The International Cactaceae Systematics Group (ICSG) of the International Organization for Succulent Plant Study recognises four Subfamilies and two of those are subdivided into a number of Tribes. Within this framework are placed the 1,400-1,500 species and subspecies grouped into 125-130 genera. It is at the genus and species level that things get a little chaotic.
You see many ‘varieties’ of cactus species for sale as plants and seeds but nowadays these clonal variations are generally not recognized so don’t be tempted to buy them without checking on The Plant List or another reliable resource. In botany, variety ranks below subspecies and is used to recognize a distinct feature found throughout the species, such as v albiflora or v roseiflora for white and pink flowers, respectively. Distinctive features that are restricted to specimens from a geographical area may set those plants apart as a subspecies but more often than not these differences are not sufficiently significant to merit this. If you eyes have not already glazed over you can read more about it on Wikipedia.
Many cactus species have been in cultivation for decades and a significant number may have unwittingly been hybridized. A species with a field number (the geographic reference of the person who collected the plant or seed) may be more reliable but even some of those have been passed between collectors since the 1970s.
My Flickr albums on the Cactaceae are based on the current classification, with a focus on several genera where I have quite a few species in my collection.
Let’s get the two smallest subfamilies out of the way quickly.
MAIHUENIOIDEAE
This subfamily only has two species (Maihuenia patagonica, M. poeppegii), of which I have both but none of my plants has flowered yet. DNA analysis has confirmed that this genus is not closely related to any other cactus genera and they are probably closest to the original plant from which cacti evolved. Neither is an easy plant to grow and while I have raised patagonica from seed, they are slow growing and still very small.
There is only one photograph – below – no album.
PERESKIOIDEAE
I used to have a thing about the genus Pereskia (the only genus in this subfamily) but I have gone off them in recent years. There are around 17 species and they are something of a curiosity in looking superficially like rose bushes, complete with thorn-like structures and flowers reminiscent of simple roses. I gave up growing them when I found out the P. aculeata that I had been allowing to scramble up the inside of my greenhouse was unlikely to flower as it has to get pretty large and will then only flowers on new growth. There’s one in Edinburgh Botanical Garden that flowers but it’s hard to see them way up in the roof of the glasshouse.
There are several smaller species that flower, including the two in this album. They are deciduous in the UK when you stop watering them. I have tried keeping them going throughout the winter with gentle watering by they don’t seem to like it.
If you want to learn more about pereskias, read this article on Wikipedia.
OPUNTIOIDEAE
This is a large subfamily of segmented cacti, some of which can grow exceedingly large but many remain small and compact. They are generally not child-friendly cacti, and the moniker of ‘bunny ears’ given to one of them does not imply that they are the slightest bit cuddly!
Many cactus growers don’t like opuntias as they are generally very spiny, often with barbed spines, and some have fine hair-like needles called glochids which are unpleasantly irritating and hard to remove from the skin. The pads or segments have a habit of dropping after they have impaled their spines in your skin or attached themselves to your clothing, but many will fall off without any encouragement, particularly when the plant is dry in the winter. (It is a method of vegetative propagation in the wild.) For this reason it’s difficult to grow a plant of any size and you end up with lots of unwanted cuttings. But don’t me me put you off trying . . .
The genus Pterocactus is worth growing as they are generally harmless and are interesting in appearance. Some species grow from an underground caudex and are thought to be deciduous in the wild, losing their aerial parts in the dry season and regrowing when the rains return.
CACTOIDEAE
Around 80% of cactus species belong in this subfamily and it is by far the most diverse in terms of the appearance, ranging from epiphytic (forest-dwelling) cacti to some of the slowest growing, rarest and largest ones.
These are the cacti which most people grow but there are many species, not all of which are easy to cultivate, and only a few make good windowsill plants. To accompany the albums there is a little information about each genus or group of genera. There is a mixed bag of genera at the end.
Ariocarpus
This really is a genus for the more experienced grower, and definitely greenhouse cacti. They are slow growing, taking many years to get to a reasonable size, and although you can buy more mature specimens from nurseries but be prepared to pay a high price. There are 6 species and a further 8 subspecies, although you often see varieties and forms sold as being different.
Astrophytum
There is probably no cactus genus that has been hybridized as much as this one simply for its appearance rather than flowers. Despite there being only 5 species and a further 6 subspecies (some of which are genetic variants) there is a big industry in creating ever-more bizarre-looking hybrids. The interesting surface flocking (trichomes, or hairs) and interestingly shaped bodies, coupled with the ease of cross-pollination and ready germination of seed, has made them ideal candidates for this.
I have bought a few over the years and am currently growing some hybrids from seed out of curiosity. Despite their popularity they are not always easy plants for the beginner and are sensitive to over-watering and can rot easily.
They need careful watering but, with the right attention will flower freely. All the flowers are yellow, sometimes with a red throat, but that is a variable feature.
Copiapoa
This is a genus of around 40 species and subspecies, many of which look rather similar and all have the same yellow, slightly ragged flowers. They come from northern Chile and their restricted geographical area may account for their lack of variation. That said, there are a few really striking species. All are slow growing but do well in cultivation and are generally easy.
Echinocereus
I have to confess that I’m not a huge fan of this genus and don’t have many. Despite the beautiful flowers, the plants often look rather scrappy. However, I’m in a minority as they are very popular, being quite small and easy to grow. You will see from this album that the flowers more than make up for their general appearance.
