Rhubarb is a rather curious plant. Since it’s grown for its edible leaves (actually, we consume only the leaf stalk or petiole), gardeners consider it a vegetable. But, in the kitchen, its tart sweetness means it’s usually used in desserts (pies, crumbles, cheesecakes, stewed rhubarb, etc.) and preserves (jams and jellies), so many cooks classify it as a fruit!
For the purposes of this article, though, let’s call rhubarb a vegetable. After all, if you’re reading an article in a gardening blog, you’re probably a gardener!
That said, rhubarb is still not a typical vegetable. Almost all vegetables we grow are either annuals or are treated as such, sown in the spring for a harvest the same season. However, rhubarb is a long-lived perennial that won’t even give you anything to harvest before the second or even third year.
A Look Back in Time
Rhubarb comes from Asia, possibly Siberia, but has been cultivated by the Chinese for at least 3000 years, although not as a vegetable. Rather, it was used as a medicinal plant, its roots having cathartic and laxative properties.
It’s believed to have been introduced to Europe via Turkey, having been carried there from China over the Silk Road, still for medicinal purposes. Moreover, the name testifies to this introduction, because the word rhubarb comes from the Greek rha barbaron, which means “barbarian rhubarb” (rha being the word for Greece’s native rhubarb species). Essentially, “barbarian” meant, in this case, “foreign,” an indication that it was seen as a novelty from a distant country.
Oddly, although rhubarb was cultivated as a medicinal plant for centuries, it is only since the end of the eighteenth century that it has been consumed as a vegetable … or as a fruit, if you prefer. Previously, it was considered too toxic for use in cooking.
The botanical name of garden rhubarb used to be Rheum rhabarbarum, the Siberian species mentioned. However, DNA tests have shown that most modern rhubarbs are actually hybrids, a cross between R. rhabarbarum and an unknown species. Thus, most taxonomists accept R. x hybridum as the proper botanical name for today’s plant, although some old cultivars might still be pure R. rhabarbarum.
Rhubarb is certainly an imposing plant, easily 4 to 5 feet (120–150 cm) high and 3 to 4 feet (90–120 cm) wide under average garden conditions. It bears a cluster of very large, medium-green, heart-shaped leaves, strongly ribbed, with very thick green or red stalks. At the end of spring, it produces tall and highly attractive flower stalks of white or greenish-white flowers.
Where to Grow Rhubarb?
Since we’ve (sort of) agreed that rhubarb is a vegetable, logically it ought to be grown in the vegetable garden. But that’s probably not the best spot for it. Its long-lived nature (it’s a perennial) and large size make it a poor choice for the traditional vegetable bed that is otherwise emptied and cleaned in the fall, then replanted each spring.
Its permanence means you can’t use a rototiller near it (if that’s your habit), plus due to its size and very dense foliage, it tends to shade out sun-loving neighboring vegetables. And a single rhubarb plant would more than fill a square-foot garden!
I, like many long-time gardeners, have long since moved rhubarb to the flower garden where it resides, perfectly contented, among other perennials that share the same needs. And it is doubly suited to the flower garden in that its huge wavy leaves, often with red stalks, and its particularly fine panicles of upright flowers, make rhubarb a very attractive plant.
And while we’re at it, no, you don’t have to remove the flower stalk before the flowers bloom, despite the long-held puritanical belief that since rhubarb flowers are attractive and give joy, they should be destroyed. Nor does letting rhubarb bloom weaken the plant. Like any typical perennial, it can easily bloom yearly and still produce a whole cohort of fine leaves. You may, however, want to remove the flower stalk after the blooms fade to prevent any seeds from maturing, as they can result in an abundance of unwanted, self-sown baby rhubarb plants.
Planting and Care
Plant rhubarb in the sun or, failing that, in partial shade, in a rich, deep, relatively moist, but still well-drained soil.
Usually, divisions, called crowns or roots, are sold in the spring, although autumn is also a fine time to plant them. Dig a hole wider than the crown and deep enough so the eyes (buds) will be eyes covered 2 to 4 inches (5 to 10 cm) deep, spacing the plants about 4 to 5 feet (1.2 to 1.5 m) apart. Mix plenty of compost or well-aged manure into the soil dug out. If the drainage is poor, consider planting in a raised bed. Set the division in place and fill with the enriched soil, then tamp down fairly firmly and water well.
