“I liken the best mystery clues to the self-sowing annuals in my border … understated but essential.”

— Marta McDowell, “Gardening Can be Murder,” 2023

All month I’ve been writing about the ways in which gardens and plants have been featured in the plots of mystery novels. Gardens, both divine and desolate, have been the settings for crimes; rare garden plants (and human greed) have been the motive for crimes; poisonous (and in some cases man-eating) plants have provided the means to commit crimes.

But as McDowell explains in a chapter titled “Clues,” the gamut of horticultural devices employed in whodunits has also included forensic botany — the science of examining plant-based evidence — to solve crimes.

A case in point is “Sad Cypress” (1940), one of Agatha Christie’s novels in her Hercules Poirot series. Elinor Carlisle, accused of murdering her elderly aunt, remarks to Detective Poirot that her aunt’s nurse had a scratch on her wrist right after the body was found. When questioned about it, Nurse Hopkins said the scratch had come from brushing up against a trellised rose by the front door. However, Poirot revisits the scene of the crime and discovers that the rose in question, an old garden rose called “Zephirine Drouhin,” is completely thornless.

So much for the credibility of Nurse Hopkins.

In Tony Hillerman’s more recent mystery novel “The Wailing Wind” (2002), Officer Bernadette Manuelito finds a murder victim’s clothing covered in rabbitbrush seeds and spiked burs called puncture vine wedged in the soles of his shoes. Bernie’s uncle, a Navajo healer, tells her puncture vine (also called goat head due to the shape of the burs) requires more moisture and different soil than rabbitbrush. This clue turns out to be important evidence that helps Bernie find the killer and solve the case.

For those who prefer reading about real-life cases involving forensic botany, “Murder Most Florid” (2019) by Dr. Mark Spencer offers a first-person account of how he used plants or plant parts as evidence to solve murders, sexual assault cases, burglaries and arson. Spencer’s book demonstrates that age-old botanical science has an important place in investigating crimes beside more recent methods such as DNA analysis and forensic genetic genealogy.

I love watching television crime shows, and some of my favorite episodes have involved forensic botany. During its first season in 1996, “Forensic Files” aired a segment called “Planted Evidence” in which a young woman was murdered outside of Phoenix, Arizona. Investigators found a beeper close to the body, which led them to their suspect. However, the man said his beeper had been stolen by the victim, and there was no other physical evidence that placed him at the scene.

That is, until investigators found two seed pods from a palo verde (Parkinsonia florida) tree in the bed of the suspect’s truck. For the first time in a criminal case in the U.S., plant geneticists were able to profile the DNA of the pods from the truck and match them to the DNA of a particular palo verde tree where the body was found, leading to the suspect’s conviction and life sentence. Amazing!

In fact, I learned plant forensics has been recognized as a valid tool in criminal investigations since 1935, during the trial of Bruno Hauptmann for the kidnapping and murder of Charles and Anne Lindbergh’s 20-month-old son, Charles Jr. One of the pieces of evidence gathered at the Lindbergh’s home was a homemade wooden ladder found outside the baby’s bedroom window. U.S. Forest Service scientist Arthur Koehler identified the type of wood used to make the ladder, which led to the discovery of where the wood was milled and where it was sold. From there, the FBI traced the sale of the wood to Hauptmann — a key piece of evidence that led to his conviction and execution.

Since then, hundreds of criminal investigations have used forensic botany to provide evidence. A forensic botanist helped solve a murder case in Florida by establishing the time of death from the length and thickness of plant roots that had grown over and through the victim’s remains.

Another case in Florida was solved when a forensic botanist identified fragments from five different plants on a blanket in the suspect’s car and linked those fragments to the same five plant species growing in the area where the victim had been sexually assaulted.

Plant pollen identified on a suspect’s shirt helped place him at the scene of another sexual assault in New Zealand.

Plant cells found in the stomach contents of a young boy found in London revealed he had been given a lethal potion made from the seeds of Calabar beans (Physostigma venenosum). This leguminous plant is native to tropical regions of Africa, and this information helped to identify where the boy and a possible suspect had come from.

Each of these cases involved scientists with specific knowledge in systematics (plant classification), palynology (study of pollens), dendrochronology (study of tree rings), limnology (study of aquatic environments) and/or ecology (study of ecosystems).

Of course, gardeners like you and me don’t have occasion to use our botanical knowledge to help solve big crimes; however, we can use our keen sense of observation and problem-solving skills to crack more modest transgressions occurring in our growing spaces.

For example, lately I’ve been investigating the untimely death of some of the plant starts in my raised vegetable bed. First, I used basic plant classification knowledge to identify the victims: strawberries (Fragaria x ananassa) and butter lettuce (Latuca sativa “Drunken Woman”). Next, I profiled possible suspects: common predators of strawberries and leafy greens are earwigs and snails/slugs. Both suspects primarily feed at night and leave large holes in plant leaves.

My stakeout at the scene of the crime, after dark and armed with a headlamp, was successful in catching the culprits in the act. By this, I mean I found two snails mating beside my strawberry plants, but the circumstantial evidence certainly points to the amorous duo’s guilt in bringing about the demise of some of my young plants. In my mind, this case is solved.