Jorge, Plants

Darwin’s illustration of the tree of life from On the Origin of Species.

The difference between genus, species, variety and cultivar is that each are different taxonomical ranks, containing populations of organisms with genetic similarities. These ranks reflect the ultimate goal of taxonomy, which is to lay out the tree of life, accurately documenting the relationships between organisms, both living and dead, tracing life back to a single ancestor. In this article, we seek to explain both why populations have been placed in certain ranks and the naming conventions used, which allow the easy identification of organisms.

Scientific name or species name – Prunus incisa

Comprised of genus Prunus and specific epithet incisa. Epithets usually refer to a feature of the plant (serrulata – little-saw, which refers to the shape of the leaves), but sometimes its origin (nipponica – Japan) or discoverer (sargentii – discovered by Charles Sargent). Genus is capitalised while its specific epithet is lower case italicized, just like its variety. Often genus is abbreviated to save time (P. incisa).  

Genus is the highest taxonomic rank you’d likely come across when browsing for plants. Genera are easy to learn. Prunus, for example, contains plums, cherries, peaches, nectarines, apricots and almonds.

Genera are hotly debated and sometimes revised. Taxons – a population of organisms – can be monophyletic, paraphyletic or polyphyletic. In monophyletic groups all species are descended from a common ancestor; paraphyletic groups contain all the descendents of a common ancestor minus one or more monophyletic groups; and finally polyphyletic groups are characterised by convergent features or habits of scientific interest. Today, taxonomists seek to avoid polyphyletic groups, believing taxons should reflect evolutionary relationships. Despite this, polyphyletic groupings persist, because of their usefulness to researchers studying similarities spread across evolutionary groups.

One recent study found that Prunus is monophyletic with all species descending from a single eurasian ancestor. Prunus, however, can be divided further into several subgenera. Historically these taxons would be based on morphology, although today they are often based on genetics. Thus subgenera are also disputed. An example of a subgenus is the Prunus subg. Padus that includes Prunus padus – a species of cherry native to the UK. As with genus, subgenus is also capitalised.

The scientific epithet completes the species name, distinguishing the plant from others in the genus. But what is a species? One definition states a species is a group of similar individuals which can reproduce successfully with each other while at the same time being reproductively isolated from other similar species. This definition leaves it up to scientists to decide when a group of individuals is distinct with some placing greater weight on genetics, others more obvious characteristics such as their morphology.

When a group of individuals becomes geographically isolated, it will begin to develop unique traits, making it distinct from the rest of the species. These distinct groups are known as varieties. Over time, they may become so different from the parent group that they are unable to breed, leading to the creation of a new species. Often, however, a variety come into contact with its parent group, resulting in an influx of genes that erodes their distinct features, reintegrating it into the greater species group.

Variety – P. nipponica var. kurilensis

The example in question, var. kurilensis is from the Kuril Islands – an island chain North of Japan, which is significantly colder than the Japanese mainland. It is extremely hardy and one of the few ornamental cherries suitable for the Nordic countries’ climate. Varieties are true to type as their seeds produce offspring with the same unique characteristics of the parent plant. Generally, plants aren’t advertised by their variety with nurseries preferring cultivars.

Cultivars are distinct from varieties in that they do not occur naturally in the wild. Cultivars are selected by humans for specific characteristics and are propagated through vegetative cuttings i.e. cloning. Propagation by seed will often lead to something different from the parent plant and as such they aren’t true to type.

Cultivars can be created through mutation breeding and hybridisation. Sometimes hybridisation programs can take years involving multiple crosses that each add a desirable trait as in the case of the Malus ‘Evereste’ – a cultivar resistant to fire blight, apple scab and powdery mildew. Mutation breeding involves bombarding plants with radiation as to induce mutations (new traits). An example of this is the ‘Rio Star’ grapefruit that is red in colour and produces more flesh and juice than varieties found in the wild. Cultivated varieties are more expensive than natural varieties due to the cost involved in development.

Cultivar – Prunus x incam ‘Okame’ / Prunus x incam cv. Okame

Cultivars are often capitalised and placed in single quote brackets, although sometimes they are written formally and preceded by an abbreviation. In the case of hybrids an x is placed before the second epithet as in the case ‘Okame’ that is a cross between the incisa and campanulatus.

If you are interested in learning more about taxonomy, please read our article: Plant Taxonomy: a History.

Jorge at PrimroseJorge works in the Primrose marketing team. He is an avid reader, although struggles to stick to one topic!

His ideal afternoon would involve a long walk, before settling down for scones.

Jorge is a journeyman gardener with experience in growing crops.

See all of Jorge’s posts.

