Parts of a plant name include:
MCM: capitalize and italicize. It is the same in singular or plural form; do not add “s.”
Laurentia, Heuchera …
MCM: not capitalized but italicized.
Laurentia fluviatilis, Heuchera sanguinea …
An editor may refer to multiple species using the abbreviation spp.
For example: Buxus spp.
If, however, there is a single plant, and the species and/or variety is unknown, then use sp., as in,
“Shown here is a large juniper (Juniperus sp.) …”
Note: When listing multiple species of the same genus, abbreviate the genus after first reference:
Clematis terniflora, C. macropetala, C. recta.
The genus and species names may be followed by:
• Subspecies: a naturally occurring, distinct variant of a species, indicated by subsp. in roman type. This is a higher division than “variety.”
Prunus lusitanica subsp. azorica
• Varieties and forms: minor subdivisions of a species, differing slightly in their botanical structure. Indicated by var. and f. in roman type.
The pink variety of the Pacific dogwood is Cornus florida var. rubra.
• Hybrids: (or crosses) naturally or artificially produced offspring of genetically distinct parent plants. Use a Dimension X to indicate hybrids.
Viola x wittrockiana
• Cultivars: selected or artificially raised, distinct variants of species, subspecies, varieties, forms, and hybrids. Cultivars will not come true from seed, so are usually propagated asexually by cloning. Denoted by single quotes, capped, no ital. If punctuation follows the cultivar name, the punctuation is placed outside the single quotation mark:
Rhododendron ‘Elsie Frye’, Sedum ‘Autumn Joy’, and Lilium ‘Côte d’ Azur’.
Note: It is not always necessary to include both cultivar name and species name. Use is at the editor’s discretion.
Aster amellus ‘King George’
Aster ‘King George’
Also, it is possible to have a cultivar of a variety:
Berberis thunbergia var. atropurpurea ‘Chenault’
• Mix, Series, Strain: Multiples of a single cultivar. When using strain, mix, or series with a cultivar name, such as Consolida Cloudy Skies Mix, don’t use single quotes around the cultivar name (‘Cloudy Skies’ in this example). When naming a specific color in the cultivar group, do use single quotes (Consolida ‘Cloudy Skies Blue’). See also Trademarked Plants.