Plant Classification of Trees and Shrubs

Genus. Species. Hybrid names. Cultivars. The following article on plant classification is for the more scientifically-minded and botanically-inclined who may desire to dive a bit deeper than our usual posts on the topics of landscaping, pruning, or transplanting. This time around, we’re sharing this resource with you directly from The Complete Encyclopedia of Trees and Shrubs – a fantastic and thorough reference guide that contains in-depth descriptions of cultivation requirements for pruning and planting various trees and shrubs. While we recommend having a copy of this guide for your own landscaping needs, first enjoy this passage on the taxonomical categorization of trees and shrubs on your property…

Although trees and shrubs can be classified in many different ways, the only classification that has a really wide application is what biologists call natural classification, that is a classification based on the branching of the evolutionary tree.

The advantage of a classification that reflects evolutionary branching patterns is that is allows the highest level of prediction of a plant’s properties. For example, if we recognize that a particular shrub belongs to the legume family (Fabaceae) on the basis of its pea-like flowers and pods with a row of seeds, then we know that there is a strong likelihood that its roots will have nodules containing nitrogen-fixing bacteria that allow the plant to thrive in infertile soils. The ability to produce these nodules must have arisen in one of the earliest ancestors of the whole family and has been inherited by nearly all surviving members – but it is not one of the features by which we recognize the family.The rules governing the scientific naming of plants are laid down in the International Code of Botanical Nomenclature (ICBN), updated at 5-year intervals by international congresses of botanists. But this code only gives rules about the correct form of names (mostly of Latin or Greek origin) and or priority among competing names, making no attempt to lay down which classification should be followed.

Major Groups of Plants

The great majority of trees and shrubs described here are flowering plants – that is, they belong to a large natural group in the kingdom, believe to have descended from a single ancestor that first appeared at some time during the Age of Dinosaurs, perhaps around 140 million years ago. The term angiosperms is also used for the flowering plants by biologists. They are characterized by having seeds fully enclosed in a fruit and by possession of flowers, which appear to have co-evolved with the insects that in most cases are required to pollinate them. There are at least 300,000 species of flowering plants in the world, of which more than half are likely to be trees or shrubs.

But trees and shrubs include one other major group of plants, namely the conifers. These comprise only about 600 present-day species, but some of those species form vast forests in far northern regions. Conifers are characterized by having their seeds sitting on the scales of cones, and their pollen is also borne in small sacs on the scales of other smaller cones, not in flowers. Their evolutionary history goes further back than flowering plants, to around 180 million years ago.

Some very interesting plant groups may also be included in a broad definition of trees and shrubs. Among the oldest are the tree ferns, arguably no proper trees or shrubs, but in contrast the ginko is without doubt a tree. The single species Ginko biloba is the sole survivor of a more diverse group that first appears in the fossil record almost 250 million years ago. It was traditionally grouped with the conifers but is now known to have branched off before these evolved their present characteristics.

The cycads are plants with palm-like fronds and large cones. Together with the conifers and ginko they are part of the broader group known as gymnosperms, or naked-seeded plants. The cycads are of a similar geological antiquity to the conifers, though now reduced to about 200 species scattered through warmer regions of the world. Popular articles sometimes call the cycads “dinosaur food”.

There are another three small groups that have been classed as gymnosperms but which molecular evidence now shows to be small offshoots of the common stock that diverged into the gymnosperms and angiosperms. These are each represented by a single genus, namely Ephedra, Gnetum and the bizarre Welwitschia, each of them with no close relatives.

Levels of Classification

All the plants covered in this book are what botanists term vascular plants, characterized by stems with distinct zones of conducting tissues that are able to carry water some distance from roots to leaves. The ferns and their allies (such as club-mosses) are the most primitive vascular plants.

Vascular plants are next subdivided into those reproducing by spores, the pteridophytes, and those reproducing by seeds, the spermatophytes. In this particular context, the only pteridophytes are tree ferns: all other trees and shrubs are spermatophytes.

Traditionally the angiosperms (flowering plants) are divided into two great classes, the dicotyledons and the monocotyledons. Nearly all woody plants belong to the first of these, which includes about two-thirds of all flowering plants species. Dicotyledons are usually recognizable for having net-veined leaves, flower parts in fours or fives, and two seed leaves (cotyledons). Monocotyledons usually have parallel-veined leaves, flower parts in threes, and a single seed leaf. Monocotyledons covered in this book include palms, yuccas, aloes, dracaenas, cordylines, strelitzias and bananas.

