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Plant names can be complicated. It seems like it should be easy enough to refer to a plant and have another person know what you’re talking about. But as soon as you get into plant-related conversations you quickly end up in a tangle of Latin, confusion, and even misinformation.
All this mess is down to the fact that there are several different systems for naming and categorising plants, which each have different motivations and requirements. No single system is correct or incorrect, it’s just that they were each created to serve different purposes. The trouble is, there’s a lot of overlap in the words used by the different systems, so it’s not always clear which is in place at any time. People switch between them arbitrarily or don’t even realise there are different systems involved. The result is that people are often talking to each other at total cross-purposes, even though they might both think they’re speaking the same language.
In this article I’m going to explain the different naming systems, why they originated, and how they are used. My aim is to explain how and why there are such frequent misunderstandings and conflicts about the names of plants. I can’t single-handedly prevent those misunderstandings from happening, but I hope that the more information people have the better equipped we’ll all be to communicate.
Botany is the area of science that studies plants. One important component of that is naming and classifying plants into categories based on how they are related to each other. All of the classifications botanists make are based on wild plants as they occur naturally, and generally have nothing to do with cultivated plants.
The primary goal of science – including botany – is simply to learn and advance knowledge. So the motivations behind the botanical systems of classifying plants are:
- Making it easy to keep track of the continual updates that come with new research, and
- Making it easy to share information between professionals.
A species is the basic ‘unit’ of categorising plants in botany. It defines one type of plant where all individuals are similar enough to each other to be considered a matching set. The exact definition of a species is difficult to pin down, but generally it means a population that are all able to reproduce with each other and have fertile offspring.
A genus is a group of species that are closely related to each other. If a species is like one individual person, then a genus is like a set of siblings. Any species can only belong to one genus – just like any person can only belong to one set of biological parents.
A family is a group of genera (genuses) that are related to each other. To extend the analogy of relatives, a family is like a whole set of cousins in different sibling groups. Any genus can only belong to a single family.
Above the family category, there are more and bigger categorisations too – extending all the way up to the kingdom, which is the group containing all plants. But for most purposes, we only need to worry about species and genera, and occasionally families.
There are also groupings below the level of species – like subspecies. These are groups with slightly different traits but which still belong to the same species. They often occur when different populations are geographically isolated, but haven’t quite developed into distinct species.
To create or change a botanical group, researchers make a sample of preserved (dried) plant material associated with a description. They publish a scientific paper and, if other researchers agree with the study, it becomes accepted. This process makes sure that the scientific world is always up-to-date on the correct botanical groups – someone gives evidence, other researchers make sure the evidence is good, and it becomes added to the shared ‘canon’ of scientific knowledge.
Because clear and unambiguous communication is really important in science, there are strict rules and systems in place to make sure botanists can agree on how to refer to and identify species. These rules are laid out in the International Code of Nomenclature for algae, fungi, and plants (ICN). It’s a very long document with lots of technical details, but I’ll explain a few of the most important parts.
An individual species is always identified by a two-part name, for example Ceropegia linearis. The first part of the name (Ceropegia) refers to the genus. That name can be used on its own to identify the whole group of related species. The second part of the name (linearis) refers to the species. That name can’t be used on its own, it’s only meaningful in the context of the correct genus. When there is a subspecies it’s written as a third part of the name with the abbreviation “subsp.” to separate it, for example Ceropegia linearis subsp. woodii.
These binomial (two-part) names are written in italics by convention. The genus name is always capitalised, and the species name is always lower-case. If the name has been written out in full already then subsequent uses can abbreviate the genus to just the first initial, for example C. linearis. Scientific names are sometimes called “Latin names” because they are treated grammatically as Latin, and are often constructed from Latin words.
Probably the single most important rule in the ICN is that every species has only one valid name, and equally that every name only applies to one species. When a species gets reclassified into a different genus, its old name is no longer valid. Instead the old name is classed as a synonym. This means that, anywhere the old name is used (for example, in a book or article which was published before the reclassification), it is considered equivalent to the new name. So future studies can still refer to research which used the old name, confident in the knowledge that it refers to the same species.
The “one name per species” rule doesn’t just come into play when a single species is moved from one genus to another. It’s also to prevent situations where two people independently study a species and create their own new names for it. When this is found to have happened, only one of the names is considered valid (almost always the first one to have been published), and any others are defined as synonyms in the same way.
Because of the constantly-changing nature of science, it’s pretty impossible for one person to keep up with the valid names of every species. But luckily, thanks to the internet there are sites that do it for you! I recommend Plants of the World Online, which is a database maintained by the Royal Botanic Gardens, Kew (a world-leading botanical authority). You can search for any genus or species name, and find out whether it is currently considered valid, or whether it’s a synonym for something else. And most importantly, see references to the publications that made and explained those decisions – because science is about trusting good research, not just trusting authority.
Horticulture is the art and practice of cultivating plants for human use. That includes anything from food, to medicine, to ornamental gardens (and of course houseplants!). Where botanists are primarily interested in studying how things naturally are, horticulturists are generally focused on what can be done.
