Recently, I showed a friend a small cactus with a flower on it and she asked, "What sort of flower is it?" I explained that I don't stick flowers onto my cacti and this was the flower of the cactus. Cacti are angiosperms, flowering plants. They all flower, except for a few screwed up ones. A mature cactus in good conditions should flower every year (perhaps I should explain that I'm only talking about real cacti, the family Cactaceae, not things like Agaves). Some will have a spectacular display for a brief time while others will have a few flowers for much of the year. Therefore it's easy to have flowering cacti, you just have to have mature plants under fairly good conditions. There are many cacti that never grow very big so it's easy to have a mature plant. You've probably noticed that florists and garden centres often have a number of small, roundish cacti with flowers (not stuck on ones). This article is going to be about these small, mostly round, terrestrial (i.e. ones that grow on the ground in deserts or semi-deserts rather than on trees in rain forests) cacti of the sub-family Cactoideae (not prickly pears, chollas etc).
There are some general advantages of this group. They mostly have the fleshy stems and spines you expect of cacti. They're fairly tolerant of neglect and dry, bright conditions. You can get of lot of them into a small space, a windowsill, small greenhouse or a small garden or balcony in a warm climate (a few of them are hardy in very cold climates). In a warm climate they can be used as small, flowering perennials. There are a large number to collect (many people specialize in collecting this group or even particular genera and they normally comprise a large proportion of a general cactus collection). You can often grow flowering size plants from seeds in a couple of years. They have lots of pretty flowers!
I'm going to be using a lot of Latin names as most of these plants don't have common names, you can just Google them for pictures and more information. The first word in the name is the genus and most plants with this first name should be similar. The genus is often abbreviated to the first letter, as long it's obvious.
Generally these plants grow better if they have a cool (between about 0 and 5C) winter rest and they don't like growing all year in a centrally heated house or a tropical climate but there are some exceptions.
It's often said that the North American species are more interesting plants while the South American ones have nicer flowers (the only cacti thought to naturally occur outside the Americas are epiphytes).
I'll start in North America with the huge and varied genus Echinocereus. These typically have sausage shaped stems rather than being strictly globular, and typically they have large magenta flowers with green stigmas. Many are tolerant of low temperatures so they're good plants for greenhouses and gardens that experience moderate frosts (the hardiest species are probably E. viridiflorus and E. triglochidatus). Some can grow into huge clumps. Some of the more interesting species are: E. reichenbachii with pectinate spines which have different coloured bands: E. knipellianus, 6 ribbed, globular, weakly spined stems; E. ochoterenae/subinermis (very similar) with globular grey stems and yellow flowers; E. pentalophus with sprawling stems. The subgenus Wilcoxia (sometimes classified as a different genus) has long thin stems and tuberous roots. E. scirus is probably the best bet for tropical conditions.
The other main, mostly north American genus is Mammillaria. Many people specialize in this genus. These typically have small (about 1cm across) flowers in rings round the top. The distinctive feature is the large tubercles (bump like projections from the stem). The flowers are produced from between the tubercles, rather than from the spine-baring areol at the tubercle tip. Otherwise they're very varied, from minute to over 1M high, with a number of cylindrical stemmed species. Unfortunately the larger flowered species (e.g. M. saboea and M. gluezowiana) tend to be hard to grow. Recommended species: M. boccasana and M. prolifera, long white hair and nearly indestructible; M. zeilmanniana, lots of magenta flowers; M. plumosa, fluffy white feathery spines; M. elongata, long stems with yellowish spines. Actually you're really spoilt for choice here. Exceptions to the large flowered species being difficult to grow rule are the subgenus Dolichothele (sometimes classified as a different genus) that have big yellow flowers. Probably the best bets for tropical conditions are M. nivosa (one of the few South American species) or the sub-genus Cochemia.
There are several genera related to Mammillaria, including Coryphantha and Escorbaria. These differ in having flowers from near the growing tip and groves (and sometimes nectar secreting glands) on the tubercals. This group includes what is probably the hardiest globular cactus, Escorbaria vivipara the only globular cactus found in Canada!
