Yorkshire, England, is one of the few places in the world where rhubarb is harvested the old-fashioned way: by candlelight. Every November, after the first long frost, farmers transplant their crop into windowless heated sheds, plunging thousands of plants into total darkness. The stress triggers alarmingly rapid growth. Under the right conditions, rhubarb can grow more than an inch per day. That’s fast enough to hear it.

Farmers have relied on this recipe of frost, darkness and warmth for more than 200 years. The combo tricks this bright-pink vegetable into growing sweeter, more tender stalks—a product called “forced rhubarb.” Yorkshire’s Rhubarb Triangle—a nine-square-mile patchwork of green fields and hedgerows in northern England—once produced about 90 percent of the world’s forced rhubarb. And now, like Parmesan or Champagne, Yorkshire Forced Rhubarb has a protected designation of origin from the European Union.

Today, the Rhubarb Triangle attracts thousands of tourists and foodies each year. You’d never guess that, until very recently, the industry was at risk of dying out altogether.

“The rhubarb forcing industry has shrunk back dramatically,” says Ben Asquith, the last in a long family of rhubarb growers in West Yorkshire. “Forty years ago we had over 70 families growing the crop.” Today, fewer than a dozen keep the tradition alive—but a rebound could be on the horizon.

A sweet discovery

According to legend, the method for forcing rhubarb was first discovered by accident. The year was 1817, and the setting was the Chelsea Physic Garden, a now-venerated London botanical yard that was founded in 1673 by the Worshipful Society of Apothecaries, a group dedicated to the propagation and study of medicinal plants. Rhubarb, which has been used in traditional Chinese medicine since at least 270 B.C.E, was among the society’s early collections.

packing rhubarb
Farm worker Marie Emery (left) packs forced rhubarb, which was harvested by candlelight, on Robert Tomlinson's farm in the Rhubarb Triangle. Oli Scarff/AFP via Getty Images

One day, a horticulturalist allegedly dropped a bucket over a rhubarb plant. A few weeks later, he discovered that the plant had responded to the darkness by growing blindingly fast, sending up dead-straight stalks in an unnerving shade of pink. Over time, growers discovered that these stalks were not only prettier than the traditional green-streaked maroon—they also tasted much better. The rapid growth stretched out the plant’s cells, making them more flexible and therefore more tender. Better yet, the plant fueled this growth by injecting stored sugars into the stalk, resulting in a much sweeter crop.

By 1877, farmers in Yorkshire—a region of northern England that was already a rhubarb-growing epicenter—had taken up the practice in earnest. Growers scaled up the bucket method, building long, windowless sheds that could accommodate huge numbers of plants. Every winter, they would pull rhubarb plants from the wet fields, place them on the ground or in racks or replant them in the dirt floor in the heated sheds, and wait. Within days, the growth would begin.

How it works

Rhubarb isn’t the only plant that reacts this way. Other plants like sea kale, asparagus and endive can also be forced, according to Jeanne Osnas, an ecologist, affiliate faculty member at the University of Alaska Anchorage and co-author of The Botanist in the Kitchen blog. With all of these vegetables, the abnormally rapid growth starts when the plant receives a series of special cues. First, there’s the frost.

“When the plant cools down or warms up, it affects the chemical environment inside the plant,” says Osnas. “This allows different processes to turn on or off. You can think of each of these processes as a cascading series of events—like a domino chain.”

In this case, the frost kicks off a cascade of chemical reactions that primes the plant, telling it to keep an eye out for its next cue: the warmth and a change in the light. But, without eyes, how exactly does it sense that light?

Harvesting rhubarb by candlelight
In commercial sheds, farmers have to check on the crop vigilantly to make sure it doesn’t rocket right past the point of harvest. To do this, they use candles. Monty Rakusen/Getty Images

Easy, says Osnas. Like many rudimentary organisms, plants rely on several different types of photoreceptors. One is a protein, a large molecule called a phytochrome that reacts to light availability. When the light changes, the molecule twitches.

“They really are little light switches,” Osnas says. “They actually do change shape based on light. It’s pretty sweet.” In rhubarb’s case, the shapeshifting phytochromes and other photoreceptors tell the plant that it’s dark, which kicks off another chemical cascade—this time telling the rhubarb to grow, grow, grow.

Once it starts, the process happens brutally fast. After all, it’s an act of desperation, explains Helena Dove, a horticulturalist and kitchen-garden educator at London’s Royal Botanic Gardens, Kew.

“When you take the light away from the rhubarb and add warmth, it thinks it’s spring,” she says. But without light, the plant is critically deprived. “It panics.”

Over the next five to eight weeks, the plant throws everything it has at a single bid for survival: Find light, and find it at any cost.

“It has to grow upward, but it can’t photosynthesize to make energy,” Dove says. “So it has to use its stored energy reserves to grow.”

To fuel this lightless growth, the plant’s rhizome—or tuber-like underground root—releases the precious carbohydrates it’s spent all winter hoarding. The result is a literal sugar rush. Glucose surges up into the stalk, giving the plant the energy it needs to shoot upward.

Oldroyd's Farm
By the 1950s and ’60s, rhubarb demand had plunged, and rhubarb growers were leaving the industry en masse. Now, just nine remain. Janet Oldroyd Hulme’s family is one of them. John Giles - PA Images/PA Images via Getty Images

With limited energy to spend, the plant takes as many shortcuts as it can. Making new cell walls, for example, takes a lot of fuel, says Dove. Instead, the plant elongates its existing cells, making it taller but weaker—and therefore more tender to discerning palates. The plant also grows much straighter, an attempt to shorten the distance it has to grow to reach the sun.