An annual application of compost will suffice as a source of organic matter, but it may also be wise to apply an all-purpose slow-release organic fertilizer, following the dose recommended on the label, early in the spring, as rhubarb is a quite a heavy feeder.
Mulching, although not obligatory, helps keep the soil cool and slightly moist, much to the plant’s liking.
Other than the occasional watering in times of drought, rhubarb usually pretty much takes care of itself through the spring and summer.
Rhubarb Likes it Cool
Rhubarb is a typical perennial in that it loses its leaves in the fall and needs somewhat of a winter to succeed: several weeks at less than 40° F (5 ° C) are necessary to stimulate healthy spring growth and it can easily tolerate temperatures as low as -40 ° F (-40 ° C) with no special protection, so it will readily grow in hardiness zones 3 to 7.
You can also grow rhubarb in (huge) containers if you can manage to keep it fairly cool. A half-barrel would be about the right size.
It’s not nearly as easy to grow rhubarb in mild climates (zone 8 and above), although extra-thick mulch and partial shade can lead to some success in cooler parts of zone 8. Sometimes gardeners in hot climates grow rhubarb as an annual, sowing seeds indoors under air conditioning in the summer and planting the seedlings out when temperatures cool off in the fall. The leaves won’t be that big, but there will be harvestable petioles. If so, harvest all the leaves you want between early winter and spring, when heat will do the plants in anyway, forcing you to start more.
When Things Go Wrong
You can often let rhubarb grow pretty much on its own for 20 years or more, but if you see that yours starting to decline, which can happen as early as after 5 or 6 years under some conditions, it’s usually because it has produced several offsets and is starting to get too crowded. If so, don’t hesitate to dig it out and divide it: that will usually give it a new lease on life. You can divide and replant rhubarb in either spring or fall.
Rhubarb rarely suffers very seriously from pests. Also, since a bit of slug, flea beetle or aphid damage is not likely to affect the part you want to harvest, that is, the leaf stalk, you can simply ignore minor infestations. Diseases are more common and much more likely to occur on plants grown in soil that is too humid or spots lacking good air circulation. Usually, the treatment is easy enough: just transplant them into a more rhubarb-friendly environment!
Rhubarb From Seed
Although rhubarb is traditionally grown from divisions, you can also raise it from seed. Rhubarb seeds are not that widely available, but can be found in several specialist seed catalogs. If so, sow it in late spring or early summer for a harvest that will likely begin in its third or fourth year.
This is where patience, mentioned in the title, comes in. You’ll need it if you want to grow rhubarb, because, unless you’ve managed to purchase a full-grown rhubarb plant, you won’t have any harvest the first year from the divisions usually sown and likely not more than a leaf or two the second. It’s really only when the plant is well-established and growing bountifully that you begin harvesting, and that is usually in year three.
From then on, though, you can harvest up to two thirds of the stalks each spring, leaving the smaller ones so the plant can replenish itself. Traditionally, you harvest rhubarb in the spring, when leaves reach their full length. It is better to pull each leaf free with a twisting motion, because by cutting with a knife you risk transmitting diseases.
Of course, that’s the age-old way, but you can in fact harvest rhubarb right through the summer and into fall if you didn’t do it in the spring. That’s how market gardeners manage to supply rhubarb stalks off-season. And in late fall, even if you did do a thorough spring harvest, all those leaves that the plant no longer needs and will soon be turning brown still have edible petioles and that’s kind of a waste, so I suggest combining fall cleanup with a second harvest in September (Northern Hemisphere), while the leaves are still green. True enough, the second harvest may be a bit tougher and bitterer (and require more sugar) if the summer was hot and dry, but will still be very tasty after a cooler, fairly rainy summer.
In some countries, rhubarb is traditionally blanched, that is, forced under cover so it grows with no light. This gives a slightly earlier harvest of pale growth that is sweeter than regular rhubarb. You simply have to cover the plants with an opaque container early in the spring. In fact, there are even rhubarb forcing pots, traditionally made of terra cotta, you can buy. Put them in place at the end of winter and leave the containers in place for about 6 to 8 weeks, until the leaves, still pale and yellow, start to expand inside, then harvest away!
