How Are Plants Classified?

The universe of plants that exist on earth is very large. To better understand and study plants, it is necessary to have a method of classifying them. There are many ways that plants have been classified throughout time.

Classifying Based on Growth Cycles

Plants are classified into three classes based on how long it takes to complete their life cycle, specifically the number of growing seasons.

Plants that complete their lifecycle (sprout, grow, flower, produce seeds and die) in one growing season or one calendar year is known as an annual. For example, they can be further divided into rainy season annuals, winter season annuals, and summer season annuals.

Biennials are plants that require two growing seasons or two calendar years to complete their lifecycle. Typically, they sprout and complete vegetative growth during the first growing season and flower, produce seeds, and die in the second growing season.

Perennials are plants that grow year after year once planted. These plants typically produce flowers and seeds each year after reaching maturity. Perennials are further classified into herbaceous and woody perennials. If the fleshy, above -ground portions of the plant die back each winter and new stems emerge each spring, then the perennial plant is said to be herbaceous. Perennials such as shrubs and trees whose top growth becomes hard and fibrous, persisting year-round, are said to be woody.

Classifying Based on Form or Structure

Plants come in many forms or structures. Some common groups of plants based on these forms or structure include vines, ground cover, shrubs, and trees.

Vines are plants with long, thin woody stems that grow along the ground or requires the support of other plants or structures to grow upward. Those vines capable of climbing do so by twining around supporting plants or structures or using tendrils which twine or have adhesive tips.

Ground cover plants grow close to the ground and form a dense soil cover to help control erosion and prevent weeds. They range in height from an inch to two or three feet.

Shrubs are woody perennial plants that have many branches arising from the base of the plant near ground level. They have no single main trunk and are smaller than trees. They survive for many years, though less than trees.

Trees are plants that have a main trunk that bears wood branches, twigs, and leaves typically some distance above the ground. Trees typically survive for many years.

Classifying Based on Leaf/Needle Retention

Most perennials are classified as either deciduous or evergreen.

Deciduous plants lose all their leaves once per year, typically in the cold seasons of late fall/winter. But certain deciduous trees can shed their leaves annually during the dry season.

Evergreen plants maintain leaves year-round. While they do shed leaves, they do not do so all at once. Older leaves are periodically shed but there are still many other leaves on an evergreen plant when this happens.

Semi-deciduous or semi-evergreen is a term for plants who lose their leaves for only a very short period each year as the old leaves fall off and new growth appears. This occurs in certain tropical and subtropical woody perennials.

Classifying Based on Climate Adaptation

Perennial plants can be classified based on their ability to adapt to changes in climates.

Tropical plants originate between the Tropic of Cancer and Tropic of Capricorn close to the equator where it is never cold. Such plants do not tolerate cold weather well and can be severely damaged or killed by freezing temperatures.

Subtropical plants grow in climates like those of tropical plants. However, unlike tropical plants, subtropical plants can survive brief exposures to temperatures at or slightly below freezing.

Hardy plants are those that have adapted to long periods of below freezing temperatures. How long they can endure cold weather and the minimum temperatures they can survive depends on their degree of hardiness.

Other non-perennials can also be classified based on the temperatures they can tolerate. For example, those that can tolerate brief periods of freezing temperatures are called hardy, while those that cannot are called tender. Plants can also be classified by their temperature preference for optimal growth. For example, cool season crops produce the best yield during the cooler months of the year while warm season crops produce the best yield during the warmer summer months.

Classification by Uses

One ancient way to classify plants was based on their use. Plants were grouped based on whether they were sources of food, spices, fibers to make clothing, sources of drugs, or had ornamental value. The horticultural plants that are sources of food are further divided into fruits, vegetables, herbs, and nuts. However, there are inherent problems with this system of classification, as the same plant may have different uses in different parts of the world. And some plants do not fit neatly into a single grouping.

Scientific Plant Classification

The most precise way to classify or categorize plants is using the scientific system of classification. This classification system was designed for use in classifying all living organisms, not just plants. It creates a hierarchy of categories and subcategories based on characteristics of each organism. Built into the categories is information about not only the specific species of organism, but also its closest relatives and evolution.

