How technical are you prepared to be? Few people among the public are aware of the trace evidence field of forensic palynology, the use of pollens and spores as evidence. It has been used for such things as determining whether a body has been moved or when the homicide happened (based on seasons). It could easily be the key to blowing up a fabrication of more gross evidence, like leaves, that might more readily come to an actor's mind.
But the classic uses of plant evidence is in time of death, based on what was in bloom or in leaf when the evidence became associated with the victim. Forensic botanists have been used to do things like show that a body had been deposited before a suspect moved away, because of the age of plants growing up through the skull after it had deteriorated to the point of allowing it.
Anyone brushing against a bloom would get pollen on their clothing. Walking through a field fills your pant cuffs with seeds and such. Cases have turned on such things as seasonal changes in shading foliage that caused differences in bleaching on tissue. Tree DNA featured in a case on A&E Real Crime.
He might also discover an insect or insect carcass and know that it wold be found only on particular plants or in particular environments.
There are Internet resources that recount cases from palynologists and botanists, so you can adapt something without becoming an expert yourself. And remember that vehicles readily pick up plant materials when off the pavement. A leafy twig trapped in a joint of a car body might provide both identity of the plant, which might not be common, and how recently it had been captured.
