At one point in Agatha Christie’s 1940 detective novel Sad Cypress, the mustachioed investigator Hercule Poirot visits Elinor Carlisle, a woman accused of killing her wealthy aunt. Asked about the crime scene, Carlisle recalls a detail that hadn’t crossed her mind before: the nurse caring for her aunt had a cut on her wrist, which she said came from a rose. This nugget of information seems trivial and useless, but then Poirot has a brain blast: the type of rose that covers the aunt’s estate is called Zéphirine Drouhin. It was first introduced in 1868 by a French breeder, and it’s completely thornless. The nurse, in other words, had lied. Why?

Sad Cypress is hardly the only murder mystery to revolve around a flower. As writer and landscape historian Marta McDowell observes in her new book, Gardening Can Be Murder: How Poisonous Poppies, Sinister Shovels, and Grim Gardens Have Inspired Mystery Writers, gardens are popular settings in crime and detective stories, especially those published in the late nineteenth and early twentieth centuries. The mysterious people that tend to them also feature commonly in these stories. Sometimes they’re the victims, sometimes the killers, sometimes the unlikely heroes who save the day. Gardens invariably lend setting, motivation, and symbolism. On occasion, as in Sad Cypress, gardening even provides the all-important clues to solving the crime.

Like those clues, the link between homicide and horticulture isn’t obvious. As a literary genre, murder mysteries matured during the Industrial Revolution, when rural villages turned into smog-covered suburbs replete with seedy establishments and dodgy alleyways. The first modern detective story, Edgar Allan Poe’s 1841 “Murders in the Rue Morgue,” takes place not in a forest, but on the cobbled streets of Paris. Its private eye, C. Auguste Dupin, was based on a real-life inner-city criminal turned law enforcer named Eugène-François Vidocq, whose work with the Paris Police Prefecture became a blueprint for those most urban of institutions, Scotland Yard.

But just because something isn’t obvious, that doesn’t mean it’s non-existent. Like the roots of an old tree, explanations for the murderous appeal of gardening dig deep, reaching disciplines as diverse as theology, sociology, and—of course—herbology.

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The number of horticulturally inclined murder mysteries goes beyond coincidence. There are classics like Carolyn Keene’s 1933 The Password to Larkspur Lane, in which teenage sleuth Nancy Drew competes in a flower show, and contemporary takes like Francine Mathews’s The White Garden, about a landscaper working for the lover of Virginia Woolf. In non-fiction, Susan Orlean’s The Orchid Thief comes to mind, as does Adaptation, a cerebral quasi-telling of the story for screen by Charlie Kaufman. Many scenes in Knives Out, one of cinema’s most successful murder mysteries in decades, were filmed in Borderland State Park, in a suburb southwest of Boston; its sequel, Glass Onion, takes place on a Greek island with an immaculately kept lawn.

Many fictional detectives, however different their eccentric personalities, share an interest in plants. These include Brother Cadfael of Edith Pargeter’s The Cadfael Chronicles, a globetrotting monk who, between solving medieval crimes, can be found inside his monastery’s herb garden, as well as China Bayles, protagonist of Susan Wittig Albert’s 2018 novel Queen Anne’s Lace; Bayles is a burned-out attorney who quits the courtroom to open her own spice store. Then there is Nero Wolfe, the New York City private eye dreamt up by Rex Stout, who in between cases (chronicled in 47 books so far) can be found nursing orchids on the roof of his brownstone.

The first fictional detective with a green thumb, McDowell writes, was Wilkie Collins’s Sergeant Cuff. First appearing in the 1868 novel The Moonstone, which “legitimized detective fiction in the English language,” the sergeant’s rough exterior hides a tender affection for roses. Joining the ranks in 1927 is Christie’s Miss Jane Marple, the Sherlock Holmes of St. Mary Mead, an astute elderly lady who picks up on the latest rumors while pruning away in her Japanese garden.

Although Holmes, the most famous of famous detectives, doesn’t show any particular fondness for rare flowers (he prefers playing the violin and indulging in cocaine), he does have an expert knowledge of plant-based poisons—a quality McDowell traces to the author’s time as a medical student; one of Arthur Conan Doyle’s first pieces of writing was a research paper on the toxicity of Carolina jasmine, which he tested on himself.

Gardens are especially commonplace in murder mysteries involving poison. Poison makes for great whodunits, not least because it establishes a degree of separation between the killer and the crime, and each writer has their personal favorite. While some prefer strychnine (as Christie did in her debut, The Mysterious Affair at Styles), others go for hemlock (see Elizabeth George’s Missing Joseph), while still others settle on more obscure poisonous plants like Yew, Foxglove, Castor Bean, and Monkshood.

In addition to poisons, gardens provide a host of inconspicuous tools to both commit a murder and cover it up. When, in Marion Chesney’s 1994 novel The Potted Gardener, a horticultural vandal graduates from dahlias and koi to stalking human targets, weed killer becomes the weapon of choice. Another book worth mentioning here is Ann Ripley’s 1971 hit Mulch, where severed body parts are found stuffed inside big, brown leaf bags.

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Perhaps gardens feature so prominently in murder mysteries due to the many ominous cultural connotations associated with them. Before early twentieth-century progressives began promoting parks and gardens to improve the living standards of the working poor, they were symbols of wealth and decadence. As the industrial historian S. Martin Gaskell writes in an article titled “Gardens for the Working Class: Victorian Practical Pleasure,” the word “garden” denoted

on the one hand, something that was the personal and private preserve of the upper classes, whether in immediate proximity to the great house or in locked London square, or, on the other, a place of gratification and amusement, frequently associated with a drinking establishment, nearly always a scene of dissipation, and more than usually of ill-repute.

