Recognising succulent plants and cactus: a guide to identification and correct classification

Giving a succulent plant a scientific name, in other words correctly identifying a succulent plant, is far from simple. The classification, in general, the so-called taxonomy, is the prerogative of researchers, scholars, true enthusiasts and, in some cases, real maniacs of order. In that boundless land that is the classification of plants – and in the specific case of succulents – there is often great confusion, to the point that it is not uncommon to see the continuous re-denomination of the same plant, first included in one genus, then placed in another, then moved to another one. Some plants are classified in one way by some researchers and differently by others; some tend to simplify and reduce the number of genera, while others are inclined to divide plants as much as possible, according to their peculiarities, into several genera, species, subspecies, forms and varieties. Diatribes between authors and researchers about the classification of this or that plant are the order of the day. Thus, the common grower and the ‘collector’ of succulents are forced to rely on plant tags or try to classify them by comparing the specimen with pictures found on the Internet or in books.

In the following article, we see how to find your way and what you need to know to begin to identify and classify succulent plants. We also learn how to distinguish a cactus from any other succulent plant and how to identify the best-known succulent families.

Starting to orient yourself
Stapelia hirsuta
Stapelia hirsuta

Giving the correct scientific name to a plant (classification) is not an easy task: you need good knowledge, at least of the most popular succulent families, then a good dose of experience and finally much, much patience. Approaching the subject of classification gradually it’s possible to learn how to orient oneself among succulents, and soon to distinguish a Euphorbia from any cactus, a Crassula from a Sempervivum, an Agave from an Echeveria or a Lithops. That said, I don’t like much classifications: I’m not passionate about them, and if I find it right to identify my plants, at the same time I don’t feel the need to match each of my plants with a tag indicating in detail the name of the genus, the species, the subspecies, the field number and the author of the discovery of this or that one. It is enough for me to know the genus to which each plant belongs and possibly the species so that I can identify it correctly and note this first information on the tags. Other information is welcome, but I’m not going to lose my head. I just appreciate the plant for its shape, its thorns or its flowers.

Of course, over time I have learned to distinguish a succulent plant from a cactus without problems and to place the succulent plants in the main families: it is a “commitment” within everyone’s reach, and with a minimum of experience you will soon get good results. Getting a primary classification at least, it will be useful in the cultivation of that specific plant, since not all succulents need the same in terms of temperature, exposure, watering, soil, etc…

Families, genera and species

Firstly, to understand the classification, you need to know the basic groupings. Without being too specific and starting from the top, plants are grouped by families (which can be divided into subfamilies, tribes and sub tribes). The families are basically “macro groups” into which it was decided to include the plants according to affinity. Limiting to the main ones, in the “succulent area” we have the family of Cactaceae, that of Euphorbiaceae, that of Crassulaceae, then Agavaceae, Asclepiadaceae, Aizoaceae, and so on. As you can easily guess from these examples, these families end with the suffix -eae.

Families are divided into genera, groupings that include species with some common characteristics, such as the type of inflorescence, for example. Then, each genus is divided into several species, which are the most “targeted” groupings and classify the plants in more detail. The genus is written with a capital initial (e.g. Echinocactus), while the species, which often derives its name from the place where that plant was discovered or from the surname of the person who classified it first, or from some salient characteristic of the plant itself, is written with a lowercase initial (e.g. grusonii).

Summarizing and taking, for example, a cactus we all know, the correct classification is the following: family Cactaceae, genus Echinocactus, species grusonii.

A Euphorbia can instead be classified as follows: family Euphorbiaceae, genus Euphorbia, species candelabrum.

This first classification can be complicated when variations have been identified in the same species, variations usually related to the growing range or to simple mutations concerning the type-form. So there will be subspecies, forms, intermediate species. Often, these detailed classifications are completed with the so-called “field numbers“, i.e. data indicating the location where that species or subspecies was discovered. Just to stay at our first example, we could find an Echinocactus grusonii intermediate form (since this plant has spines slightly shorter than the species-type, but longer than the inermis variety).

Cactaceae
Cespitosa
Cactus: caespitose form

At first, the things seem to be complicated and in fact, going deep into it, we realise they’re quite knotty, but these primary indications allow us at least to make the first, fundamental distinctions. The plants included in the family of Cactaceae, for example, have almost unique characteristics compared to all other succulent plants (i.e. plants able to accumulate water reserves in the stem, or in the roots or leaves). However, the first aspect to keep in mind is that all cacti are succulent plants, while not all succulent plants are cacti.