The flowering plants are usually treated as a division in the formal hierarchy of plant classification, and the monocotyledons and dicotyledons as the two classes belonging to it.

The most important levels in this hierarchy are shown in the following list, with the tree Magnolia campbellii subsp. mollicomata taken as an example to illustrate where it belongs at each level of the hierarchy.

  • Division – Angionspermae
  • Class – Dicotyledonae
  • Subclass – Magnoliidae
  • Order – Magnoliales
  • Family – Magnoliaceae
  • Subfamily – Magnolioideae
  • Tribe – Magnolieae
  • Genus – Magnolia
  • Subgenus – Yulania
  • Section – Yulania
  • Species – Magnolia campbellii
  • Subspecies – Magnolia campbellii subsp. mollicomata

The levels picked out in bold are the ones commonly used by both botanists and gardeners.

Below the level of species there is great confusion in plant names. Subspecies is the only rank below species shown in the list above, as this is the primary subdivision of a wild species and is sometimes regarded as a species still in the process of evolving. Historically, though, it is the rank of variety (strictly speaking “var.” stands for the Latin varietas) that has been most often used, but this is full of ambiguity.

And then there is the level of form, which in the botanical system comes below subspecies and variety (the abbreviation “f.” correctly means the Latin forma, which is the same thing. Botanists generally use this level to signify an aberration from the typical state that may occur repeatedly in the wild and involves only one character. An example of this is Gleditsia triacanthos f. inermis (thornless honey locust), whose only difference from other forms of this species is the absence of thorns, a feature which is observed to recur in wild populations.

Hybrid Names

The International Cod of Botanical Nomenclature states that the name of a plant known to be a hybrid between two species should be of similar form to a species name but with the multiplication sign x inserted in front of the specific epithet. Thus the hybrid between the horse chestnut species Aesculus hippocastanum and Aesculus pavia is Aesculus x carnea.

And once a name is published for that particular combination of species, then any hybrid konwn to have that same parentage must take the same hybrid botanical name, though it may be distinguished by a different cultivar name (see the following text). The multiplication sign can also be used to indicated a hybrid even in the absence of a published hybrid name, by placing it between the names of the two parent species; an example of this is Rhododendron aurigeranum x R. zoelleri.

There are cases of hybrids occurring between plants of different genera, and these may be named with a generic hybrid name. This is signified by placing the multiplication sign before the hybrid genus name, for example x Crataemespilus grandiflora for the hybrid between Crataegus monogyna and Mespilus germanica. Note that such names are usually formed by combining parts of the names of the parent genera.

Of much rarer occurrence are graft hybrids, which result when tissues of two different plants get mixed together at a graft union, without any sexual combination of genes. The best known cases are generic graft hybrids, and they are designated by putting a + sign before the graft hybrid name, which is formed in a similar way to a normal generic hybrid. One such involves again Crataegus and Mespilus and has been named + Crataegomespilus to distinguish it from Crataemespilus.

Cultivars

The concept of a cultivar as a distinct kind of name only crystallized after about 1950, though plant breeders had obviously been using non-Latin names since the mid-nineteenth century – for example, Camellia “Aspasia”. In 1953, the International Code of Nomenclature for Cultivated Plants was published, in which cultivar was defined. In the case of trees and shrubs a cultivar is nearly always a single clone and must therefore be propagated by cuttings, division, grafting, budding or tissue culture. Although numerous closes may arise during a breeding program, it is only when one is selected for its superior qualities and named, that for practical purposes a cultivar comes into existence.

A cultivar name differs from a botanical epithet (of a variety or subspecies) in its style of printing. It has initial capitals and is not italicized, and is usually enclosed in single quotation marks. An alternative to quotes is to precede it by the abbreviation “cv”. It may follow a species or hybrid name (Acer palmatum “Sango-kaku”, Acer x conspicuum “Silver Vein”), a subspecies or varietal name (Acer pectinatum subsp. forrestii “Alice”), or may follow immediately after a genus name (Rosa “Iceberg”). This last style is usual when more than 2 species are involved in the parentage of a cultivar, or where the parentage is unknown.

One of the features of cultivar names is that they can be attached to whatever botanical name is thought to reflect their true genetic origin, without the need of formal publication and regardless of changes in botanical classification. So, if the shrubby Senecio species from New Zealand are now reclassified into the genus Brachyglottis, their cultivars are all shifted automatically into the genus, and Senecio “Sunshine” becomes Brachyglottis “Sunshine”.

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