The original scientific classification system was published by Carolus Linnaeus in the 1700s. He is known as “the father of taxonomy." It originally grouped organisms into a hierarchy of categories from broadest to most specific as:

  • Kingdom
    • Phylum
      • Class
        • Order
          • Family
            • Genus
              • Species

This early system was based on morphological characteristics (what something looks like). Over the years more and more sublevels have been added to the above hierarchy to account for new differences observed in organisms. In recent years, there has been a radical shift in how organisms are grouped due primarily to DNA technologies.

When classifying plants, the broadest category is the kingdom, named Plantae. All plants are a member of the Plantae kingdom.

The next category level is this hierarchy is phylum. It is at this level where the distinction is made between gymnosperms (phylum name Coniferophyta), angiosperms (phylum name Anthophyta), and other high-level plant types.

Angiosperms are further divided into two classes: dicotyledons or dicots (class name Magnoliopsida) and monocotyledons or monocots (class name Liliopsida). Each of these classes of flowering plants has certain common characteristics. For example, monocot seedlings emerge from the soil with a single “seed leaf” or cotyledon, where dicot seedlings emerge from the soil with two cotyledons. Monocots typically have sepals and/or petals that occur in threes or multiples of threes where dicots typically have sepals and petals that occur in fours, fives, or multiples thereof. The vascular bundles of monocots are typically scattered throughout the stem where those of dicots form concentric circles.

At each level of this scientific classification system, certain characteristics are used to group similar plants. As you move deeper into the classification system from kingdom to phylum to class to order to family to genus to species, those characteristics get more and more specific. This classification system is useful when attempting to identify an unknown plant or trying to understand characteristics and problems of a given plant. It can also be helpful in determining plants which might be suitable for grafting or for breeding together to create a new hybrid.

Scientific Names for Plants

Carolus Linnaeus changed the way plants were named when he published Species Plantarum in 1753. Rather than naming plants using long, Latin names with many words to describe various aspects of the plant, he shortened the names of plants to just two words like a person’s last and first name. The genus is listed first and identifies a group to which the plant belongs, like a person’s last name identifies a family to which they belong. The specific epithet follows the genus to identify the specific plant in the genus, like a person’s first name identifies the specific family member. Together, the genus plus specific epithet identify a species of plant. This naming strategy is called the Latin system of binomial nomenclature.

Sometimes a genus is followed by “sp.” which represents a singular species within that genus or a singular species whose specific epithet is not known. References are also sometimes made to a genus followed by “spp.” That is an abbreviation for the plural form of species and used to refer to all species in that genus.

Scientific names should always be italicized or underlined in written documents. The genus names should be capitalized while the specific epithets should always be lower case, as in Acer palmatum (Japanese maple).

Plants can have a third name called the cultivar. Cultivar is short for “cultivated variety”. These cultivars differ from the species typically in some characteristic, like the color of the flower, color of the leaves or height of the plant. They are almost always the result of cultivation (propagated through vegetative cuttings instead of seeds) and rarely occur in nature. They are not true-to-seed, meaning their seeds are not guaranteed to yield a plant with the same characteristics as the cultivar from which it comes. Cultivar names should be enclosed with single quotes and the first letter of each word capitalized, like Latingenus latinspecies ‘Cultivar Name’. An example is Acer palmatum 'Bihou,' or the Bihou Japanese Maple.

Other plants may have an “x” between the Genus and specific epithet indicating that the species is a hybrid created by crossing two species in the same genus. For example, Latingenus x latinspeciesnew indicates that the new latinspeciesnew species was created by cross breeding two other species form the Latingenus genus.

Some plants have a third Latin name preceeded by “var.” like Latingenus latinspecies var. latinvarietym, which indicates a variety. Varieties are true-to-seed meaning that (unlike cultivars), plants grown from seeds of the variety will inherit and retain the unique characteristic that makes that variety different from the species.

Some plants may also have a trademarked marketing name like MytrademarkednameTM. This can sometimes be confused with a cultivar name, though the marketing name is not part of the scientific name of the plant.

Though Latin names can be intimidating, they often offer various clues about the plant being named such as its geographic origin, shape, size, color, and more. But sometimes they can mislead.

It should be noted that a plant may have several common names that may differ depending on geographic region. Multiple plants may share the same common name, and they may or may not be related. What is constant is that a plant will have a single scientific name.