One of the most useful classification systems is utilizing plant taxonomy.  Taxonomy is the science of systematically naming and organizing organisms into similar groups.  Plant taxonomy is an old science that uses the gross morphology (physical characteristics, i.e., leaf shape, fruit form, etc.) of plants to separate them into similar groups. Quite often the characteristics that distinguished the plants become a part of their name.  For example, Quercus alba is a white oak, named because the underside of the leaf is white.

The science of plant taxonomy is being absorbed into the new science of systematics.  The development of more sophisticated microscopes and laboratory chemical analyses has made this new science possible.  Systematics is based on the evolutionary similarities of plants such as chemical make-up and reproductive features. 

It should be noted that plant taxonomic classification changes with continuing research, so inconsistencies in nomenclature will be found among different textbooks.

An overview of plant taxonomy helps the gardener understand the basis of many of our cultural practices.  For example, fire blight is a disease of the Rose family, therefore it is helpful to recognize members of the Rose family to diagnosis this disease.

Common Taxonomic Divisions

The scientific system of classification has all living things divided into groups called taxa (singular, taxon).  Plants are in the Kingdom of Plantae.  Other Kingdoms include Fungi, Protista (one-celled organisms including yeasts, bacteria, and protozoans), and Animalia (animals).
The plant kingdom is divided into two groups: broyophytes (including mosses and liverworts) and vascular plants (plants with a vascular system of xylem and phloem).

Vascular plants (sometimes called higher plants) are divided into two subgroups: seedless and seeded.  These sub-groups divide into Phyla (plural of phylum).  Phylum names end in “phyta”.  The seedless phyla include the Pterophyta (ferns).  Seeded phyla include Cycadophyta (cycads), Ginkgophyta (ginkgo tree), Coniferophyta (conifers), and Anthophyta (angiosperms). 

Angiosperms are divided into two taxa, monocotyledon (monocots) and dicotyledon (dicots).  Distinguishing between monocots and dicots is a common practice in landscape management as some of our common herbicides work at the monocot/dicot level.  For example, lawn weed sprays (such as 2,4-D and Dicamba) kill dicots (broadleaf plants like dandelions) but not monocots (the grass).  Other herbicides will kill monocots but not dicots, allowing the gardener to kill grass (a monocot) in the shrub or flowerbed (dicots).

Additional taxa in descending order include class, order, family, genus and species.

Families of higher plants are separated from one another by characteristics inherent in their reproductive structures (flowers, fruit, and seed).  Families have primary importance in gardening as family members generally share comparable cultural requirements and similar insect and disease problems.  Disease management and cultural techniques are often discussed at the family level.

Genera (plural of genus)are groupings whose members have more characteristics in common with each other than they do with other genera within the same family.  Similarity of flowers and fruits is the most widely used feature, although roots, stems, buds, and leaves are also used.

Common names typically apply to genera.  For example Acer is the genus of maples, Fraxinus is the ash, and Juniperus is the junipers.

Species generally refers to interbreeding sub-groups of genus or groupings of individual plants that adhere to essential identification characteristics but display sufficient variation so as not to be categorized as replicas of one another.  The specific epithet name is always used in conjunction with the genus.

Plant Names in the Binomial System

Plants are named using a binomial system.  The Genus name comes first and is analogous to a person's LAST name ("family" name).  The specific epithet name follows the genus name and is a more specific identifier.  Many plants share the same specific epithet.  It would be analogous to a person's first name.

Genus	Species
Smith	Frank
Quercus	rubra

Following the genus and species is the cultivar or variety name.  This is an even more specific identifier, similar to a person's middle name.

Genus	Species	Cultivar
Smith	Frank	Joseph
Quercus	rubra	'Aurea'

When genus and specific epithet names are written, they should always be underlined or italicized to denote they are Latin words.  However, cultivar and variety names are not italicized.  The genus name is always capitalized, but the specific epithet name is not. 

The singular and plural spelling of species is the same.  In writing, the abbreviation “sp.” following the genus indicates a single unidentified species and “spp.” indicates multiple species.  For example, “Acer sp.” would indicate an unidentified species of maple, and “Acer spp.” refers to multiple species in the maple genus.  The “sp.” or “spp.” are not underlined or italicized.

Wild and naturally occurring plants are named under the rules of the International Code of Botanical Nomenclature.  Cultivated plants are named according to the same principles found in the International Code of Nomenclature of Cultivated Plants.

Genus and specific epithet names are universal, being used worldwide.  Since taxonomy classifies living organisms, there will be some inconsistencies between books. 

On the other hand, common names are often local in use and many times don’t clearly identify the specific plant.  For example, Liriodendron tulipfera is known as the Tulip Tree in the north and as Yellow Poplar in the south.  Carpinus caroliniana goes by American Hornbeam, Blue Beech, Musclewood, Water Beech, and Ironwood.  The European White Lily, Nymphaea alba, has 15 English common names, 44 French common names, 105 German common names, and 81 Dutch common names.

Sub-Groupings of Genus and Specific Epithet        

Variety or Subspecies is a sub-grouping of species assigned to individuals displaying unique differences in natural populations.  The differences are inheritable and reproduce true-to-type in each generation.  Variety is denoted as var and subspecies is denoted as ssp.

For example cauliflower and cabbage are varieties of the same species Brassica oleracea.  The thornless variety of honeylocust would be written Gleditsia triacanthos var. inermis.

Cultivar is a species sub-grouping of cultivated plants (“cultivated variety”) which display rather unique differences and, when reproduced by seeds or cuttings, retain its distinguishing characteristics.

For example, Early Girland Big Boy are cultivars of tomatoes.  In technical writing, the cultivar name follows the genus and specific epithet and is always capitalized and written inside single quotes but not italicized.  For example, October Glory Red Maple is Acer rubrum ‘October Glory’.

It is possible to have a cultivar of a variety.  For example, Cornus florida var. rubra ‘Cherokee Chief’.

Note: Cultivar names must be enclosed in single quotes when following a species name.  Variety and subspecies names must be denoted with var. or ssp. when following a species name.  For example, Oenothera macrocarpa ssp. incana.  The use of trinomials Gleditsia triacanthos inermis is improper usage in scientific nomenclature.

Strain is a sub-group of cultivar with specific characteristics, like resistance to a disease or better color.  For example “Early GirlVFN” tomato.

Clone is a sub-group of cultivar derived by asexual propagation (i.e., cuttings).  The offspring have one parent and therefore are identical to the parent because no exchange of genetic materials has occurred.

Line is a sub-group of cultivar propagated by seed

Form is based on selection by growth habit, not reproducible by seed.  For example, Columnar Norway Maple.

---

Examples of taxonomic classification

---

---

                 Monocots                                                   Dicot

---

Additional Information – CMG GardenNotes on How Plants Grow (Botany):

#121 Horticulture Classification
#122 Taxonomy
#131 Plant Structures: Cells, Tissues, and Structures
#132 Plant Structures: Roots
#133 Plant Structures: Stems
#134 Plant Structures: Leaves
#135 Plant Structures: Flowers
#136 Plant Structures: Fruit
#137 Plant Structures: Seeds
#141 Plant Growth: Photosynthesis, Respiration and Transpiration
#142 Plant Growth: Light
#143 Plant Growth: Temperature
#144 Plant Growth: Water
#145 Plant Growth: Hormones

[ top ]