The industry is based on things like creating new plants, preventing pests and diseases, propagating plants efficiently, and – of course – selling them. So the main motivations for the way plants are classified in horticulture are:
- Making it easy for growers to care for and keep track of their own plants, and
- Making it easy to advertise plants to customers – including people who know little or nothing about the industry itself.
The basic ‘unit’ of categorising plants in horticulture is the cultivar. Like the species, it also defines one type of plant where all individuals are similar enough to each other to be a matching set. The difference is that this similarity is not determined by whether they can reproduce together or how they relate in the wild. Instead the similarity is purely defined by having specific desirable traits which are maintained when propagated by humans. Depending on the cultivar, those traits might be things like particularly tasty fruit, resistance to certain diseases, or unique flower colours.
Unlike the definitions of wild species, cultivars are defined by their description and not their genetics. If you breed together two plants of a certain species, the offspring are always – by definition! – members of that species. But the same does not apply to cultivars. If a plant is propagated and some of the resulting new plants don’t fit the cultivar description, they are not considered the same cultivar. Similarly, if a completely different plant is bred or spontaneously mutates into a form that’s indistinguishable from another cultivar, then they are considered the same even though they might be genetically separate.
Some cultivars are reproduced by selective breeding – parents with desirable traits are bred together to produce seeds that have the same traits. Other cultivars are propagated exclusively by asexual methods like cuttings or divisions, which means that every plant of that cultivar is genetically identical. No distinction is made between these different types of cultivars, they simply have to be stable when propagated in an appropriate way.
Although cultivars can be propagated in different ways, it still has to be the plant itself which maintains the desired traits. That means that a plant cannot be considered a cultivar if its distinctive characteristics require chemical treatments or other inputs to maintain. That’s why the controversial Philodendron “Pink Congo” is not a cultivar – its pink leaves are produced by chemical treatment and they return to green when the treatment wears off.
There is also a wider category called the cultivar group. This is, as you might guess, a group of multiple individual cultivars. But again, unlike botanical classifications, these are not so strictly defined. A group can be created if it’s convenient to put a set of cultivars together – for example, if they all have flowers of a certain colour. But there’s nothing stopping a cultivar from being put into multiple cultivar groups if it fits all of their descriptions. They do not have to be mutually exclusive in the way genera are.
Because horticulture as an industry has such a different emphasis to science, the systems for creating and keeping track of cultivars are also very different. Whereas science relies on peer-reviewed journals and evidence, horticulture has very little need for that. Instead, growers simply create names in order to describe and market their own plants. There is no requirement for cultivars to be reviewed or confirmed by other professionals, or to be published or documented in any particular way.
There are organisations whose aim is to record all cultivars within certain botanical groups. And there are guidelines for defining and naming a new cultivar (the International Code of Nomenclature for Cultivated Plants – more in the next section). But all these groups and guidelines are completely voluntary, and many growers simply don’t bother to submit their cultivars to any organisation, or use the advised format for labelling. They don’t need to follow those rules in order to grow and sell plants successfully, so it’s much less universal than the systems enforced by the scientific method in botany.
A note on the word “variety”
I’ve deliberately avoided using the word “variety” in this article to avoid ambiguity, because it’s used to mean diffrent things in botany and horitculture.
A botanical variety is a naturally-occuring form of a plant. It’s a category below the level of species (similar to the subspecies). But in the context of horticulture, people often say “variety” when they actually mean cultivar (a deliberately cultivated form of a plant). To avoid any confusion, I always either say “botanical variety” or “cultivar” – depending on what I actually mean.
The horticulture industry has its own set of rules for naming cultivars, laid out in the aptly named International Code of Nomenclature for Cultivated Plants (ICNCP). It’s partially based on the botanical ICN, and comes with a rule that every member of an ICNCP-named group (a cultivar) is also a member of an ICN-named group (a species). There is a lot of detail in the document, so I’ll simplify the most relevant points. (You can also read a plain english version here).
A cultivar is identified with a name in two parts, for example Ceropegia ‘Silver Glory’. The first part is the scientific genus the plant belongs to. It can be given as the scientific name of just the genus, but it can also be the full scientific name of the species or subspecies (if applicable). It can even be given as an ‘unambiguous’ common name (more on those later) for the genus or species. So all of these are accepted ways of labelling the same cultivar according to the ICNCP:
- Ceropegia ‘Silver Glory’
- Ceropegia linearis ‘Silver Glory’
- Ceropegia linearis subsp. woodii ‘Silver Glory’
- String of hearts ‘Silver Glory’
The second part (‘Silver Glory’) is the cultivar itself. This can either be a single cultivar name, or a cultivar group. If it’s a single cultivar, it should always be written in regular text (not italic), in single quotes, and in title case (the first letter of each word is capitalised). If it’s a cultivar group it should be in regular text in title case, and without quotes. Unlike scientific names, cultivar names must not be entirely Latin.
The ICNCP has a “one plant per name” rule for cultivars, just like the ICN’s rule for botanical groups. Each cultivar identifier has to be unique within a genus of plants (you can’t have more than one ‘Silver Glory’ cultivar of Ceropegia), but it can be duplicated in a different genus.
