to eat 9 to 18 lb (4 to 8 kg) of rhubarb blades to reach a lethal dose. Its toxicity largely comes from oxalic acid, a product which humans readily ingest in small quantities (it’s found in many tart-tasting foods, including spinach, sorrel and beets, as well as tea, chocolate, nuts, parsley and poppy seeds) and indeed, our own bodies produce oxalic acid. That said, people suffering from gout or kidney stones should avoid foods containing oxalic acid, and that even includes rhubarb stalks, otherwise considered safe to eat because they have only about one tenth of the oxalic acid of the leaf blade.
- Rhubarb is believed to be poisonous to pets. To what degree, though, is unclear. Quite likely, as with humans, the leaf blade is toxic, while the stalk may not be or may be less so. However, rhubarb poisoning in pets is so rare (most animals dislike its taste and won’t eat it) that the details are unsure. It’s unlikely to be deadly, but eating enough of it could make them sick.
- The word “rhubarb” is sometimes used to imitate the sound of a crowd. In the world of radio, television and films, groups of actors often repeat the word “rhubarb” as a background noise, creating the effect of a somewhat menacing crowd.
- Is rhubarb a fruit or a vegetable? Gardeners have long argued with cooks over that one! Botanically, it’s a vegetable, as we grow it for its edible foliage, not for its “fruit” (which are dry seeds and highly unpalatable). However, it is commonly used cooked in desserts (pies, compotes, crumbles, etc.), a domain where fruits (strawberries, apples, etc.) usually reign, so many people claim it as a fruit. In fact, it is called “pie plant” in some areas. In New York state, it has even been legally declared a fruit.
- In many areas, rhubarb is grown in hothouses. Not because it needs protection from the cold, but so it can be forced into growth early and thus brought to market during the winter, before outdoor cultivated rhubarb is available. The grower harvests mature crowns from outdoor fields and transfers them to a structure heated to about 55ºF (13ºC) for forcing. This rhubarb is grown entirely in the dark, leading to yellow leaves and brilliantly red stalks. “Hothouse rhubarb” is generally tenderer and sweeter than outdoor rhubarb and, since it’s an exclusivity, far, far more expensive.
- You can force rhubarb yourself with a pail. At winter’s end, cover it with a dark coloured pail, pot or bucket (you may need to weigh it down with a brick or rock), excluding all light. In the darkness and warmth (the container will heat up in the sun, helping keep the plant warmer), it will sprout extra early. When the pressure of the growing leaves starts to lift the pail, harvest the yellow leaved stalks and enjoy.
- In spite of claims the contrary, green rhubarb is not poisonous, nor do you have to wait for the stalks to turn red before you harvest them. Individual rhubarb cultivars can have green, pink or red petioles, just like people can have blue, brown, hazel, etc. eyes. The coloration has no effect on their taste nor their toxicity: they’re all edible. True enough, the taste clearly varies from cultivar to cultivar and some are indeed sweeter (or less tart) than others, but petiole color is not one of the factors that influences taste.
- You can harvest rhubarb all summer. Typically, rhubarb is harvested from spring to perhaps the earliest days of summer, when the petioles are most tender, but the belief that rhubarb petioles become poisonous in summer is untrue. If you don’t harvest in spring, you can do so anytime time until fall. Just expect the petioles to be a bit tougher. And do note you have to leave enough foliage on the plant for it to build up its energy for next year’s harvest.
- You shouldn’t harvest rhubarb the first year … unless you’re growing it as an annual. Let it grow and build up its energy in year 1 and even in year 2, harvest only moderately, perhaps two leaves per plant, leaving at least 5 strong ones to carry out photosynthesis over the summer. By year 3, harvesting can begin in earnest, but make sure you still leave at least 5 large leaves to carry on.
- You don’t have to remove the flowers, although many gardeners will claim otherwise. They’re very attractive, so let them bloom to their heart’s content and enjoy the show. Producing seed does use up quite a bit of energy, though, and could weaken the plant. So, if your plant is not very vigorous, just cut the flower stem back after it has finished blooming and allow the leaves to continue to grow.
- Rhubarb with hollow stalks is usually caused by either a mineral deficiency or unusually hot weather. If you suspect a deficiency, fertilize with a complete fertilizer like fish or seaweed fertilizer. As to keeping the plant cooler, about the only thing you can do is use mulch and keep the soil a bit moist. Even if the stalks are hollow, they remain edible.
- Rhubarb is long-lived. It’s one of the most perennial of all vegetables. Plants can live for 60 years or more. However, it commonly begins to decline after 20 years or so, sometimes sooner. If so, try dividing it, then replanting in a new spot in compost-enriched, well-drained soil in full sun. That will give it a new lease on life.
If you want to learn more about rhubarb, read this article: Rhubarb: for the Patient Gardener.