Current Issues, Jorge, Plants

A 1644 edition of Theophrastus’ Historia Plantarum.

Plant taxonomy, or systematics, is one of the oldest biological disciplines, tracing back thousands of years to when the identification of medicinal, edible, poisonous plants as well as those suitable for crafting would prove essential for survival and later man’s mastery over the environment.

Paradigmatic to history of science were the ideas of Aristotle, in particular the science of logic. This method influenced systematists who sought to identify the essence of living things by examining many specimens and discarding variable characteristics and establishing constant characteristics. This, of course, does not work well for biology with species exhibiting significant variation between individuals. Thus, improved understanding required the emergence of empiricists, who did not believe in the essence of each form.

Other early historic figures include Theophrastus and Dioscorides, who were both Greek, but lived hundreds of years apart in classical Greece and the Roman period respectively. Theophrastus wrote hundreds of manuscripts describing plants including two large botanical treatise Enquiry into Plants and On the Causes of Plants. His works are the first surviving documents to describe plant parts, reproduction and sensitivity to climate as well as classify them by their properties as medicinal, edible and herbal for example. Dioscorides travelled widely as a physician in the Roman army and classified over five-hundred plants by their medicinal properties in his five volume De Materia Medica. Unlike Theophrastus, whose work was lost to the West till the renaissance, Dioscorides’ pharmacopoeia remained the primary botanical text for nearly fifteen hundred years.

Aristotle made immeasurable contributions to numerous fields. Despite this, many of his scientific ideas were off the mark and became entrenched after becoming part of Church’s official doctrine, which sent thinkers down blind alleys and forbade freethinking.

It took to the 1600s for the next major advance in taxonomy with John Ray’s Methodus Plantarum Nova that published details of eighteen thousand species classified by their morphology – that is an organism’s form and structure. Previously, many taxonomic systems were arbitrary, sorting plants alphabetically or by their medical properties; although he has an interesting precursor in Andrea Cesalpino, who classified plants according to their fruit or seeds. Ray was devoted in his study of botany and based his system on all of a plant’s structural characteristics, including internal autonomy. He was also a cleric and can be viewed as an early parson-naturalist who saw science as an extension of his religious work, with God wishing for man to understand his creations by collecting and classifying organisms.

Next came Joseph Pitton de Tournefort’s Eléments de botanique, ou Méthode pour reconnaître les Plantes that while not particularly original and somewhat flawed was both well written and structured and would prove highly influential as an educational textbook, especially for the father of modern taxonomy Carl Linnaeus.

Linnaeus proved revolutionary, creating the taxonomical system in use today, laid out in the works Systema Naturae and Species Plantarum. He established the binominal system of nomenclature – that is, the use of a two part name for each species, consisting of the genus name and scientific epithet. This proved a huge advance over the long, excessively descriptive names used previously such as Rosa sylvestris inodora seu canina and Rosa sylvestra alba cum rubore, which now read simply as Rosa canina. It was in fact essential with the massive influx of species originating from the hitherto unexplored regions of Africa, Asia and the Americas in need of classification.

(These older names were influenced by the Aristotelian definition of form, split into genus – the general thing described – and the differentia, which gave its special characteristics. The major problem with this was as more species were discovered the differentia became longer and longer, hence the impractical name for the dog rose above.)

John Ray saw the natural world as static, its wonders evident of intelligent design.

The publication of Charles Darwin’s The Origin of Species and his theory of evolution would again prove paradigmatic. Classifying plants by their morphology was clearly limited as organisms can possess similar characteristics but be unrelated. It was now the task of systematists to use classifications to reflect evolutionary history, placing closely related organisms together, and identifying unique species.

It wasn’t until the 1960s that systematists could accurately classify organisms according to their evolutionary history with the work of Walter Zimmerman and Willi Hennig in the preceding decades that established an objective criteria for determining the shared genetic attributes of living and fossil organisms. It was in this decade also that revolutions in molecular biology provided methods for determining the molecular structure of proteins and amino acids. It was techniques such as these that allowed systematists to supplement their analysis by comparing organisms’ genetic codes and identifying changes in genetic code.

Today systematists use multiple sources of evidence to establish a plant’s evolutionary history such as morphology, biochemistry, paleobotany (plant fossils), physiology (internal activities – i.e. photosynthesis), ecology (plants and their environment), biogeography (plant distribution), and molecular systematics (analysis of genetic code). This has been enabled with advances in computing that have allowed the analysis of large datasets.

Carl Linneas characterically posing with a plant.

Scientists estimate that there are ten to one hundred million species, so establishing their evolutionary history is a monumental undertaking. Currently, plant taxonomy is controlled by the International Codes of Botanical Nomenclature (ICBN) published by the International Association of Plant Taxonomy (IAPT), who revise codes at every International Botanic Congress. It should be stated that even with all the advances in understanding, scientists still disagree how to best classify organisms. For example what is a species?