There’s also a corresponding “one name per plant” rule. This means that each cultivar only has one accepted name. If multiple names have been used for the same cultivar, then only one of them (usually the first-published, but sometimes just the most widely-recognised) is considered correct, and the others are considered synonyms.
As I mentioned above, these are guidelines, but they are often not followed. Plant growers are more focused on producing and selling nice plants than on keeping up with scientifically-precise rules for labelling them. Although the ICNCP is the closest thing to a set of universal rules, its implementation is very far from universal. In spite of the guidelines, these are all common in horticulture:
- Using outdated scientific names (because it’s the name you’ve always used, why suddenly change it and confuse your customers?)
- Creating new or duplicate cultivar names (because you’ve come up with a better name that will help your plant sell, and who cares if someone else calls it something different?)
- Writing labels in a completely different format (because a garbled combination of italics, latin, and quote marks is a lot of effort, so why bother if customers like catchy names?)
There’s nothing wrong with these choices, they’re perfectly justified for the context. I’m just giving an explanation of why things are this way, and why plant labels are often less ‘scientific’ than I and other nerds like me might like.
Although botanists and horticulturists are the biggest single plant-related communities, they are still only a tiny fraction of the population. Most people who buy, see, use, and talk about plants are not specialists in botany or horticulture. And this huge chunk of the population have yet another system (if you can call it that) for grouping and naming plants.
Unlike the others, there’s nothing organising this system at all. There aren’t even any attempts at guidelines – it’s a free-for-all! Just like the way language as a whole develops. People create names and categories that are useful, they spread among families and communities, they get adjusted over time and they constantly mutate. They’re often at least partially informed by information coming from horticulture and botany, but they’re also very often completely separate.
In day-to-day life, people categorise plants in all kinds of different ways.
- Sometimes the groups have a practical definition (“weeds” – any plant you don’t want to grow there)
- Sometimes they’re according to visual traits (“flowers” – any plant that makes blooms that grab more attention than the foliage)
- Sometimes they correspond to one of the other classifications (“aroids” – plants from the botanical family Araceae)
And there are countless others too. Some of these ways of classifying plants are specific to a certain location, a certain family, or a certain culture. Some of them are unique to one individual person. Some are very widespread or near-universal. The only thing they all have in common is that they don’t have any kind of formal or precise definitions, which is how they differ from either botanical or horticultural classifications.
Any word or phrase used to identify a plant that is not either an ICN-compliant scientific name or an ICNCP-compliant cultivar name is called a common name. These are often ambiguous, repetitive, unclear, misleading, or very locality-specific. I would be here all day if I tried to explain all the possible problems common names can have.
Don’t take it the wrong way, common names are great! They are vitally important and they serve a very useful function, enabling people to communicate about plants in certain contexts. Within families or communities or specific groups, common names work perfectly well.
But in a context where we are often talking to strangers from across the country or across the world, they don’t work so well. Often, people will assume that their own familiar name is the only common name for a plant – or the only name at all. Their own internal classification of plants and names might be very organised, but it might also be entirely different from someone else’s. And people don’t necessarily realise this is the case, assuming their internal classification is universal. Which is why we end up with conversations like this:
Person A: Split-leaf philodendron and Monstera deliciosa are the same thing!
Person B: Split-leaf philodendron and Monstera deliciosa are different species!
And they are both correct. Because Person A knows “split-leaf philodendron” as a common name for Monstera deliciosa, and Person B knows “split-leaf philodendron” as a common name for Thaumatophyllum bipinnatifidum. And there’s no such thing as an incorrect common name! Because if it’s a name, and people use it, then it’s a common name. No matter how strange or confusing it might be to someone who doesn’t use it, no matter whether it’s used for multiple different species, no matter if it contains a misleading scientific name that belongs to a completely different group… It’s commonly used, so it’s a common name.
Common names also often fail to distinguish between the botanical level of classification (wild species, genera, and families) and the horticultural level of classification (human-made cultivars and cultivar groups). For example, the species Epipremnum aureum has a common name of “golden pothos”. There’s a particular cultivar of Epipremnum aureum with yellow-variegated leaves which also has a common name of “golden pothos”. And there’s a different cultivar of Epipremnum aureum with lime-green leaves which has a common name of “neon pothos”.
So, is “neon pothos” a type of “golden pothos”? Well, yes, and no, because it depends on who’s asking and what names their internal classification uses for cultivars and the species they belong to! It all comes down to the subjectivity of common names. They can be very useful in the right context, but they can also be very unhelpful without the right context.
There is nothing wrong with common names. Many people are only familiar with common names, and might not know anything about the formal systems of botanical names and cultivars, and that’s fine. It’s completely okay to refer to plants using whatever names are familiar and useful to you.
But in a conversation with other people, always remember that the names that are familiar and useful to you might not be the names that are familiar and useful to anyone else. If it’s a conversation about identifying plants or discussing how they relate to each other, then be sure to consider how subjective and variable common names can be. The ICN and ICNCP systems for botanical and horticultural names are the closest thing to an international language and the most unambiguous option to use when in any doubt.