All succulent plants can accumulate water reserves, and cacti fall into this broad category of plants. The Cactaceae, however, are easily identifiable thanks to a specific element, which is not the presence of thorns, since many cacti are without them and many other plants, even non-succulent (think of roses), have thorns. First of all, what distinguishes a cactus from any other succulent plant is the absence of leaves. With the evolution, the cacti have eliminated the leaves, organs through which the transpiration of liquids occurs very quickly. For this reason, to adapt to arid climates, cacti have developed to accumulate water inside the stem or in the roots, avoiding to disperse liquids through the leaves.

Pereskia in fiore
Pereskia in bloom

The only exception is Pereskia and Pereskiopsis, genera considered the link between succulent plants and cacti, a sort of “ancestors” of cacti. According to the italian expert Giuseppe Lodi, the Pereskia “are not real succulent plants” (G. Lodi, “Le mie piante grasse”, Edagricole), even if most researchers include them in the Cactaceae family. In fact, Pereskia and Pereskiopsis are particular plants, more similar to shrubs than to cacti: they have fleshy central branches and large, flat, non-fleshy, bright green leaves.

What makes these two genera included in the Cactaceae family is the presence, along the central branches, of the “areole”, the real common denominator of all cacti. The areoles are fundamental “joints” present on the stem of all the Cactaceae: from here the thorns develop and the flowers sprout. Areoles can be found along the coasts that divide the stem or on top of small protuberances (called tubercles), always around the stem, but in cacti will never lack and it’s the first useful element to distinguish any succulent from a real cactus.

Globosa
Cactus: globose form

Learning to distinguish the various genera of the Cactaceae family is the first step towards the identification of the plant. First of all, it is useful to know the forms that can assume the cactus: it goes from the classic globular shape, that is spherical, to the columnar one, which over time can branch and produce the characteristic “arms”. However, it should be considered that many columnar cacti can emit branches only after many years and, as in the case of the famous saguaro (the Carnegiea gigantea, plant symbol of Arizona), it happens only in nature and not in pot cultivation.

Colonnare ramificato
Columnar cactus

So be careful not to confuse the columnar cacti with some Euphorbia, which can have an elongated bearing with ramifications from the main bodies (for example Euphorbia enopla and Euphorbia resinifera). To distinguish at first sight a cactus from a Euphorbia you have to observe the blooms: the Euphorbias do not produce real flowers but tiny inflorescences, usually gathered in clusters.

Among the other forms that cacti can assume there is the caespitose one, with a central body surrounded by suckers, then the leafy one, that is with flat and fleshy stems (as for Epiphyllum), the prostrate one, with elongated and drooping stems, like snakes (for example, Aporocactus).

Finally, the typical shape of the Opuntias: with the characteristic “blades” that develop from the central body to form large clusters (this is the case of the common prickly pear). A genus similar to the Opuntia is Tephrocactus, which produces short cylindrical or almost spherical stems, often covered with long thin thorns and almost paper-like consistency.

Opuntia
The shape of Opuntia

To give a name to the genus of the various Cactaceae, you need patience and experience: with the time you will be able at first glance to frame the genus to which belongs any cactus.

For those who have little familiarity with taxonomy, it will undoubtedly be even more complicated to identify the species, but even this, over time and with research and comparisons with photographs and cards on books, is not an impossible task. A little effort and observation skills are certainly needed, but overall we can say that the primary taxonomy is within everyone’s reach.

Some kinds of Cactaceae more easily recognisable have some characteristics of the trunk or the blooms in common. The cacti that produce small flowers arranged in a crown around the apex of the plant, for example, belong to the genus Mammillaria. Globose plants with wide and flat thorns, very robust, hooked, usually of a colour between red and yellow (with the older ones tending to brown, greyish), can belong to the genus Ferocactus. Globose plants with strong but not hooked thorns can instead belong to the genus Echinocactus.

Ricandente
Drooping posture

Plants with nocturnal blooms, with big flowers with a long stem, are almost all included in the genus Echinopsis, while small plants, usually accessed and with very showy blooms, with flowers with medium stem length, can belong to the genus Rebutia and Sulcorebutia. Globose cacti, more or less thorny, with flower calyxes without thorns or awn, belong to the genus Gymnocalycium, while small or medium-sized cacti with flat stems, triangular tubercles and fleshy and taproot root, belong to the genus Ariocarpus.

In short, it’s requested a lot of experience with great patience, and if some doubts remain, in particular on the genus of Cactaceae to be identified, you need to wait for the flowering, which is always very useful in the classification of plants. It is also important to remember that sometimes the codification can be even more difficult because of the abnormal growth of the Cactaceae. In these cases, we speak of “monstrosity”, including phenomena such as variegation or fasciation.

