How plants are named

It would in many ways be very convenient if every plant had a single name. There would be no mixing up your verbena (Verbena officinalis) with your lemon verbena (Aloysia citradora) or your hemlock (Conium maculatum) with your hemlock (Tsuga spp.).

Unfortunately, it's not that simple. Dialects are regional, but plants can be ubiquitous, spanning countries and even continents. Different names spring up for the same plant depending on where you go and who you speak to.

Geum urbanum, for example, is referred to variously as wood avens, herb Bennet, colewort, St Benedict's herb and (my personal favourite) cloveroot. Worse, Alliaria petiolata has a panoply of names, including jack-by-the-hedge, jack-in-the-bush, sauce-alone, penny hedge, poor man's mustard, garlic mustard and garlic root (even though it's about as distantly related to garlic as possibly it could be). And there are plants with more names than that!

Geum urbanum, a versatile weed with many names

The taxonomic system

That's why an inspiring Swede named Carl Linnaeus set about giving every single individual species its own, unique technical and scientific name. That way, no one species could be mixed up with another, and you would always know what plant you're referring to.

That system is known as binomial nomenclature, which, in short, means that each plant is given two names that, when used together, uniquely identify it. This will be familiar from biology lessons at school. Who can forget the mnemonic King Philip (or David) Came Over For a Good Sherry, which helps us learn the various levels - kingdoms, phyla, classes, orders, families, genera and species - into which life as we know it is divided?

Under binomial nomenclature, a plant (or, indeed, an animal, fungus or bacterium - the system works for all life) is identified by citing its genus and its species.

• Genus is the last-but-one level of the traditional taxonomic system. It's effectively a grouping of plants that are so similar they sit naturally alongside one other. They may look the same, taste the same, grow in the same way, etc.

• Species is the last level of the traditional taxonomic system. It is the level that uniquely identifies a particular organism. All members of a single species are the same plant, so much so that one of the key defining characteristics of a species is often said to be that members of the species can produce fertile offspring, but they cannot produce fertile offspring with members of a different species.

As time has gone on, this system of classification has proven far too rigid. How likely was it ever that all plant life could be sub-divided only six times? Traditionally plants (as other life) were sorted into these six, rather convenient categories based almost purely on their outward appearance.

With the benefit of science, we have now discovered more and more plants and connections between them, and we are now able to crack open the genetic code of all life, meaning we can place plants more accurately within the taxonomic system. Plants are now increasingly grouped more based on the similarity of their DNA (a process called "phylogenetics") or their evolutionary history (as process known as "cladistics") than on appearance alone. Indeed, phylogenetics and cladistics often go naturally and inescapably hand in hand.

This has resulted in two key changes:

• It has been necessary to create many more levels than the traditional seven (kingdom, phylum (or, more often for plants, division), class, order, family, genus, species). Enter tribes, clades, sections, series, sub-species, varieties and forms. Often even that is not enough, as we see infra-orders, sub-families and the like.

• As their genetic code becomes clearer, plants are often moved from one genus to another, sometimes even from an entire family to another. New families, genera and species are constantly being created to better group together related plants. So the binomial name you once gave a plant may now be something quite different.

Don't let this put you off. Generally speaking, although it's interesting, as an amateur you don't need to know the complete taxonomic breakdown for a given plant. Just the genus and species (and possibly the sub-species or variety) will do! And if a plant's binomial name changes, fear not: the old name is often still used, or given in addition, to help with the transition.

Fraxinus excelsior (European ash), with seed pods ("keys")

How binomial nomenclature works

Taxonomic names are designed to be universal, rather than biased towards a single language. For this reason, they are usually taken from Ancient Greek or Latin, or a combination of both, with the reasoning that both of those languages have inspired scientific and technical jargon in various languages. Of course, in reality, this "universalism" remains highly Europocentric, but it seems to serve well enough.

Thus, the taxonomic name for a plant may well stem from its name in Greek or Latin, or from some descriptive factor that derives from a Greek or Latin word. Take, for example, the European ash, whose taxonomic name - Fraxinus excelsior - is formed from the Latin word for "ash tree" (Fraxinus) and the Latin word for "higher up" (excelsior).