Echinopsis
This genus is another taxonomic minefield, with the related genera Lobivia (small and clumping) and Trichocereus (columnar) now usually lumped in here. The funnel-shaped flowers of Echinopsis in particular are stunning and only let down by the fact that they appear for a night and a day and then they are gone.
There are many Echinopsis hybrids – the so-called Schick hybrids are collected by many people – as well as Lobivia. Here the emphasis is on the flowers, many of which are beautiful.
Epiphytic (forest) cacti
Species in this group have evolved to live in places that are moist but where there is little available water. Despite the name, not all epiphytic cacti grow on trees. A number of lithophytic; that is, they grow in rock crevices. The familiar Christmas/Easter cactus (different species) have been hybridized extensively, as has the genus Epiphyllum. Along with related genera such as Rhipsalis, Disocactus and Selenicereus, they make up an interesting group but are something of an acquire taste in the cactus world.
Most growers have some but few specialise in them. The flat-stemmed, spineless, forest-dwelling, dangly species need less light and can do well indoor so long as they are not blasted by central heating. The spiny-but-safe genera such as Selenicereus and Pfeiffera may dangle or scramble, depending on species.
The flowers vary from tiny in many Rhipsalis species, to the world’s biggest cactus flower – Selenicereus grandiflorus (see album).
Eriosyce (including Islaya, Neochilenia, Neoporteria)
This group of related cacti has undergone considerable revision in recent years, to the extent that all are now subsumed into the one genus: Eriosyce. That is not to say you won’t see the other genera used extensively so you need to be careful when buying a plant you don’t think you already have.
Take a look at what CactiGuide has to say on the matter as Daiv explains it better than I can. I have grown a number of what were said to be Islaya species over the years but they are all regarded as Eriosyce islayensis, but I have labelled the photographs with the names I bought them as.
Gymnocalycium
A popular genus of smaller-growing, mostly easy, and free-flowering cacti often found for sale in garden centres. The unique feature of the genus is the scales on the flower stalks, but at the species level things become more difficult. I recommend buying Graham Charles’ book Gymnocalycium in Habitat and Culture if you get serious about them. He lists only about 50-60 species and subspecies, and no varieties.
Here is a test flipbook:
Mammillaria
This is the largest cactus genus with over 200 species and subspecies. Over the years a few species have been put into other genera, such as Mammillopsis, Mammilloydia, Bartschella, Cochemiea and Solisia, but all are now back into the one genus according to current opinion. The genus has been divided into six Subgenera but over 90% of species sit in just one of those. Under taxonomic rules this largest Subgenus has to be called (confusingly) Mammillaria because that’s where sits the ‘type species’ (the first one named when the genus was created: M. mammillaris). This Subgenus is further broken down into four Sections which in turn are further split into a number of Series (both are standard taxonomic terms).
While you may think this unnecessary, the desire to make sense of the world around us is no less relevant in the wonderful world of cacti.
The genus is rightly popular because most are easy to grow and flower well. The majority have the typical ring of flowers around the top of the plant but a number have much larger flowers in smaller numbers. The key taxonomic feature of the genus is that all species have prominent tubercles, between which the flowers emerge. Some species are solitary, others clump, some are spherical, others are columnar and a few hang down. However, species in this genus are always identifiable from their general appearance and are a definite must for the novice collector.
Parodia, Notocactus, Wigginsia
There is some controversy about the recent lumping of all species into the genus Parodia because there are some distinctive features that link some of the previously genera. For example, Wigginsia species are heavily ribbed. However, DNA analysis shows them to be closely related. A distinctive (but not unique) feature are the furry flower bud, the protruding stigma (often in a contrasting colour), and the furry and sometimes spiny fruit. The genus is not too difficult to spot but the flowers help to confirm this. Most are either yellow or red.
Rebutia, Sulcorebutia, Aylostera, Weingartia
This is another confusing group of cacti which, in turn are related to Lobivia and Echinopsis (which I have grouped separately). The UK National Collection is held by a private collector in Scotland who has their own view on the classification so I have left the naming of my plants more or less as it was when I obtained them.
They are free-flowering, colourful and easy cacti to grow and will delight you with their lovely flowers. They come from the Andes and are pretty tolerant of lower temperatures. Most will clump but they are magnets for red spider mite for some reason, so follow my tips on preventing this.
Thelocactus
There is little controversy over the taxonomy of this small genus. I’m not a ‘set’ collector but I do have nearly all of them, not a difficult task when there are only about 12 species and 14 or so subspecies. They flower readily from the crown and the flowers (from white to yellow to pink) are comparatively large and may start appearing in some species within three years from seed (at the same size as adult plants, so they look huge).
Turbinicarpus
These small, free-flowering and interesting looking cacti are rightly popular with collectors. A few are very distinctive but many look superficially similar which make identification a challenge, even for experts. There are currently around 24 species and about as many subspecies. One species in particular – T. schmeidickeanus – has over 10 subspecies. Nevertheless, this is a genus worth growing; they are easy, if somewhat slow, from seed. The genus Gymnocactus is now subsumed into this genus.
Other genera in the Cactoideae
There are many other genera worth considering when building a collection. Some have only a few species, other have many but take up a lot of space (either horizontally or vertically) and are therefore unpopular, while a minority are rare and difficult to grow and much sought-after, despite the risk and the cost.
The best place to look for a fairly definitive list of the genera is CactiGuide. Daiv is very much a supporter of the current thinking on classification which is not to split species into separate genera unnecessarily (disparagingly called lumpers by the splitters).