Growing for weeks without light can be hard on the plant and usually rhubarb plants thus treated are given a year off with no forcing so they can recuperate. Even so, forcing rhubarb is in decline. I suspect gardeners are tired of going through all the extra work … and expense (terra cotta forcing pots are not cheap!)
You can also force rhubarb for a midwinter harvest by potting it up in the fall and growing it under cover. This technique is usually used by market gardeners who want to produce extra-early rhubarb for customers willing to pay a premium for the first rhubarb of spring, but the home gardener can do it too. If so, pot up a mature plant in the fall, but don’t bring it indoors right away: allow it to undergo at least a few degrees of frost outdoors. Then store the pot cool and dark, in a garage or root cellar for example, for at least two months, keeping the soil at least slightly moist at all times. Then expose it to gentle heat (about 15 ° C) and up the leaves will come. The stalks will be ready to harvest in about a month.
How Poisonous Is Rhubarb?
Most gardeners know that you should only eat the stalk of a rhubarb leaf, never the blade, because it’s poisonous. But how poisonous is it really?
In fact, very little is known about the subject. The truth is toxicologists don’t even know what causes the toxicity!
It used to be said that the problem was oxalic acid … and indeed, a lot of sources still make that claim. While in low doses, as in rhubarb stalks, oxalic acid gives the crop a delicious acidulous taste, it becomes toxic when in too great a concentration. (As the saying goes, the poison is in the dose!) However, although rhubarb leaf blades are about twice as rich in oxalic acid as their stalks, they’re still not as rich in oxalic acid as other plants we regularly eat, like spinach or chives. You would need to eat many pounds of rhubarb leaf blades to suffer from oxalic acid poisoning!
There have been very few cases of poisoning due to eating rhubarb leaf blades (I could only find mentions of two) and neither was truly unambiguous. They might have been caused by something else. Some toxicologists suggest that the true cause of rhubarb leaf poisoning, if indeed it exists, might be due to the anthraquinone glycosides it contains. But no one really knows and toxicologists simply don’t go around feeding people potentially poisonous leaves to find out!
As a result, the cause of rhubarb leaf-blade toxicity and even its degree of toxicity remain essentially a mystery. The truth may one day come out, but in the meantime, I say don’t take a chance: eat only the leaf stalks!
You Can Compost Rhubarb Leaves!
One thing is certain: there is no need to worry about putting rhubarb leaf blades into the compost bin, despite the popular belief that because they are poisonous, they’ll harm the compost’s beneficial organisms. In fact, microbes are able to break down almost any natural toxin and besides, seem to absolutely adore rhubarb leaves, which they digest very quickly. You can even add rhubarb leaves to a compost pile that’s maturing a bit slowly in order to give it a boost!
Do Rhubarb Leaves Make a Good Insecticide?
Decoctions of rhubarb leaves are a popular homemade insecticide. They’re made by boiling the leaves in water, then pouring the cooled and strained solution into a spray bottle along with a little insecticidal soap for better adhesion. Then you spray it onto insect-infested plants. It’s said to be especially effective against aphids.
Is rhubarb spray effective? Some people claim so, others have less success with it. I don’t think anyone has ever done a bonafide study, one that would, of course, include a control group, to check it out. If the idea of a homemade insecticide pleases you, try it and see.
I use a similar treatment in my garden. I dilute insecticidal soap in water, but skip the the rhubarb leaf decoction, and that works very well!
For an atypical vegetable, you’ll find a surprisingly large number rhubarb cultivars, most available only from seed. I have no particular recommendations. Ideally, you’d visit a farmer’s market where you can do a taste test, then choose your favorite.
In the past, green-stalked varieties, like ‘Riverside Giant’ or ‘Turkish’, were the most popular, but these days, rhubarb with red stalks, such as ‘Canada Red’, ‘Valentine’ or ‘Macdonald’ have definitely taken over the market. The popular variety ‘Victoria’ is sort of half and half: red at the base, green at the top.
Enjoy growing this charming, easy-to-grow plant and savor your first stewed rhubarb … in about 3 years’ time!