11 Introduction to Dichotomous Keys

A dichotomous key is a useful tool for the identification of things not known to the observer; for example, unfamiliar plant species. The typical dichotomous key, as shown in the example below, is made up of a series of descriptions, features or characteristics, arranged in pairs (couplets) of contrasting alternative choices (e.g., hairy vs. not hairy, bigger than a breadbox vs. not bigger than a breadbox, etc.). Each couplet is worked through sequentially until the correct determination is made.

Starting at the first couplet, choose which of the two alternatives best suits an object or thing, and proceed to the couplet number or answer indicated by that choice. The number of things being considered is reduced at each successive step in the key so that by a process of elimination the correct determination is made.

Example Dichotomous Key

Example objects to identify: apple tree, water-lily, fir tree, dandelion, astroturf, seaweed.

• 1.a. found in water …………………………………………………………………………………….. go to 2
• 1.b. found on land ……………………………………………………………………………………… go to 3
• 2.a. grows in salt water ……………………………………………………………………………… seaweed
• 2.b. does not grow in salt water ………………………………………………………………… water-lily
• 3.a. a real plant ………………………………………………………………………………………….. go to 4
• 3.b. not a real plant ……………………………………………………………………………………. astroturf
• 4.a. grows more than 50 m tall …………………………………………………………………… fir tree
• 4.b. grows less than 50 m tall …………………………………………………………………….. go to 5
• 5.a. produces yellow flowers ……………………………………………………………………. dandelion
• 5.b. does not produce yellow flowers ……………………………………………………… apple tree

Or, the couplets may be grouped like this:

• 1.a. found in water …………………………………………………………………………………… 2
• 2.a. grows in salt water …………………………………………………………………………….. seaweed
• 2.b. does not grow in salt water ……………………………………………………………….. water-lily
• 1.b. found on land …………………………………………………………………………………….. 3
• 3.a. real plant …………………………………………………………………………………………… 4
• 4.a. grows more than 50 m tall …………………………………………………………………. fir tree
• 4.b. grows less than 50 m tall …………………………………………………………………… 5
• 5.a. produces yellow flowers ……………………………………………………………………. dandelion
• 5.b. does not produce yellow flowers ………………………………………………………. apple tree
• 3.b. not a real plant …………………………………………………………………………………… astroturf

Dichotomous keys may be simple or complex depending on what is being identified. For example, distinguishing obvious visible characteristics, such as structures for water movement in woody or herbaceous plant growth is straightforward. However, it should be noted that leaves, flowers, and fruit will not typically be available at the same time and return visits may be needed. Furthermore, differentiating minute plant parts such as reproductive structures requires the use of a hand lens or low magnification microscope for inspection and a thorough understanding of the descriptive terminology used in a dichotomous key.

An example of a dichotomous key for plant identification is available online at Oregon State University Dichotomous Key [New Tab].