One definition, known as the Biological Species Concept, defines a species as a “group of similar individuals which can reproduce successfully with each other while at the same time being reproductively isolated from other similar species”. The problem with this is identifying the point at which a particular population is distinctive from its parent species, as there are infinite possibilities to choose. Another definition, known as the Phylogenetic Species Concept, places more weight on the genetic differences between populations and their evolutionary history. Again the problem with this is that scientists can identify numerous genetically distinct populations, greatly increasing the number of known species.

To conclude, plant taxonomy is an ongoing project that will likely never end due to divisions about the importance of a particular characteristic and the discovery of new species and fossils. Nevertheless, the work to date has produced a logical system of classification that makes identifying plants and their relatives relatively easy.

If you would like to know more about the challenges of classification a great overview can be found here. If you would like a simple overview of the classification of plants, a table can be found here. If you would like to know more about taxonomy, especially the ranks you are likely to come across  when browsing for plants, please read our article: What is the Difference Between Genus, Species, Variety and Cultivar?

Jorge at PrimroseJorge works in the Primrose marketing team. He is an avid reader, although struggles to stick to one topic!

His ideal afternoon would involve a long walk, before settling down for scones.

Jorge is a journeyman gardener with experience in growing crops.

See all of Jorge’s posts.

Gardening, Jorge, Planting, Trees

Before we continue, it should be stated that while bare root and containerised trees each have their own advantages and disadvantages, they will not significantly affect a tree’s health, so you can be happy knowing your tree will one day achieve its potential. Rather, it is your own preferences regarding price and time of planting that will make an option worthwhile.

What’s the difference?

Containerised trees are supplied in containers, while bare root trees are supplied without soil with their roots carefully wrapped in plastic. Trees can only be extracted from soil when they are dormant; dormancy occurs from late-Autumn to early-Spring when the tree sheds its leaves. Depriving a tree of nutrients during Spring and Summer is highly detrimental to its health as the tree will try to grow, but be unable.

So this is where the first difference is. Bare roots can be supplied only when the tree is dormant, while containerised can be supplied and planted anytime during the year. (Although, it is worthwhile to first research the time of year a particular species best establishes itself. And, in general, it is not recommended to plant trees in summer when they grow at their fastest rate as without established roots, it may fail to establish.)

As bare roots are supplied without soil, they are lighter and cheaper to transport, which makes them significantly cheaper (30-50%) than potted varieties. Hence, bare roots can be great value for money.

Next, as bare roots grow in the ground, their roots spread out in a natural fashion, which allows them to establish themselves effectively, giving them adequate access to soil from which they acquire their nutrients. Sometimes a containerised plant’s roots have inadequate room to grow, resulting in spiralisation, whereby their roots grow in spiral at the bottom of the pot, which puts it in a poor position come planting. Although, this usually only occurs in garden centres, rather than nurseries that will upgrade a tree’s pot as it ages.

It has been argued that containerised trees are better at establishing themselves when planted as they are supplied with nutrients throughout the transplanting process. Bare roots, on the other hand, often lose a chunk of their roots when transplanted, which can lead to water stress. However, this argument doesn’t really hold up, because the tree’s roots are wrapped with compost or hydrogel. Furthermore they are usually supplied as one or two year old trees, which ensures the roots are adequate for the above-ground matter. Altogether, providing the tree is well wrapped and planted promptly, it will be fine. If you can’t plant immediately, it is recommend to leave the tree in water, possibly with the addition of liquid fertiliser.

Planting trees is not recommended in Summer, nor when the ground is frozen, so containerised can’t be planted anytime and it is recommended to buy bare roots when they are first available in November.

This leads to another advantage of containerised trees: they do not have to be planted immediately. This can be useful if you wish to gift a tree. And as containerised can be purchased whenever, you can purchase a deciduous in summer when it looks best. Furthermore, containerised are pruned so will have a nice shape on arrival. Bare roots on the other hand aren’t, which on-the-flipside can be useful if you wish to train a tree, as in the case of many fruits. Lastly, there are many options of containerised trees. One can purchase, for example, a 9 year old ornamental in 55L pot that can provide an immediate uplift to a garden.

Overall, all trees will flourish, providing they are looked after. Bare roots are cheaper and can be trained into a fan, espalier or cordon, but they are only available as one or two year old trees and can only be planted in the Winter. Containerised trees can be planted whenever, look better on arrival and come in a range of sizes, but they are more expensive and can be harder to train.

