The Cactaceae — the cactus family — are the iconic succulents of the Americas: spiny, drought-defying, and so distinctive in form that the word “cactus” has become a universal shorthand for toughness and self-sufficiency. With approximately 155 accepted genera and 1,400–1,800 species (depending on the taxonomic authority followed), this is one of the most recognisable plant families on Earth, yet one of the most taxonomically turbulent — a family where molecular phylogenetics has upended decades of morphology-based classification, where an “extraordinarily high proportion” of genera have been shown to be non-monophyletic, and where the internal structure continues to be revised with each new study.
For growers and collectors, the Cactaceae offer an almost infinite palette: golden barrels that sit in the desert sun for centuries, night-blooming columns whose flowers last only hours, trailing forest cacti that drape from tropical branches, pincushion-sized miniatures that flower at thumbnail size, and towering saguaros that define the American Southwest. This guide provides a comprehensive overview of the family’s biology, its current classification, and the major genera of interest to cultivators.
Defining the Cactaceae — The Areole
What makes a cactus a cactus? One structure, found in no other plant family: the areole. An areole is a small, cushion-like, often woolly or hairy structure on the surface of the stem from which spines, flowers, new branches, and (in some species) roots emerge. Areoles are modified, highly condensed lateral shoots — they are homologous to the axillary buds of ordinary plants, but so radically transformed that they are unique to the Cactaceae.
The areole is the single reliable diagnostic character of the family. Any plant with areoles is a cactus. Any plant without them — regardless of how spiny or cactus-like it appears — is not. This is why succulent euphorbias, which mimic cacti through convergent evolution but lack areoles, are immediately excluded. The distinction is detailed in our article Cactus or Euphorbia? How to Tell Them Apart.
Classification — A Family in Flux
Historical overview
The Cactaceae were placed in the order Caryophyllales by APG (1998, 2003, 2016) — a surprising position alongside the pinks (Caryophyllaceae), beets (Amaranthaceae), and ice plants (Aizoaceae), but strongly supported by molecular data. Within the Caryophyllales, the closest relatives of the cacti are the Portulacaceae and related families in the “succulent clade” identified by Manhart & Rettig (1994).
The internal classification of the Cactaceae has been notoriously unstable. The International Cactaceae Systematics Group (ICSG), set up in 1984, produced a consensus classification used by Anderson (2001) that recognised four subfamilies: Pereskioideae, Opuntioideae, Maihuenioideae, and Cactoideae. Molecular studies by Nyffeler (2002), Bárcenas et al. (2011), and Hernández-Hernández et al. (2011) broadly supported this framework but revealed that a very high proportion of genera and tribes were not monophyletic.
The 2025 phylogenomic revolution
A landmark 2025 study using the Angiosperms353 phylogenomic markers — sampling 170 species covering nearly 90% of genera — produced the most comprehensive molecular phylogeny of Cactaceae to date. This study, using coalescent-based gene tree–species tree reconciliation, formally recognises six subfamilies (not four), accepting 155 genera:
- Pereskioideae — leafy, scarcely succulent shrubs and trees; the ancestral grade of Cactaceae
- Leuenbergerioideae — newly segregated from Pereskioideae; monogeneric (Leuenbergeria)
- Maihuenioideae — monogeneric (Maihuenia); low, cushion-forming plants from Patagonia
- Blossfeldioideae — newly segregated; monogeneric (Blossfeldia); the smallest cactus in the world
- Opuntioideae — the prickly pears and chollas; 3 tribes
- Cactoideae — the core cacti; by far the largest subfamily; 8 tribes
Four of the six subfamilies are monogeneric (containing a single genus each), reflecting the pattern seen across the angiosperms of species-poor “orphan lineages” scattered among explosively diverse clades.
The Six Subfamilies
Subfamily Pereskioideae — the leafy ancestors
Pereskia (sensu stricto) comprises a handful of species of leafy, scarcely succulent shrubs and small trees from tropical America. They retain broad, functional leaves — an ancestral trait lost by all other cacti — and have areoles bearing spines, confirming their placement in the Cactaceae despite their un-cactus-like appearance. Molecular data show that Pereskia in the traditional sense is paraphyletic (it does not include all descendants of a common ancestor), which led to the segregation of Leuenbergeria. Pereskia sensu stricto represents the ancestral grade of the family — the plants closest to what the earliest cacti looked like before the evolution of stem succulence.
Subfamily Leuenbergerioideae
A monogeneric subfamily segregated from the former Pereskioideae. Leuenbergeria comprises a few species of leafy, spiny trees from the Caribbean and tropical South America. Like Pereskia, they retain functional leaves, but molecular data place them as a distinct lineage from Pereskia sensu stricto.
Subfamily Maihuenioideae — the Patagonian cushions
A monogeneric subfamily containing only Maihuenia — a low, dense, cushion-forming plant from southern Argentina and Chile (Patagonia) that looks more like a moss cushion than a cactus. Maihuenia retains small, cylindrical, persistent leaves — a feature shared with the ancestral Pereskioideae but absent from all other cacti. It is ecologically specialised and extremely frost-hardy, surviving the harsh, wind-swept conditions of the Patagonian steppe. Its isolated phylogenetic position and unique combination of characters justify its placement in its own subfamily.
