A tree is defined as a woody perennial, which in its natural state, has a distinct trunk. The NPS provides further detail on the nomenclature of Trees as follows:
Each tree must be specified by giving its full BOTANICAL name. The botanical name is specific to the plant and avoids the confusion which can occur in using COMMON names. The botanical name relies on the use of the binomial system. This system, invented by Linnaeus in the eighteenth century, has enabled botanists to name and classify plants throughout the world. By this means Acer pseudoplatanus is the international name for what in the U.K. is known as sycamore but in the U.S. is called a Plane. Its botanical name gives absolute clarity to ensure that the correct plant is supplied.
In the example above, sycamore is a species within the genus Acer which includes many other species of ‘maple’, the Norway maple, Acer platanoides being an example. Generic names are always spelt using an upper case initial letter when using a combination of upper and lower case letters, e.g. Acer platanoides
A species may be defined as “a sub-division of a genus consisting of plants which have the same constant and distinctive characters and which have the capacity to interbreed amongst themselves.”
The species is the basic unit of classification and nomenclature Quercus petraea, or Sessile Oak is a species within the genus Quercus which includes all the oaks worldwide. By the use of specific epithets, the range of different Oak trees can be identified; Quercus robur, which is commonly called the Pendunculate Oak or the English Oak (but only in the U.K.!), Quercus cerris, the Turkey Oak, Quercus ilex, the Evergreen or Holm Oak, Quercus rubra, the Red Oak and so on. All of these species are sufficiently similar to be identified as within the genus Quercus, but sufficiently different to be separate species.
The specific name may refer to a characteristic feature of the plant; Quercus rubra is the Red Oak; Populus alba is the White Poplar. Specific names are always spelt using lower case letters when using a combination of upper and lower case letters, e.g. Quercus rubra.
Subspecies, Varieties and forma
These may be defined as naturally occurring “sub-divisions of a species, consisting of plants which differ in some heritable characters such as form, colour or season, from what is regarded as typical of the species.”
Study of any species identifies variation within that species. Identification of subspecies, varietas (or more commonly ‘‘variety’’) or forma is the recognition of this variation by name. The tree, Sorbus domestica has a variant with apple-shaped fruits, Sorbus domestica maliformis. The addition of the third word within the name, not only identifies the plant but also, in this instance, describes the particular characteristic of this variety. For the botanists there are reasons to identify subspecies, varieties or forma, but for plant specification purposes, they can be broadly considered to be the same.
Subspecies, varieties and forma are not common in commercial, amenity horticulture. These are identified by the third word within the name which, in the case of subspecies, varieties or forma is written in lower case letters when a combination of upper and lower case is used. Usually the qualification within the name, e.g. variety (var.) subspecies (subsp.) or forma (f.) is not included, e.g. Sorbus domestica maliformis
Cultivars (cvs.) and Clones
A cultivar may be defined as ‘‘an internationally agreed term for a cultivated variety’’ and a clone as ‘‘genetically uniform group of plants originating from a single plant by vegetative propagation.’’
A variety within a species is usually identified form the typical form by botanical characteristics which may or may not be relevant to its selection and use within horticulture or landscape work. A cultivar is a plant which has been specifically selected by man because it exhibits different characteristics from the typical species and is worth maintaining in cultivation by vegetative propagation or by seed in the case of true breeding cultivars. Acer platanoides ‘Crimson King’ is a cultivar of Acer platanoides, the Norway Maple selected and cultivated for its deep purple leaves. In this instance the change in leaf colour arose by chance and it is easier to maintain in cultivation by vegetative means than relying on seed. Clones are the vegetatively produced offspring from a single parent and are, therefore, identical to the parent in all respects. Many cultivars are clones, propagated vegetatively in order to perpetuate the characteristics of the original selection.
In the example above, Acer platanoides ‘Crimson King’ is exactly like the green leafed Acer platanoides except for its leaf colour. It is easier to identify that it is a variation of a particular species and the cultivar name qualifies the species name. However, through selection, hybridisation and mutation, it is not always easy to link a cultivar name to a species or hybrid. An example is Prunus ‘Snow Goose’. Its exact botanical origins are in doubt but it is a cultivar within the genus Prunus which has been selected and continues to be propagated. In these instances the species name is left out.
Cultivars are common in commercial and amenity horticulture. Whilst the genus, species and variety is identified by the use of italic type face and the use of a combination of upper and lower case letters, cultivar names are in Roman letters, with an upper case initial and is within single inverted commas. e.g. Fraxinus excelsior ‘‘Westhof’s Glorie’ or Sorbus ‘Sheerwater Seedling’.
A hybrid may be defined as ‘‘a plant raised by the crossing of two genetically distinct plants’’. Since the characteristics of a species are maintained through breeding between individuals within the species, it can be surmised that hybridisation is uncommon in nature. Hybrids may result from the crossing of species (bi/specific) or even more rarely, the crossing of genera (bi-generic). Aesculus x carnea is the result of crossing the species, Aesculus hippocastanum with Aesculus pavia. The resultant plants exhibit characteristics of both parents but differ between themselves and so have been given a range of cultivar names, e.g. Aesculus x carnea ‘Briotii’. The well known Leyland Cypress, x Cupressocyparis leylandii is an example of a bi-generic hybrid. In this instance, the cross was between two species in different genera, Chamaecyparis nootkatensis and Cupressus macrocarpa.
Hybrids are identified by the symbol ‘x’ , which for a bi-specific hybrid occurs before the specific name, e.g. Sorbus x thuringiaca and for a bi-generic hybrid to occurs before the generic name, e.g. x Crataemespilus
It is recommended that common names are only used in addition to the Botanical Name and never used alone. Many plants of our landscapes have a very wide range of Common Names. Geoffrey Grigson in Dictionary of English Plant Names (1974) records 90 different local names for Arum maculatum within the U.K. Lords and Ladies, Cuckoo Pint, Jack in the Pulpit, Starchwort, Sucky Calves are just the start of a long list.