In 1997, a woman in Christchurch, New Zealand, was raped in an alleyway. She described the assailant to the police and a man was arrested, but he denied having committed the crime and there was no DNA evidence. The case looked hopeless until an unexpected witness emerged—a flowering wormwood shrub that had been damaged in the struggle. Native to the Mediterranean, it’s an uncommon plant in New Zealand; and when a forensic lab examined the suspect’s jeans, they found a pollen match with 99.9% accuracy. This evidence was used at trial to sentence the man to eight years in prison.

The field of forensic botany may be less popular than fingerprint or ballistic analysis, but it’s just as useful. There are around 400,000 plant species on earth, each with a unique pollen type which can b...