“This also causes rhubarb’s red pigment to get stretched out,” says Dove. “And because the plant isn’t photosynthesizing, it’s less green. So it becomes this off-the-charts kind of pink—a proper Barbie pink.”

In just a couple months, the rhubarb reaches up to three feet tall. It grows so fast in a home garden that it can knock the lid off a traditional forcing pot, Dove says. In commercial sheds, farmers have to check on the crop vigilantly to make sure it doesn’t rocket right past the point of harvest. To do this, they use candles. The plant can detect anything much brighter than that, explains longtime rhubarb grower Janet Oldroyd Hulme, and if it does, the forcing process is disrupted.

“Even a couple of seconds of bright light in the middle of the night can completely reset a plant’s sense of its seasonality,” adds Osnas.

The farmers don’t chance it. Instead, they set wax candles on metal spikes among the plants. They light the wicks with care, and tiptoe up and down the rows by the flickering light of the flames.

A dramatic downfall

In the late 1800s, the forcing process revolutionized rhubarb production, giving farmers a fast-maturing, premium product that was extremely valuable. People were already into rhubarb, of course. In the 1500s, traditional rhubarb was considered extremely exotic; at its peak, it cost more than opium or saffron. But by the 1800s, costs had come down, and rhubarb still retained its allure. Plus, this new forced rhubarb was even better.

By the 1930s, the number of forced-rhubarb growers in Yorkshire’s Rhubarb Triangle had ballooned to about 200. Then, during World War II, the government set regulations to restrict the price of rhubarb to just a shilling per pound, in an effort to make it accessible to more people. Throughout the war, it was used in crumbles, jams and desserts, or roasted alongside other vegetables. At the time, no one guessed this government-sponsored explosion in use would bring the rhubarb industry to the brink of destruction.

Worker at Oldroyd's Farm
Valentin Vulpe harvests forced rhubarb by candlelight at Oldroyd's Farm. Danny Lawson/PA Images via Getty Images

“During the war, the government kept the price of forced rhubarb at one shilling a pound so the nation could have the benefit of forced rhubarb in winter,” Hulme says. “But by the end of the Second World War, everybody was tired of it. It was a bit tart for their tastes when sugar was rationed. And then refrigerated transport came along and brought tropical fruits to a nation that was tired of rhubarb as the staple of the diet.”

By the 1950s and ’60s, rhubarb demand had plunged, and rhubarb growers were leaving the industry en masse. Now, just nine remain. Hulme’s family is one of them.

“My father was really involved in trying to hold the growers together,” Hulme says. “He said, ‘There will be light at the end of the tunnel—we just have to hold on, because people will come back to eating rhubarb again.’ I’m so glad he lived long enough to see that he was right.”

Rhubarb on the rise

Today, rhubarb is making a comeback, in part due to changing tastes and a resurgence of interest in home baking and thoughtfully grown, local food. Yorkshire’s annual Rhubarb Festival, held each February, now attracts people from all over the world, says Hulme. Tourists come to tour the farms, sample local produce, attend demonstrations, and listen to talks and live music. Foods on offer include rhubarb pork pies, jams, gins, liqueurs, and various tarts and other desserts.

Rhubarb Festival performers
Wearing the colors of rhubarb (green, pink, red and yellow), the Rhubarb Tarts from the famous Rhubarb Triangle in West Yorkshire perform at the Wakefield Rhubarb Festival. Tessa Bunney/In Pictures via Getty Images

“It just seems to get busier and busier every year,” says Hulme. “It’s amazing to me and to other people how popular rhubarb is.”

Outside of rhubarb forcing season, tourists still come to the region, where they can tour the farms and purchase rhubarb-inspired foodstuffs year-round. Hulme says some of the interest might be because of recent studies touting rhubarb as a health food. (The plant contains a high concentration of polyphenols, for example, which help prevent the growth of cancer cells. It’s also used as a weight-loss food thanks to its high fiber and low caloric density.) She says the sour taste is also a big draw. Over the past decade or so, home baking has taken off in the United Kingdom and United States, fueled first by the runaway popularity of The Great British Baking Show and then by the proliferation of pandemic-era DIY hobbies. During this same period, Hulme says, Western gastronomic preferences have shifted away from cloyingly sweet desserts and toward dishes with more complex flavor.

Kew Gardens’ Dove also expects rhubarb to shine in the face of global warming.

“People are looking for crops that can be self-sufficient and grow well in a changing climate,” she says. “Perennials [like rhubarb] are it. They have these big roots. If it’s dry, they can find water really deep down. If it’s wet, they have enough roots to take up the water. They’re incredibly resilient.” Plus, she says, since perennials come back annually, there’s no need to sow seeds or transplant seeds year after year—a process that involves disrupting the soil, transporting seeds and using huge amounts of plastic.

“People sometimes forget about perennials, because things like peppers and tomatoes are really attractive. But our perennial plants—our artichokes, fruits, asparagus and rhubarb—will be with us for 20 or 30 years,” Dove says. “They’re incredibly efficient. In the future, I think we’ll see more people growing rhubarb all over the place.”

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