Pediocactus isn't the easiest genus to grow but they survive cold conditions. I've had P. simpsonii growing in my garden in Calgary, Canada for several years. It's survived while Echinocereus viriflorus and some supposedly hardy Opuntia haven't.
The remaining North American ones tend to be odd but slow growing and probably not the best things to start with. Echinofossulocactus (aka Stenocactus) have many narrow wavy ribs and stripy flowers. There are several species of Thelocactus with strong, colourful spines and beautiful flowers in a variety of colours. Leuchtenbergia principis has long, leaf-life tubercals and long papery spines. It also has large yellow flowers. Astrophytum, with few ribs and white flecks in addition to the ariols (the newly discovered A. caput-medusae has long, thin tubercals). Ariocarpus are spineless, with large, leaf-like tubercals, which make them look more like Sempervivums (house leeks) than cacti. Lophophora looks like a hot cross bun and has small white or pink flowers. Obregonia looks like a globe artichoke. Pelecyphora strombiliformis looks like a pinecone and P. asselliformis looks as it's covered in wood lice. Turbinicarpus (which now includes Gymnocactus and Neolloydia) are tiny cacti, mostly with few curved spines and greyish stems. The former Gymnocactus and Neolloydia species are bigger and spinier. Then there are the rugged looking and weekly spined Aztekium, Geohintonia and Strombocactus. If you want to grow this group, it's probably best to start with Echinofossulocactus, Thelocactus, Leuchtenbergia, Astrophytum myriostigma, Turbinicarpus, Lophophora (illegal in the USA because it's hallucinogenic) and Obregonia.
Now for South America! The Trichocerinae is a large group of mostly easily cultivated cacti but its classification is a mess. Echinopsis used to be a genus of cantalope-sized cacti with huge white or pink flowers that opened at night and stayed open for two days. E. eyesii (short spines and white flowers) and E. multiplex (longer spines and pink flowers) were the ones granny grew on her English cottage windowsill. My favourites are the smaller species: E. ancistrophora, E. seminuda, E. cardensiana and E. obrepanda, which are sometimes classified as Pseudolobivia. Also in Pseydolobivia is E. kermessina with yellow spines and large magenta flowers. Pushing the limits of plants we're talking about here are E. hauscha, E. grandiflora and a few similar plants with clumps of stems up to about 1m high and huge white or coloured flowers. These are variously classified as Trichocereus, Helianthocereus and Lobivia. Unfortunately Echinopsis is now often considered to include the tall cacti formally called Trichocereus and small plants with coloured flowers, usually called Lobivia. Therefore you really have to check the species when buying them.
E. mirabilis is often classified in its own genus, Setiechinopsis. This is an oddball with brown, cylindrical stems to about 20cm high and white, nocturnal flowers.
Lobivia was a genus of mostly smaller plants with brightly coloured flowers that opened in the day but they're now mostly included in Echinopsis today. E. densispina and E. winteriana have pectinate spines and straight ribs. Other species like E. pentlandii have spiralled ribs, curved spines and off centre ariols, giving them a trough, rouged look. Apart from the species sometimes classified as Trichocereus or Sohrensia (which are rather large) you can just collect loads of these.
With clumps of short, creeping, cylindrical stems and red flowers, Chameacereus silvestrii (a.k.a Lobvia silvestrii or Echinopsis chameacereus) is one of the easiest cacti to grow. It's one of several odd members of this group.
There are several groups of hybrid Echinopsis. Chamaebivia (Chameacereus X Lobivia), with sausage like stems, Paramount hybrids (Echinopsis X Lobivia) and Schlick hybrids (mostly with Helianthocereus in them), generally rather large. They all have large flowers.
Mediolobivia was a genus of plants that were small even for Lobivias with flowers near the base of the stems. They're variously classified as Lobivia, Echinopsis or Rebutia. The best-known species is M, L, E. or R. pygmea, which has short cylindrical, brownish stems with pectinate spines and flowers in a variety of colours.