Have you decided on a bare root or containerised tree? If so, Primrose has a huge range of fruit and ornamental trees, both bare root and containerised, so please have a browse.

Jorge at PrimroseJorge works in the Primrose marketing team. He is an avid reader, although struggles to stick to one topic!

His ideal afternoon would involve a long walk, before settling down for scones.

Jorge is a journeyman gardener with experience in growing crops.

See all of Jorge’s posts.

Jorge, Plants, Trees

In sharp contrast to the frosty mornings and cold evenings, fiery hues and bright coloured fruit can liven up your Autumn. Cultivars of crabapple and rowan can produce pink, yellow and red fruit, which are perfect for wildlife, while maples and beech can create gorgeous palettes of red, orange and yellow. Now without further ado, here are six trees with fantastic autumn colour.

Maple Trees

An essential part of Autumn iconography, the red maple (Acer rubrum) can be found throughout the UK’s public parks. Introduced all the way back in 1656, the tree will produce a profusion of red, orange and yellow, before turning a vivid red. Highly versatile, the tree can be found growing in a wide range of conditions in its native North America and is suitable for urban settings as it is tolerant of pollution.

Unlike its larger cousin, the japanese maple (Acer palmatum) is suitable for all gardens and can be grown in containers. Indigenous to East Asia, the tree can be found growing at heights up to 1100m, hence its other name the mountain maple. Come Autumn, the tree’s dissected leaves will turn a deep red. The palmatum has many quirky cultivars including ‘Butterfly’ with its cream-tinged green leaves that turn pink; ‘Atropurpureum’ with scarlet red Autumn foliage; and ‘Sango-kaku’ with its gorgeous coral coloured bark and stunning foliage.

Beech Trees

Picture Credit: Jean-Pol GRANDMONT licensed under CC BY 3.0.

Another typical Autumnal tree, the common beech (Fagus sylvatica) is a large, majestic tree with great spectrum of colours. Reaching up to 50 metres in the wild, although commonly 30, it was once believed that the tree is native to southern England, but not the North where it is sometimes removed. Researchers now believe the tree was first introduced to Southern England by Neolithic humans who sought its nuts for food, making it a non-native species. Trees in the North, on the other hand, were introduced by Vikings in the first millennium. Thankfully, most trees sold are limited in height by their rootstock, and are thus suitable for most gardens.

The Sweet Gum

Picture Credit: Famartin licensed under CC BY-SA 4.0.

Similar to maple and beech, the sweet gum (Liquidambar styraciflua ) is notable for its conflagration of colour with its sharp five-pointed leaves turning red, yellow and purple. First introduced to Europe in 1681 by John Banister, one of the first university trained botanist, the species is also notable for deeply ridged bark, known as alligator bark in America.

Crabapples

Severely underrated, the humble crabapple will produce attractive foliage and bright and colourful fruits, which can last well into winter. The species is hardy, versatile and great for wildlife, being native to the UK. The tree will produce beautiful blossom come Spring and serves as a great pollinator for apples. Notable cultivars include ‘Butterball’, ‘John-Downie’, ‘Evereste’ and ‘Red Sentinel’ with yellow, scarlet-orange, red and yellow-orange fruit respectively. Special is ‘Butterball’ that can produce six different colours throughout the year.

Rowan Trees

Similar to the crabapple, rowans produce fantastic coloured berries that can last well into winter, providing an essential source of sustenance for birds. Hardy and versatile the species is suitable for most soils, and with many different sized cultivars, there is a tree for every garden.The most famous is the native Sorbus aucuparia with red berries and slender leaves, which turn yellow in Autumn. Simply stunning, however, are the cultivars ‘Pink Pagoda’ and ‘Joseph Rock’. The former produces gorgeous pink berries, which are a favourite for birds, while the latter looks amazing with its deep red pinnate leaves contrasted with bright yellow berries.


Cherry Trees

Commonly thought as a tree for Spring, many cherries are ideal ornamentals for Autumn hues. One of the best has to be the ‘Autumnalis Rosea’ that will flower intermittently from November to April with clusters of small semi-double rosy-pink blossom. Other varieties such as ‘Fragrant Cloud’, ‘Sargent’s Cherry’ and ‘Umineko’ will produce fiery displays, with the Sargents among the first trees to colour up. ‘Fragrant Cloud’ is especially beautiful with its upright form. Worthy of note is the ‘Tibetan Cherry’ with its smart coppery-brown bark that will look beautiful regardless of season.

Jorge at PrimroseJorge works in the Primrose marketing team. He is an avid reader, although struggles to stick to one topic!

His ideal afternoon would involve a long walk, before settling down for scones.

Jorge is a journeyman gardener with experience in growing crops.

See all of Jorge’s posts.

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