[PHOTOGRAPH OF A SUNFLOWER]

In many cases, there will be no true Ancient Greek or Latin word for a plant, because it will not be native to Greece or Italy, or even Europe. The sunflower is a good example. All sunflower species originate from the Americas and were brought to Europe and other parts of the World some time around the turn of the 16th century.

So, the sunflower's taxonomic name is "made up" - reconstructed from bits of Greek and Latin. The sunflower genus is called Helianthus - a word formed from the Greek words for "sun" (ἥλιος hélios) and "flower" (ἄνθος ánthos). The common sunflower, with which we are all familiar, is called Helianthus annuus, the second part of its name being the Latin word for "annual".

In some cases, there is little inspiration even in the modern language, and it's been left to botanists to engage their creativity. The water dropworts, for example - an extremely poisonous genus of tall, tubular plants in the celery family - are known by the taxonomic name Oenanthe. This name stems from the Greek words for "wine" (οἶνος oínos) and "flower" (ἄνθος ánthos), giving the literal name "wineflower", in tribute to the wine-like scent of their blooms. However, they have nothing to do with actual wine, and if you drank a liquor made from them you would end up quickly and emphatically dead.

Finally, in many cases the second part of a plant's binomial name will refer not to one of its descriptive qualities, but to the name of the person who discovered or classified it. This usually takes the form of the person's name, suffixed with the letter "-i" or "-ii" in imitation of the Latin genitive (possessive) case. So, for example, the taxonomic name for the Douglas fir is Pseudotsuga menziesii. "Pseudotsuga" means "false tsuga". ("Tsuga" itself is a Japanese word; the family of trees is more commonly referred to in the UK as "hemlock". The Douglas fir is not, in fact, a fir!) "Menziesii" refers to Archibald Menzies, a Scottish naturalist who first documented the tree. Thus, "Pseudotsuga menziesii" literally means "Menzies' false hemlock".

Note that the name of the genus always has a capital letter, and the species name is always lowercase.

[PHOTOGRAPH OF AN OENANTHE SPECIES]

The binomial system is therefore a tremendously useful way to refer to plants, and you will often find that botanists - both amateur and professional, experienced and novice - will use them to avoid any confusion.

Binominal nomenclature can sometimes get quite unwieldy. It can be a pain to repeat the entire name for a plant each time, particularly because taxonomic names can sometimes get quite long! Take, for instance, the European horse chestnut (Aesculus hippocastanum), or the giant redwood (Sequioadendron giganteum). My fingers hurt typing these names just once, let alone over and over!

For brevity and out of kindness for our digits, therefore, we tend to recite the genus only the first time. After that, we simply abbreviate it each time to its first letter. So, for example: A. hippocastanum, or S. giganteum. If taxonomy works well, this abbreviation should cause no confusion at all. There should still be only one plant named S. giganteum anywhere in the world.

Sometimes people get a little sloppy and go even further, missing out even the genus abbreviation. That works OK for some species. You can call a sycamore "pseudoplatanus" instead of "Acer pseudoplatanus" without confusion, because there's no other species anywhere with the word "pseudoplatanus" in it.

But often the second part of the binomial name is not unique to the plant. Rather, it may be a word used to describe some aspect of both that plant but also the same aspect of completely unrelated plants.

A common example is sylvestris, which means "wild" or "of the forest" in Latin. This appears as the second word in the binomial name of many species, including cow parsley (Anthriscus sylvestris), wild angelica (Angelica sylvestris), woodland tobacco (Nicotiana sylvestris), wild tulip (Tulipa sylvestris), Scots pine (Pinus sylvestris), crabapple (Malus sylvestris) and snowdrop anemone (Anemone sylvestris). It might be possible to refer to one of these species simply as "sylvestris" if it has been made abundantly clear earlier in the conversation that that is the species being discussed, but otherwise it is simply impossible to know what plant is intended.