Rebutia (which is variously considered to include Aylosteria, Mediolobivia and Sulcorebutia), are very small cacti, normally forming clumps of globular stems. The flowers are produced from near the bases of the stems and are normally brightly coloured. Some of the more noteworthy species are: R. senilis with long white spines and red flowers; R. albiflora with very small, prolificially clustering stems and white flowers with pink blotches and the beautiful R. heliosa with silvery pectinate spines and orange flowers. Just go mad and get loads of them!
Sulcorebutia is similar to Rebutia but have elongated areols with pectinate spines and a slightly different flower structure. They're all worth growing!
Weingartia are a bit bigger with yellow flowers in rings round the top. There isn't much to choose between the species.
Gymnocalycium is sometimes considered to include Weingartia. They're typically wider than high with a few wide ribs and white or pink flowers from the top. G. brushii is the smallest species and looks more like a Sulcorebutia except that it's pink flowers are produced from the top. G. baldianum has pink or dark red flowers. In garden centres you will often see three sided green cacti with round, bright red tops. These are grafted cacti. The stock (bottom part) is Hylocereus (which is very sensitive to cold) and the scion (top part) is a chlorotic (lacking chlorophyll) cultivar of Gymnocalycium freidrichii, which can't photosynthesize so it has to be grafted. The normal G. freidrichii is purplish brown and grows perfectly well on its own roots. Due to their small size, you can afford to collect lots of Gymnocalyciums.
The tribe Notocactinae is also a bit of a mess with regard to classification. The genus Notocactus are typically small, easily grown cacti with large yellow flowers with red stigmas but they're now sometimes lumped into Parodia. N. uebelmannianus is noteworthy for it's purple flowers. N. leninhausii has clumps cylindrical stems, with oddly sloping tops, to about 1m high, covered in yellow spines. N. magnifica has grey stems with yellow spines and also grows quiet large.
Parodia is more varied and generally harder to grow than Notocactus but most have beautiful flowers.
Frailea are very small cacti, which tend to produce viable seeds without opening their flowers. F. pygmea is round with weak, pectinate spines and F. magnifica looks like a very small N. leninghausii.
Chile is home to two genera (some would say more) of interesting, globular cacti. Copiapoa are varied from very small to bowling ball sized (sometimes forming clumps) and they all have yellow flowers. It's probably best to get the smaller species like C. hypogea or C. humilis.
Neoporteria is very varied, often with curved spines and brownish stems. It used to include plants with smallish flowers with somewhat incurved petals but now it includes a number of plants with larger pastel coloured flowers (formally included in Neochilenia). There are a lot of interesting plants to collect in this genus but some can get large and they're a bit more temperamental than the Trichocerinae.
Matucana is an odd genus, sometimes included in Borzicactus (mostly large, cylindrical cacti). They have tubular, zygomorphic (lop sided, a bit like a Christmas cactus) flowers, which look really wrong on small, round cacti. M. roseo-alba has white spines and pastel pink flowers. M. madisonorum is the only one you seem to see in garden centres. It has grey stems with few ribs and sometimes has three spines to each areol, baring a strong resemblance to Lophophora. Its flowers are red (but there's a white flowered form).
Blossfeldia are the smallest of all cacti (although Frailea and Turbinicarpus come close). They are spineless with tuberous roots and a tendency to produce seeds without opening their whitish flowers. Not the easiest cacti to grow!
I'll end with some plants for warm climates or centrally heated homes. Melocactus are cantaloupe-sized cacti with straight ribs. As small plants they look like pretty typical cacti but then they grow a very hairy and spiny bit on the top (called a terminal cephalium) from which they produce their small flowers. Discocactus are smaller cephalium-producing cacti with large white flowers that open at night. Buiningia are similar to Melocactus but have lateral cephalia (on the side of the stem). Uebelmania are weird looking and have yellow flowers.
I hope this inspires you to get some of these interesting and often beautiful plants. You can normally find some in garden centres and most will grow to flowering size in a few years from seeds.