These competing interpretations—one gesturing toward the hidden and secretive, one toward a general debauchery—made gardens a suitable locale for crime and detective fiction, genres that, aside from providing suspense and intrigue, have long been concerned with exploring the darker sides of industrialization, urbanization, and modernization – and indeed humanity itself.

Other, arguably more relevant, connotations are rooted in religion. Whether it is the author’s intent or not, every sin committed in a garden invariably harkens to the Biblical story of Adam and Eve, who, after eating forbidden fruit, were promptly expelled from Eden. Then there’s the tale of the couple’s envious son Cain, who killed his brother Abel in the middle of a freshly plowed field. Gardens are a battleground for good and evil, a meeting place of life and death.

In the post-Puritan world of nineteenth-century America—the world of Poe and Nathaniel Hawthorne—Romantic and Gothic literature alternately painted the great outdoors as magical or occult. Forests represented wilderness, savagery, and paganism; the garden was a space where these feral forces brushed shoulders with civilization. Researching the role of nature in Gothic fiction, Tom J. Hillard, a professor of American literature at Boise State University, found these sentiments condensed by the naturalist Henry David Thoreau, who—in the midst of Walden—refers to a parliament of owls as “the low spirits and melancholy forebodings, of fallen souls that once in human shape night-walked the earth and did the deeds of darkness, now expiating their sins with their wailing hymns or threnodies in the scenery of their transgressions.”

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The link between homicide and horticulture can also be explained through history. McDowell points out that Wilkie Collins conceived of the rose-loving Cuff during a time when his native England was experiencing a rose-boom, stating that, although roses had been popular there for centuries,

The introduction of the China rose, Rosa chinensis, to Europe in the mid-1700s tipped the scale. Enthusiastic hybridizers on both sides of the Channel and across the Atlantic began crossing china roses with other known varieties. [Offspring] yielded a spectrum of color, fragrance, and habit as well as a tendency to rebloom in one season. By the reign of Victoria, a rose garden was a must, whether you were one of the landed gentry (…) or stood solidly with Sergeant Cuff in the echelons of the respectable working class.

Likewise, Christie’s decision to give Miss Marple not just any garden but a Japanese garden may be related to the Japan-British Exhibition of 1910. Organized nearly two decades before the character’s first literary appearance in the short story “The Tuesday Night Club,” the exhibition, which drew more than 8 million visitors, generated an appreciation for Japanese landscaping that didn’t dissipate until the start of World War II.

Historian of rural culture Keith Snell connects the prominence of gardening in murder mysteries to the unique social and political conditions of the interwar period, when first wave feminism propelled a new generation of women’s writers, who placed their detective stories in the kind of setting that was most familiar to them: domestic, small-town life. Inside these closed communities, gardening sleuths like Miss Marple were tasked with maintaining order—a mission that appealed to readers in times of rapid change and great uncertainty.

As Snell puts it,

The detective, like the parish clergyman, restores the pre-murder status quo, the rural idyll, the sense of village innocence and benign fair play. He or she weeds out the evil, the nasty, the malevolent, from the flowerbeds where it does not belong.

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But maybe the link between homicide and horticulture is even more direct. Maybe the reason so many detectives have a secondary passion for gardening isn’t because the activity has some deeper, hidden meaning, but—rather—because gardeners naturally make good detectives.

McDowell lists their keen eye, obsessive temperament, and penchant for deductive reasoning, all of which she illustrates by way of quoting Sherlock Holmes as he studies the holes on a hosta leaf:

‘There is nothing more deceptive than an obvious fact.’ One collects pertinent evidence—a slime trail perhaps, or damage to other plants. Critical thinking is required. While the usual suspects for hosta mutilation would be a gang of slugs, it also might have been hailstones from last night’s freak thunderstorm.

She also acknowledges the gardener’s killer instinct toward intruders and undesirables—“rare is the gardener who can approach a slug without homicidal intent”—a quality that makes them just as formidable as murderers. Green is, after all, the color of jealousy.

Beyond the gardening profession, gardens and the plants growing within them make for broadly applicable metaphors that allow mystery writers to sprinkle a dash of imagery onto their procedural plotlines. In The Moonstone, the concept of grafting—fusing one plant to another by joining their stems or root systems—mirrors the novel’s themes of race, imperialism, and unrequited love. Brother Cadfael’s walled herb garden symbolizes his own withdrawal from a life of sins, just as the lush Italian villa at the center of Mark Mills’s 2006 novel The Savage Garden symbolizes human mortality.

According to McDowell, one of the reasons that plants make for good metaphors—and clues—is that they’re often overlooked.

“People have a tendency—validated in studies of school children—to rank animals first in importance,” she writes. “We can’t see the forest, not for the trees, but for the movement of birds and other members of the animal kingdom.”

This is certainly the case in Sad Cypress, where Hercule Poirot alone is able to recall that the garden roses were thornless—a detail the other characters (and the majority of readers) do not pick up on.

Finally, it’s possible gardens are so common in mysteries simply because the process of mystery writing is itself a little like gardening.

“An engagement to write a story must in its nature be conditional; because stories grow like vegetables, and are not manufactured like a pine table,” declared Hawthorne, who spent as many days writing as he did cultivating the land around his homes.

“Perhaps if we could penetrate Nature’s secrets,” he elaborated elsewhere, “we should find that what we call weeds are more essential to the well-being of the world, than the most precious fruit or grain.”

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