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Solving the Mysteries of Plant Identification

Posted: October 5, 2015

Every year, weeds come up in various parts of my property. They go through their life cycle, perhaps supporting insect life, perhaps invading habitat and driving out native plants. I deal with them the entire growing season without any real idea of what they are and what role they play in the life of my garden.
Ocimum basilicum (sweet basil)

Ocimum basilicum (sweet basil)

This season, I decided to try to find out. The search plunged me deep into the arcane and strange world of plant taxonomy. Taxonomy is the system botanists use to classify and name plants. The language of taxonomy is Latin and Greek, so I had to buy a book with a glossary of botanical terms before I could even get started.

Every plant catalogued has a botanical name consisting of the genus name followed by the species name. Then it has one or more common names. Take sweet basil for example; its botanical name is Ocimum basilicum (fragrant basil in Greek) and its common name is sweet basil.

You have to own a field guide if you are going to collect specimens to classify. Popular field guides like Newcomb's Wildflower Guide are less technical and can get you pretty far along. Newcomb's gives the common name for the plant, with botanical name following in brackets. Newcomb and many other guides index by common name and botanical name.

A caution about collecting specimens; if you want to make yourself really sick, start fooling with weeds without gloves and a dust mask. I have had continual hay fever since I pulled up a pigweed and brought it inside to preserve it. One pigweed can pollinate an entire house in about an hour. Also, many weeds are poisonous or unpleasant in other ways. That's the main reason we don't like them. Moral - prepare yourself before you start.

Also, it helps to have a specimen collection book or plant press. If you buy one, the book will have two hard covers, two straps that wrap around the book to hold things tight, and a number of paper pages for laying out your specimens interleaved with corrugated cardboard for air circulation.

These books are easy to make from newsprint paper, cardboard, belt straps, and plywood. It helps to spray the flower with hairspray or shellac to keep the pollen from spreading. I wish I had thought of that before I brought home that pigweed.

In every field guide the characteristics of each plant are described in the language of botany. This again is something you have to learn. Leaves have margins, petioles, venation. They are sessile, glabrous... to identify a plant, you either need a glossary of botanical terms by your side or you will need to look up the words on the Internet.

Once you have your field guide and collection book, you can start collecting. Try to collect the whole plant including roots and flower. Don't collect from public lands. You can get more than enough from your own yard. Keep a notebook of where and when you collected the specimen. Take pictures of the plant in situ and get close ups of the flowers because some of them collapse almost immediately upon collection.

Flowers are the most important part in figuring out what plant you have. Although leaves and leaf arrangements are noted, they are not individualized enough for classification. Most herbaceous plants have leaves opposite each other, leaves whorled around the stem, or leaves alternating up the stem. Therefore, the best time to gather specimens is when the weed is in bloom and prepared to shower you with pollen.

Field guides identify most flowering plants by one of several plant keys, perhaps even a unique key written by the author of the field guide you are using. The keys look like outlines and ask a series of dichotomous (yes-no) questions at each level. For example, if a flower has five petals that all look identical, go to page 16 where the author lists all plants with five identical petals in its flower. If the plant has five identical petals AND the leaves are opposite each other on the stem, go to page 20 where there is a smaller group of plants to choose from.

Eventually, in theory, by the process of elimination, you arrive at the only possible candidate for your sample. In reality, it can take a lot of hunting and verifying before you can be sure you have the right answer.

After days of frustration with plant keys, I discovered there are new approaches to plant identification under construction on the Internet. The University of Missouri website groups weeds first by flower color, then by leaf arrangement. You may find a perfect picture of your sample in their photo gallery.

There is a new weed Identification tool being developed at the University of California Davis at the  Weed Research & Information Center. Instead of classifying by flower, they offer a database search tool in the form of a questionnaire. You only answer questions if you are certain of your answer. Each time you answer a question, you select for all the plants with that characteristic. The more questions you answer, the fewer plants fit the description. Right now the only search that works is for California plants. However, eventually the database will cover all the states in the country.

If you are wondering why I am spending time looking at weeds, the answer is in the outcome. I discovered growing on my property weeds that are poisonous, invasive, or high on my allergy index. I also discovered weeds that host a particular butterfly caterpillar. Think how we used to regard milkweed before we knew about monarch butterflies.

Eileen East
Penn State Master Gardener of Carbon County