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Plant Identification: Examining Leaves

Pat Breen, Oregon State Univ., Dept. of Horticulture

       To identify an item is to recognize the item and associate it with its appropriate name.  Such as, that tan automobile in front of our house is a Honda Accord.  Or, that large woody plant in the park is a tree, more specifically a Doug-fir.  Identifying a landscape or garden plant requires recognizing the plant by one or more characteristics, such as size, form, leaf shape, flower color, odor, etc., and linking that recognition with a name, either a common or so-called scientific name.  Accurate identification of a cultivated plant can be very helpful in knowing how it grows (e.g., size shape, texture, etc.) as well as how to care and protect it from pests and diseases.

       First let’s look at some common characteristics of plants that are useful in identifying them.   Now if this was a botany class dealing with plant systematics, the field of study concerned with identification, naming, classification, and evolution of plants, we would spend a good deal of time on the reproductive parts of plants, i.e., mostly the various parts of the flowers, i.e., ovary, stigma, etc.   Structural similarity of reproductive parts is an important means by which plants are categorized, grouped, named, and hence identified.  However, with many horticultural plants, especially woody plants, we may have to make an identity without regard to flowers, for often flowers are not present or are very small, and other characteristics may be more obvious. Some plants characteristics are so obvious or unique that we can recognize them without a detailed examination of the plant. Similarly, we can probably all immediately recognize a Volkswagen Beetle among a group of cars in a parking lot.

So what are some plant characteristics that can be used to identify plants?


Leaves are often the basis for identifying plants since they are so easily observed. They usually consist of two parts,

       First be aware that all the leaves on a given plant do not have the same size or even appearance [Betula papyrifera, shoot, comparison].   They may vary in size, color, and even shape [Sassafras albidum, leaves, fall] and [Malus sargentii, leaves, fall]; those that receive much sun may look different from those in heavy shade.   So when trying to determine the identity of a plant by its leaves, make sure you examine many leaves and attempt to determine what might be considered “typical” leaf characteristics.  Although basketball players may vary in size, shape, and color, a “typical” physical characteristic of a basketball player often is “tallness”.

Broad vs. narrow leaves

Leaves can be divided into categories of broad and narrow.

Leaf attachment

The pattern by which leaves are attached to a stem or twig is also a useful characteristic in plant identification.   There are two large groups, alternate and opposite patterns, and a third less common pattern, whorled.

Simple and compound leaves

Leaves may have a single undivided blade or a blade that is divided into parts.

Remember,

It is not always easy to find the bud at the base of a petiole, it may not be visible early in the growing seaon and sometimes a mature bud is “hidden”, such as being enclosed by the petiole base, such as in Look at the entire shoot to determine what is a leaf, don’t just look at the end of a branch.  Since a bud is at the base of each leaf, it is possible to determine the leaf arrangement (i.e., alternate, opposite, etc.) of a deciduous plant in winter by looking at the arrangement of buds on a bare twig, e.g., Red Maple, [Acer rubrum, shoot branches and buds, winter].



Leaf lobes

       Leaves may be lobed or not lobed.  A lobe may be defined as a curved or rounded projection.  With leaves there is no clear distinction between shallow lobes and deep teeth.  A main vein is often visible in a lobe, this may not occur in teeth.

Leaf margin

       Another important leaf characteristic for plant identification is the edge or margin of a leaf or leaflet.  Leaves have either smooth edges, called entire, or small notches or “teeth” along the margin.

Other leaf characteristics to consider, especially if using a botanical key. Non-leaf characteristics are also useful in attempting to identify woody plants, these include:
Some characteristics of narrow leaf plants.

      Two groups, scale-like and needle leaves.

Scale-like leaves are usually small, short and overlap; they are common in several genera of conifers including junipers (Juniperus), falscypress (Chamaecyparis) and arborvitae (Thuja), for example, Arborvitae and Western Red Cedar (T. plicata) , [Thuja orientalis, branchlets, comparison].  Often scale-like leaves are displayed as two, three or four per node.  A hand lens or low power microscope is often necessary to make this determination.

Differences in scale-leaves can be used in distinguishing the following "cedars" native to Oregon (none of which are true Cedars, i.e., Cedrus). Needle leaves, also common in conifers, they are attached to twigs in several ways:
To identify an unknown plant using the characteristics descried above, in additional to others, one could us a traditional dichotomous key or a computer data base of plants, such the   Oregon State University Woody Plant Data Base.

References:
Cope, E.A. Muenscher’s Key to Woody Plants: An Expanded Guide to Native and Cultivated Species. 2001. Cornell University Press, Ithaca, New York.

Partyka, R.E., J.W. Rimelspach, B.G. Joyner, and S.A. Carver. 1980. Woody Ornamentals: Plants and Problems. ChemLawn Corp. Columbus, Ohio.



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