Classification To Species Level
Classification Below Species Level
How To Write Plant Names (Properly!)
How To Write Cultivar Names (Properly!)
Hybrids Between Genera
Type Specimens and Standard Specimens/Folios
Some Major British, Irish and foreign Herbaria
The classification of plants (and animals) is called TAXONOMY.
TAXONOMY involves a hierarchy of categories ranging downwards from very large groupings to very small ones.
E.g. the classification of the geranium Geranium endressii is:

Division Anthophyta  

Class Dicotyledones  

Family Geraniaceae  

Genus Geranium  

Species endressii
Each of the various groupings in this list (Anthophyta, Dicotyledones, Geraniaceae, Geranium, endressii) is called a TAXON - plural TAXA.
In normal everyday use the full classification is not given, instead the plant is referred to by a Latin name made up of the Genus + species, so:
     Geranium endressii
Variations within the species which occur in nature are classified into infra-specific taxa - mostly subspecies, varietas (= botanical variety), and forma (form).
Variations which are selected in cultivation and deliberately propagated are called cultivated varieties (cultivar for short), or simply varieties.
REMEMBER! Variety or cultivar is not the same as varietas or botanical variety.
The Latin name of a plant is the name of the Genus (always with an initial capital) followed by the name of the species (always with a small initial letter), usually written in italics.
E.g. creeping buttercup:
Ranunculus repens correct
ranunculus repens wrong!
Ranunculus Repens wrong!

If the species has infraspecific categories e.g. in asparagus, which has two subspecies, they are written as:

Asparagus officinalis subsp. prostratus correct
Asparagus officinalis subsp. officinalis correct
Asparagus officinalis prostratus wrong!
Asparagus Officinalis Prostratus wrong!
Asparagus Officinalis subsp. Prostratus wrong!
Asparagus officinalis officinalis wrong!

The rank of the infraspecific category must always be stated ie is it subspecies, varietas, or form etc? [Abbreviated as subsp. (or ssp.), var. and f. The plurals are subspp. (or sspp.), vars., and ff.]

Hence in creeping willow there are a number of botanical varieties which are written as so:

Salix repens var. repens

Salix repens var. fusca

Salix repens var. argentea

BUT NOT: Salix repens repens, Salix repens fusca or Salix repens argentea.


Selected forms of plants which are grown in gardens and which do not occur in the wild are called cultivars or cultivated varieties or simply varieties.

Until recently their names were written in two ways, both of which were correct, and will be found in older literature:

Geranium endressii cv. A.T. Johnson

or Geranium endressii ‘A. T. Johnson’

Ipheion uniflora cv. Wisley Blue

or Ipheion uniflora ‘Wisley Blue’

However, a recent change in the Rules means that you must from now on only quote cultivar names within single inverted commas i.e. Geranium endressii ‘A. T. Johnson’ and Ipheion uniflora ‘Wisley Blue’, etc. Do not use the abbreviation cv.

Note that cultivar names must begin with capital letters.

Note also that modern cultivar names are now always in a modern language, not Latin, and are not written in italics like the Latin names.

(But old cultivar names invented before these rules may be in Latin form; they are still valid. Old common Latin cultivar names are ‘alba’ for white-flowered varieties and ‘flore pleno’ for double forms.)

If the species is uncertain, it may be omitted, hence this is still correct:

Rosa ‘Lord Penzance’

Rhododendron ‘Pink Pebble’

Note that if you are referring to a long list of species or varieties from the same genus, you can abbreviate the genus name to an initial letter, e.g. Sorbus japonica, S. ‘Joseph Rock’, S. intermedia, S. insignis ‘Bellona’.


The same plant can have more than one Latin name. The reasons for this are quite complicated and depend on several different factors. For example a difference of opinion between experts as to whether a genus or species should be split into two. An example is the Rosebay Willowherb Epilobium angustifolium which some botanists call Chamerion angustifolium, depending on whether the genus Epilobium should be split or not into Epilobium and Chamerion - simply a matter of opinion. Another factor is that the valid name for a taxon is the earliest published one, and the rediscovery of a "forgotten" name from old literature can cause the abandonment of a later but more familiar name. Such changes can seem trivial and annoying but are ultimately aimed at stabilising nomenclature.

Similarly cultivars can have more than one name, but in this instance only the earliest name can be correct.

Many books will give a list of the common synonyms of a plant to facilitate tracing it in different reference works.


