Botanists have classified plants into an orderly, ranked system reflecting similarities among the world's plant life. The plant kingdom is broken down into groups that are less and less inclusive: division, class, order, and then the groups defined below, which are the ones of most significance to gardeners. The plant kingdom includes algae, mosses, ferns, and seed-bearing plants such as wildflowers, trees, shrubs, and grasses.
Each plant belongs to a family. All members of a particular family share certain groups of characteristics that are not found in other families. Although family names - Rosaceae (rose family), Liliaceae (lily family) - are not as important to gardeners as are the groups into which the families are divided, sometimes knowing the family name contributes to an understanding of a plant's cultural requirements.
A plant family is divided into groups of more closely related plants, each group called a genus (the plural is genera). Sometimes a family will contain only one genus: the Ginkgoaceae contains only the genus Gingko; at the other extreme, the composite family (Compositae) contains around 950 genera. The first word in a plant's botanical name is the name of the genus to which the plants belongs: for example, Gingko, Liquidambar, Primula. The genus name always starts with a capital letter.
Each genus is subdivided into groups of individuals called species. This is the second word in a plant's botanical name. A few genera contain only one species (i.e. Gingko biloba), but more often a genus contains two or more species. Each species is a generally distinct entity, reproducing from seed with only a small amount of variation. Species in a genus share many common features but differ in one or more minor characteristics. In addition members of a species do not usually interbreed successfully with members of a different species. Species names begin with lower-case letters.
Sometimes a third word in a botanical name is present, and indicates a subspecies or variety. In the strictest sense, a subspecies is more inclusive than a variety. Subspecies is often used to denote a geographical variant of a species, but in general usage, subspecies and variety have become virtually interchangeable. Subspecies or varieties retain most characteristics of their species while differing in some particular way, such as flower color or leaf size. Of major biological importance is the fact that varieties or subspecies of a given species can easily interbreed and form perfectly fertile offspring in the same way that two varieties (or breeds) of dogs can. Subspecies often is shortened to subsp. or ssp.; variety to var.
Cultivar: (horticultural variety or clone)
Many cultivars are of the hybrid orgin. They are usually listed by genus name followed by cultivar name, as Rosa 'Chrysler Imperial'. Some have been found as wild plants (sometimes known as sports) that show some superior and recognizable characteristic and have been perpetuated by vegetative propagation only. Cultivar names are usually written within single quotation marks. (i.e., Penstemon fruticosus ssp. fruticosus 'Purple Haze')
This is a distinct plant resulting from a cross between two species, subspecies, varieties, cultivars, strains - or any combination or even between two plants belonging to different genera. Some occur in the wild (such as Haliminocistus, a hybrid between a species of Halimium and a species of Cistus), but more often hybrids are the deliberate product of a horticultural experiment.
Many popular annuals and some perennials are sold as strains, such as State Fair zinnias. Plants in a strain usually share similar growth characteristics like height or flower color. They are often the result of a series of breeding and selection trials until the plants breed true for the desired characteristics. It must be noted that such characteristics can easily be lost from the next generation if the plants are allowed to be pollinated openly. Strains can therefore be regarded as varieties or subspecies that have been produced quickly and artifically whereas the variety and subspecies have developed naturallyin plant populations over scores of generations.