Anthriscus sylvestris (cow parsley (UK) or wild chervil (US))

Other examples include the following, which, in some cases (as in Latin) may alternatively end in "-us", "-a", "-um", "-is" or "-e":

• albus ("white"), as in Populus alba (white poplar)
• angustifolius ("narrow-leaved"), as in Lavandula angustifolia (English lavender)
• aquaticus ("aquatic"), as in Ipomoea aquatica (water spinach)
• arvensis ("of the field"), as in Torilis arvensis (spreading hedge-parsley)
• cordatus ("heart-shaped"), as in Tilia cordata (small-leaved lime/linden)
• dulcis ("sweet"), as in Prunus dulcis (almond)
• glutinosus ("sticky"), as in Alnus glutinosa (common alder)
• hirsutus ("hairy"), as in Hypericum hirsutum (hairy St John's wort)
• japonicus ("Japanese"), as in Lonicera japonica (Japanese honeysuckle)
• maculatus ("spotted, blotched"), as in Conium maculatum (hemlock, poison hemlock)
• maritimus ("maritime"), as in Eryngium maritimum (sea holly)
• montanus ("mountain"), as in Ranunculus montanus (mountain buttercup)
• niger/nigra ("black"), as in Pinus nigra (black pine, Corsican pine)
• occidentalis ("Western"), as in Trifolium occidentale (western clover)
• officinalis ("official"), as in Salvia officinalis (common sage)
• orientalis ("Eastern"), as in Helleborus orientalis (Lenten rose)
• parviflorus ("small-flowered"), as in Malva parviflora (Egyptian mallow, cheeseweed)
• rēpens ("creeping"), as in Ranunculus repens (creeping buttercup)
• rěpens ("unexpected"), as in Trifolium repens (white clover)
• sativus ("cultivated"), as in Pastinaca sativa (parsnip, wild parsnip)
• vulgaris ("common"), as in Ligustrum vulgare (common privet)

So, if you're using the binomial name, do try always to include both parts.

Sub-species, varieties, forms and hybrids

Well, that was all a bit too easy, wasn't it? But it goes just a little bit further. Sometimes, even within a single species, we see different types of plant and we want to distinguish between them.

Sub-species are different populations of the same species that inhabit different areas and show different appearance. Generally speaking, two sub-species don't interbreed with each other, not because this is biologically impossible, but because they just don't live near each other. However, as members of the same species, it would be perfectly possible for them to do so.

A good example is Prunus domestica - the European plum - which can be divided into several subspecies, including (in the UK) Prunus domestica ssp. domestica (common plum), Prunus domestica ssp. institita (damson), Prunus domestica ssp. intermedia (Victoria plum), Prunus domestica ssp. italica (gage) and Prunus domestica ssp. syriaca (mirabelle).

[PHOTOGRAPH OF A PLUM]

Varieties are naturally occurring variations within a species. All species exhibit some degree of individuality, or else every specimen would simply be a clone of every other plant! But sometimes a group or collection of plants shows such a consistent variation that it's possible to place it within a separate variety.

To take the example above: the gage (Prunus domestica ssp. italica) can be divided into a number of varieties, including the greengage (P. domestica ssp. italica var. claudiana), the Ontario plum (P. domestica ssp. italica var. subrotunda) and others.

Forms are plants that have a minor difference from the species as a whole, such as differently-coloured leaves or flowers. Common examples seen in the UK include the copper beech (Fagus sylvatica f. purpurea) and the black elder (Sambucus nigra f. porphyrophylla).

Sambucus nigra f. porphyrophylla (black elder)

Finally, cultivars or hybrids are crosses between two or more species or sub-species that have been deliberately created by us humans. Although I said above that normally species cannot interbreed to produce fertile offspring, there are plenty of exceptions, especially within the plant kingdom. Crossing species is normally done to create a plant that has the best characteristics of its two "parents", making it stronger than either, less susceptible to disease, faster-growing, more heavily-cropping or just generally "better" in some way. Common examples you will have seen in the UK but may not have known are cultivars include the London plane (Platanus x hispanica or Platanus x acerifolia) and the common lime (Tilia x europaea).

There really is a fine line, though, between sub-species, varieties, forms and hybrids. And, indeed, given long enough, it is perfectly possible for two sub-species to continue diverging far enough that they become separate species, a process known as "speciation".

Confused?

Don't panic. You don't need to remember all this. It's best to start by just understanding the way plants work, how to tell them apart and the most common name (or names) they go by.

If you can remember the binomial name of a plant, that's great. You'll garner more respect among botanical communities and you'll avoid plenty of dispute about names that inevitably happens from time to time on social media. Most importantly, you'll be able to put beyond doubt what plant you're talking about.

It may take some time at first, but that's ok.