Hybrids are the result of crossing two or more species. Their names can be written in two ways:

Populus deltoides x P. nigra is a hybrid black poplar very frequently planted. It has been named as if it were a species in its own right, like many hybrids, as Populus x canadensis.

So Populus x canadensis = Populus deltoides x P. nigra

Hybrids can have cultivars like true species.

So in Populus x canadensis we have:

Populus x canadensis ‘Eugenei’

Populus x canadensis ‘Marilandica’

Populus x canadensis ‘Gelrica’


[The ‘x’ is a multiplication sign.]


Hybrids between species in different genera sometimes occur. The genera will be closely related. Inter-generic hybrids only occur commonly in relatively few families such as Orchidaceae (orchids) and Gramineae (grasses).


In Great Britain a grass occurs on sand dunes which is a cross between Ammophila arenaria (marram grass) and Calamagrostis epigeios (wood small-reed).

We can write this as Ammophila arenaria x Calamagrostis epigeios, but it is was originally thought to be a true species when first discovered, and was named as Ammophila baltica.

When it became apparent that it was an intergeneric hybrid, it was renamed xAmmocalamagrostis baltica. This is a more convenient name than the full hybrid formula, being considerably shorter!

xAmmocalamagrostis is an example of a hybrid genus.

Other examples are xOrchiserapias which contains hybrids of species within the orchid genera Orchis and Serapias and x Orchihamantoglossum which contains hybrids of Orchis and Himantoglossum.

Note that in intergeneric hybrids the multiplication sign comes before the hybrid genus, not between it and the species.


Graft-chimaeras (formerly often called 'graft hybrids') are a rare phenomenon arising entirely in cultivation where two species, frequently of two distinct genera, are grafted together. The chimaera arises as a branch or shoot from the point of union which contains tissues of both species, resulting in a combination of both species being expressed in the shoot. Such shoots have been vegetatively propagated and cultivated and are available to horticulturists as graft-chimaeras.

The formula for a graft-chimaera uses a + sign to connect the two "parent" species (not a multiplication sign, to show that they are not sexual hybrids). An example is: Syringa vulgaris + Syringa x chinensis.

Graft chimaeras can be given cultivar names. The example above has been named Syringa 'Correlata', They cannot be given species names.

(To complicate matters further the parents of this lilac graft-chimaera are Syringa vulgaris (Common Lilac) and Syringa x chinensis, this second parent being itself a sexual hybrid of Syringa vulgaris and S. laciniata, known as the Rouen Lilac!)

If intergeneric, graft-chimaeras can be given their own genus name (which is a combination of the two constituent generic names) preceded by a + sign.

Some common examples are:

the Cytisus purpureus + Laburnum anagyroides graft-chimaera which is +Laburnocytisus 'Adamii'
the Crataegus monogyna + Mespilus germanica graft has given rise to two distinct graft-chimaeras, named as separate cultivars:
+Crataegomespilus 'Dardarii'
+Crataegomespilus 'Jules d'Asnieres'


The name of a plant species is attached to what is called the TYPE specimen. This is the specimen which was described by the author of the name and is the ultimate authority for the description and characteristics of the species, to which any doubtful specimens can be compared.

Type specimens are also designated for infra-specific taxa like subspecies, varietas and forma.

Cultivars do not have type specimens, but it is now recommended that a STANDARD be created as a reference base for each new cultivar, in the form of an herbarium specimen or equivalent. It is also recommended that a STANDARD PORTFOLIO be created for each new cultivar, to contain this herbarium specimen plus a photograph, description and associated data, and which will act as a permanent reference file for the cultivar name. Standards and Standard Portfolios should be lodged in recognised herbaria.


Herbarium collections have international codes consisting of one or more letters.


The code for the herbarium at Glasnevin is DBN.

The code for the Trinity College, Dublin herbarium is TCD.

The code for the Royal Botanic Gardens at Kew is K.

Some herbaria have huge collections:

K (Kew) has more than 5 million specimens.

NY (New York Botanical Garden) has about 4 million specimens.

US (US National Herbarium, Smithsonian Institution, Washington) has nearly 4 million specimens.

LE (Komarov Botanical Institute, St. Petersburg) has over 5 million specimens.

In Ireland, DBN has c.500,000 specimens, TCD has c.200,000 and BEL (Ulster Museum) has c.110,000.

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