Plants can show your presence at a crime scene

How empty does a room have to be to have no witnesses?

Columbo fans will know a plant in the wrong place can yield a vital clue. Things have come on a bit since the seventies, and criminals are getting tracked by pollen. But how reliable is this? For example, if a killer walked through a park, could you place his shoes at the scene?

A gun and a footprint

Beverley Adams-Groom noticed that sometimes prosecutors use pollen analysis, but there aren’t many reference cases. So is this really evidence, or is it a convenient coincidence that looks persuasive? To test the idea, Adams-Groom set up a mock scene, without body, in a quiet corner of the University of Worcester campus. She took two samples as the police would when investigating a case.

She then asked twelve students to give her a puzzle. She asked some students to walk on the scene and some to stay off. The students decided this, so while she knew at least someone would have walked the scene, she didn’t know who. The students also chose what shoes they used. They could be clean or dirty. The student put on their shoes and walked to the crime scene over a hundred metres away. Then guilty and innocent alike, walked back. Finally Adams-Groom used some shoes from visitors to act as a negative control sample.

She found that she could indeed use the pollen collected by the shoes to identify who had actually been on the scene. The clues weren’t simply that Bob had a specific pollen on his shoe that Sue lacked. There’s also the relative quantities of pollen types that creates a ‘pollen spectrum’.

Adams-Groom points out some limitations of her study. For example, it’s a very quick return from the scene to analysis. The weather also helped with her work. Finally she had told the students where the scene was. This might not be so obvious when police investigate a scene. She notes that because pollen spectra can be variable. This means you might need many more control samples from the site to know what the palynology is like.

It also helps that the crime scene was outdoors, and so somewhere where you could expect pollen. What if the victim is indoors Another study by Nguyen and Weber asked Can pollen match shoes to a previously visited indoor location?

Nguyen and Weber use a factor that might not apply to secluded woodland. Indoors it’s not unusual to have ornamental plants. No everyone’s taste is the same though. If you have a particular love of some kinds of plant, you’re creating an unusual collection of pollen.

It’s an interesting idea. To test it Nguyen and Weber tested a flat (apartment) over several visits in different seasons. They then tested the shoes after visits to see what stuck. Disappointingly, it wasn’t always a lot. A few of the samples held less than 300 grains of pollen. Ironically, if you made a quick getaway from the scene, for example, in a car – then you helped preserve the pollen on your soles. More brazen criminals who were happy to walk from the flat could pick up more outdoor pollen on their soles. The longest escape routes were 1.8 km away from the flat.

The authors show that ornamental pollen alone is not likely to be enough to prove that a person visited a space. However, while it wasn’t in large amounts on the shoes they tested, some ornamental pollen could be found. Sometimes, even after the shoes had walked over a mile. So Nguyen and Weber say indoor pollen could still give hints to where someone has been.

Botanists seeking justice after their murder could help the police by keeping particularly unusual plants inside their house. Even a room with no eyewitnesses could still help identify a criminal.

Further reading

Adams-Groom, B. (2017). Assessment of pollen assemblages on footwear for evidence of pollen deriving from a mock crime scene: a contribution to forensic palynology. Grana, 57(3), 223–234. https://doi.org/10.1080/00173134.2017.1310293

Nguyen, P., & Weber, M. (2015). Can pollen match shoes to a previously visited indoor location? Grana, 55(2), 164–172. https://doi.org/10.1080/00173134.2015.1096955

Nguyen, P., & Weber, M. (2015). Forensic value of pollen from ornamental indoor plants. Grana, 54(3), 236–246. https://doi.org/10.1080/00173134.2015.1045024