How to Identify Plants in the Field

Part of the Biodiversity Counts Curriculum Collection.

Identifying plant specimens is the most important job in botany, according to Brian Boom, Vice President of Botanical Science and the Pfizer Curator of Botany at the New York Botanical Garden. "There's an old Chinese proverb that goes something like, 'The beginning of knowledge is knowing the names of things.' That's how scientists communicate," he said. 

As a professional botanist and an amateur entomologist, Brian told us that it is much easier to identify plants than arthropods. "There are fewer plants than arthropods, for one thing, and they're mostly bigger. And in this part of the world, they have been studied more thoroughly." That would not be true in the rain forest, where Brian has worked identifying trees, but the native plants of North America are generally well known. 

All the same, there is a discipline and a challenge involved in the identification of plant species. "Identifying involves a comparison between a known and an unknown, so you have to have a guide or a field manual or a key of some sort (the known) to compare with your specimen (the unknown)," he said. Regional field guides are available in libraries and bookstores; other sources are local museums, botanical gardens, wildlife and conservation agencies, and college and university departments of botany. 

"Identification begins with observation," Brian said. "You have to observe the qualities of the unknown, but to do that accurately—so you know what to look for when you are using a key—you need to know some plant basics: the difference between perennial and annual plants, for example, and some general information about plant parts—flowers, leaves, roots, seeds, and fruit." 

Brian described a typical dichotomous key for plant identification, which presents a series of choices to narrow down the search. "Is the specimen woody or non-woody? If it is woody, is it a tree, a shrub, or a woody vine? If it is a tree, is the leaf arrangement opposite or alternate? Are the leaves compound or simple? Do the leaves have entire margins, or are they serrated? And so on." 

Brian warned that a plant detective can make a lot of progress with this line of questioning up to a point, "but a botanist's life starts getting difficult at the species level, because you have to use flowers and fruit to distinguish between species. The vegetative features (leaves, needles) of plants are not very characteristic at higher levels of classification. There will always be difficult specimens, especially if they are sterile," that is, without flowers and fruits. "Fruits and flowers are what systematic botanists use to get to the word 'go,'" he told us. "That is the basis of the whole classification system, so if you don't have them, you have to rely on experience or guesswork to try to find out what something is." Brian recommended asking a local expert for help if you are stumped. 

Identification Tool Kit 

The tools of identification are relatively few and simple. The following tools are part of the field kit, which also includes tools and equipment for specimen collection. 

  • Field guide with keys to plants of the region 
  • A hand lens, to examine plants at close range 
  • Binoculars, to look at things high up in a tree, for example 
  • A metric ruler, to measure leaves and other small features 
  • A metric tape measure, to measure the diameter of tree trunks 
  • An altimeter, to measure the altitude of your site 
  • A compass, to determine the location of your site 

Brian Boom suggests wearing the lens and ruler on strings around your neck. Another idea is to tape an actual ruler or a photocopy of a ruler to the inside cover of your field journal. 
Even experts sometimes make mistakes when identifying plants. Brian showed us an example from his own work, and because of the careful way he kept his records, we could follow the history of the mistake and its correction by looking at his field journal and specimen sheet. He collected a specimen consisting of a twig with a few leaves in Bolivia in 1984. While still in the field, he identified it as being a member of the Euphorbiaceae, a large family commonly called the spurges, with more than 5,000 species. Most are tropical plants, but the decorative poinsettia is one family member we know in North America. "In all fairness, the plant was sterile," he said. In other words, he had no flowers, seeds, or fruit to help him identify the plant. 

"But when I got back to the lab I was able to examine it more thoroughly without a lot of mosquitoes buzzing in my ears, and I changed my identification to a species of Moraceae," a family that includes mulberries, figs, and Indian rubber trees. Then, in 1992, the world Moraceae expert took a look at Brian's specimen and said it belonged to a different genus of Moraceae. "At least I had it in the right family," Brian said. 

The specimen label Brian showed us gave the full history, including the name of the person who made each identification. Each time a correction was made, the incorrect name was crossed out. "Never erase anything," advised Brian. "Simply cross things out so others can trace the history. Sometimes when I've made a real whopper of a mistake, I might scratch it out heavily, but that's not the same as erasing," he joked. 

"Seriously, though, making mistakes is part of the process, and it's nothing to be embarrassed about. What's important is to document everything so you can go back and determine when the mistake occurred, which is why you never erase anything from a field journal either."