Euphorbiaceae

Another large family of succulents is the Euphorbiaceae or Euphorbia. The classification of these plants, living in all continents (unlike the Cactaceae, which come only from the Americas), is very difficult for the less experienced.

Euphorbia
Euphorbia

Euphorbie’s bearing is in fact very variable, and you can go from the common Euphorbia obesa to Euphorbia milii: the first one with a perfectly globular and thornless stem, which over time tends to stretch until it becomes short-cylindrical; the second one, a thorny shrub with thin branches covered with thorns.

The Euphorbias do not produce real flowers, but small inflorescences that can be of various colours, from yellow to red, to pink. In some cases they have small leaves sprouting from branched stems, in other cases, the leaves are bigger and similar to those of many other plants (e.g. the ordinary “poinsettia“, which is a Euphorbia, although not many are aware of it). Many Euphorbias have short, pointed and thin thorns, while others are without thorns. The stem can be thick and fleshy, as in Euphorbia resinifera, or formed by thin branches that are not fleshy at all.

Agavaceae
Agave
Agave, typical shape

Even the plants belonging to the Agavaceae family are very ordinary and widespread. The identification of plants included in this family is quite simple, at least if you limit yourself to the genus. The Agavaceae have a very characteristic bearing, with rosettes of leaves that almost always end with hard, pointed thorns. The “leaves” of the Agave can reach a considerable size and are invariably fleshy, while the colour can vary from dark green to bright green, up to the greyish and yellow streaked green of the classic American Agave.

Initially, it can be difficult to distinguish the Agave from some species belonging to the Aloaceae family. Some Aloe, in fact, may resemble the Agave and have triangular-shaped leaves with a fleshy texture. Also, in these cases, it’s very decisive to have experience and closely observe the plant: almost all Aloe have in fact a central branch from which the leaves branch off. In the Agave this central branch is absent, and the leaves are arranged in a rosette.

The flowering of the Agave is very impressive and take place at the end of the life cycle of the plant: from the base of the central rose comes out a stem that can reach two or three meters in height and have a diameter of 10-15 centimetres, from which the inflorescences branch off.

Crassulaceae
Crassula
Crassula ovata

They are small or medium-sized succulent plants and have a bearing that can range from the rosette of the very common Sempervivum, very robust and widespread plants, to the small shrub with fleshy leaves of Crassula ovata. These plants have thick, fleshy leaves and are thornless. The family of Crassulaceae includes many genera, some of which are particularly widespread and appreciated, such as Aeonium, Kalanchoe, Echeveria, Sempervivum, Crassula. The flowers are generally small in size and sprout, for example in the case of Sempervivum, from long branches that develop between the basal leaves.

Asclepiadaceae

The Asclepiadaceae’s one is a composite family, which includes succulent and non-succulent genres. Overall, there are thousands of species that can be traced back to this family. The succulent Asclepiadaceae is very variable, although it is usually caespitose, with short, fleshy and branched stems. The genera Stapelia, Caralluma, Huernia, Hoodia, Piaranthus are very ordinary.

Stapelia
Stapelia with flower

These plants, with stems always succulent and in some cases with short and harmless thorns, have a peculiarity that makes them easy to identify in the family of Asclepiadaceae: their flowers give off an ugly smell. They are in fact designed to attract flies, which pollinate them by laying eggs inside them. The flowers’ odour of many Asclepiadaceae remembers rotting flesh, which is why it mainly attracts flies. As for colour and size, the flowers can be variable, going from the big star-shaped flowers of Stapelia Grandiflora or Stapelia gigantea to those, always star-shaped but dark red, of Stapelia hirsuta. Other Asclepiadaceae produce small flowers with a smooth appearance and bright colours (e.g. Huernia zebrina).

Aizoaceae (and ex Mesembrianthemaceae)
Lithops
Lithops

This is a family with many species, almost all from the African Continent. Aizoaceae are all succulent and nearly always small in size. They belong to this family, in which for some years researchers have also traced what was once the family of Mesembrianthemaceae, genera that are very widespread in cultivation, such as the ordinary and appreciated Lithops (also called “stone plants” or “living rocks”, because of their peculiar look), then Conophytum, Pleiospilos, Faucaria, Carpobrotus. The bearing of Aizoaceae is highly variable, but usually, they are plants with fleshy stems that form small clusters. The blooms are small and medium in size and in many cases, in cultivation in our hemisphere, the flowers sprout in the winter months.

The graphic elaborations of this page are by Alessandra: @alle.grafica

You may also be interested in “How to repot cactus“, or “When and how fertilize cactus and succulent plants“.

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