How a Forensic Botanist Helps Crime-Scene Investigators Decode Nature's Clues

Outdoor murder investigations can be quagmires in ways controlled-environment crime scenes are not. That's because a myriad of factors can muddy the picture as investigators try to reconstruct the crime scene—from erratically changing temperatures to the number of people coming through the area.

But outdoor crimes can also bring more evidence into play: the clues of the natural world.

We spoke with David Hall, a Florida-based forensic botanist who has worked on hundreds of criminal cases, to discuss the information plant life leave behind.

How does a forensic botanist help aid investigations?

Most law-enforcement officers even aren't aware of what a botanist can do.  In most cases, a botanist can destroy an alibi, put something or someone somewhere or help establish time of death.

What's the most famous case you've worked on?

I've worked a lot of homicides. Most recently, probably Caylee Anthony in Orlando a few years ago—the Casey Anthony case.

What did you do on the Casey Anthony case?

The child (Caylee Anthony) had been missing for quite a long time, several months. When they found the body, it had been disposed; it had been wrapped in plastic and a laundry bag and tossed into the edge of a swamp.

One of the problems with cases like that is the medical examiner needs to find a time of death. In this case..the medical examiner and the forensic anthropologist were puzzled.

One of the pieces of evidence were plant roots [that had] grown over and through the remains. They asked me, if by looking at those roots, I could give them an approximate time of death. I did. It wasn't very exact, but it was [within] four to six months.

How did you narrow the window that much?

By the length and thickness of the roots, and how many there were.

Tell us about other cases you've worked on where botanical evidence helped bring a suspect into focus.

In 1995 in the St. Petersburg/Tampa area, there was a case that had every agency involved: Cheryl Ann Commesso.

Guys were cleaning up the side of an interstate. They were raking and all of a sudden a skull bounced out. They called law enforcement, and they started finding other bones.

When you're doing that, it's important to try to find all the remains, which is difficult when it's been a long time, [And] in this case it had been. A forensic anthropologist—one of the top 10 in the world— was involved in [the investigation], but he couldn't give them any time since death.

When they finally had a suspect, the suspect said, 'How do you know that lady didn't die long after I left the area?' Timing became critical.

As they were working the case, they'd collected pieces of the vegetation, some of it growing through bones.

It took a long time, but I scattered all the evidence out, and finally I found a couple of pieces of plant that had grown through bones that I could positively identify. The person had to have died and deteriorated to the point that the plant material could've gone through those bones.

I looked at those plants—they were branches—and I could age the branches by the rings: the annual rings that were produced. And so I could give the approximate time the body had been there; it had been there before the suspect [Franklin Delano Floyd] had left. He was tried and convicted [of first-degree murder in 2002 and received the death penalty].

When a body is deposited into the earth, do plants at the site grow differently?

One thing most people don't realize is when you disturb soil, plants do grow more lusciously, in general.

The reason is not that the body is producing anything that helps the plant. Body fluids are certainly not helpful for plant growth—they'll basically kill a plant. [They] are caustic, and that will stop plant growth. If a body is put on the ground it can [also] kill a plant by shading it.

It's simply that the soil has been disturbed, which produces moisture and air and light.

When they push the soil back, they almost invariably scrape plants in with it, and the plants can then be used to help determine timing. When digging the grave, you usually cut through roots. Some roots will regrow, and you can use regrowth as part of timing, too.

You said in addition to figuring out time of death, botanical evidence can help destroy alibis.

For example, I had a case in South Florida—one of my first cases ever—which was a sexual assault. It took place outdoors.

They collected a blanket from the suspect's car, and there were a couple thousand fragments of plants on the blanket: sedges (marsh plants that looks like grass and grows in bunches), grasses, small dicots (dicotyledons, a type of flowering plant). I was able to put all those plant fragments into piles, and I found there were five different plants on that blanket.

They were able to find the scene where the young lady was sexually assaulted, and I was able to match all the plants to the blanket. The suspect—he had a family—said he and his family went on picnics and that was what the blanket was for. Law enforcement asked where [they went on picnics] and he [told them]. But there were three plants on that blanket that weren't in any of those places.

I've had dozens of cases like that. That's the most common thing I do. I ask, 'How did you get that material on you?' They've got an explanation and [then] I've got an explanation. But most people don't know enough about plants to lie about them.