Subfamily Blossfeldioideae — the smallest cactus
A newly recognised monogeneric subfamily containing only Blossfeldia — a tiny, spineless, globular cactus from the Andes of Argentina and Bolivia that holds the record as the smallest cactus in the world (mature plants can be under 2 cm in diameter). Blossfeldia is remarkable for several additional features: it is poikilohydric (can lose and regain almost all its body water — a trait shared with resurrection plants), retains an rpoC1 intron that all other Cactoideae have lost, and lacks the cortical vascular bundles present in Cactoideae. Molecular data consistently place it as sister to all other Cactoideae, and its unique biological features support its recognition as a distinct subfamily rather than a basal member of Cactoideae.
Subfamily Opuntioideae — the prickly pears and chollas
Approximately 350 species in three tribes. Characterised by two synapomorphies: glochids (tiny, barbed, easily detached spines in the areoles that embed in skin and are notoriously difficult to remove) and bony seed arils. Growth forms include flat-padded prickly pears, cylindrical chollas, and low, cushion-forming tephrocacti.
Tribes and key genera:
- Tribe Opuntieae — Opuntia (prickly pears — the largest genus in the subfamily; flat pads; edible fruits in many species; extremely frost-hardy forms exist), Consolea, Brasiliopuntia, Nopalea
- Tribe Cylindropuntieae — Cylindropuntia (chollas — cylindrical stems with ferocious barbed spines), Grusonia
- Tribe Pterocacteae — Pterocactus (tuberous-rooted, sprawling cacti from Patagonia)
The Opuntioideae are ecologically and economically important: Opuntia ficus-indica (Indian fig) is cultivated worldwide for its edible fruits and pads; prickly pears are invasive weeds in Australia, South Africa, and the Mediterranean (controlled by the biological agent Cactoblastis cactorum); and several Opuntia species are among the most cold-hardy cacti known, surviving to –25 °C.
Subfamily Cactoideae — the core cacti
Approximately 1,000–1,200 species, comprising the vast majority of cultivated cacti. Characterised by the loss of leaves (with rare exceptions), the loss of the rpoC1 intron, and distinctive seed morphology. Eight tribes are recognised in the 2025 phylogenomic framework:
- Tribe Cacteae — the barrel cacti, globular cacti, and mammillarias of North America. The most species-rich tribe. Key genera: Mammillaria (~200 species — the largest genus of cacti, pincushion cacti with spiral tubercles), Echinocactus (golden barrel — E. grusonii is the most widely cultivated barrel cactus), Ferocactus (fishhook barrels), Ariocarpus (living rocks — extraordinary mimicry of limestone), Lophophora (peyote — psychoactive), Astrophytum (bishop’s cap — star-shaped, often spineless), Turbinicarpus, Thelocactus, Coryphantha
- Tribe Cereeae — the columnar cacti of South America. Key genera: Cereus (type genus of the family, tall columns), Pilosocereus (blue-stemmed columns), Gymnocalycium (globular, popular in cultivation — includes the grafting rootstock G. mihanovichii ‘Hibotan’), Rebutia (small, profusely flowering), Echinopsis (including the former Trichocereus — large-flowered, San Pedro cactus)
- Tribe Phyllocacteae — the epiphytic and semi-epiphytic cacti. Key genera: Epiphyllum (orchid cacti — spectacular nocturnal flowers), Hylocereus (dragon fruit / pitaya — major tropical fruit crop), Selenicereus (queen of the night — S. grandiflorus), Echinocereus (hedgehog cacti — cold-hardy, beautiful flowers), Disocactus
- Tribe Rhipsalideae — the mistletoe cacti. Epiphytic, spineless, trailing forest cacti. Rhipsalis (the only cactus genus native to the Old World — R. baccifera occurs in tropical Africa, Madagascar, and Sri Lanka), Hatiora, Schlumbergera (Christmas cactus / Thanksgiving cactus — one of the most commercially important flowering houseplants), Lepismium
- Tribe Notocacteae — South American globulars and columns. Parodia (including the former Notocactus — popular, easy-flowering beginner cacti), Eriosyce, Copiapoa (extreme-desert Chilean cacti — fog-dependent)
- Tribe Lymanbensonieae — a small tribe of columnar cacti
- Tribe Copiapoeae — Copiapoa and relatives from the Atacama Desert
- Tribe Fraileae — Frailea (tiny, self-pollinating South American cacti)
Diversity of Forms
Globular and barrel cacti. The classic “cactus” form: spherical to barrel-shaped, ribbed, heavily spined. Echinocactus grusonii, Ferocactus, Mammillaria, Astrophytum, Gymnocalycium, Parodia. Primarily Cacteae (North America) and Notocacteae (South America).
Columnar cacti. Tall, candelabra-like columns that define desert and arid-landscape silhouettes: Carnegiea gigantea (saguaro — up to 15 m, the symbol of the Sonoran Desert), Pachycereus pringlei (cardón — the world’s largest cactus by mass), Cereus, Pilosocereus, Echinopsis/Trichocereus.
Flat-padded cacti. Opuntia and relatives. Flattened stem segments (cladodes) that give the plant a segmented, paddle-like appearance. Includes the edible Opuntia ficus-indica.
Cylindrical chollas. Cylindropuntia. Cylindrical, heavily armed segments that detach easily and cling to passing animals — one of the most effective passive dispersal mechanisms in the plant kingdom.
Epiphytic forest cacti. Rhipsalis, Schlumbergera, Epiphyllum, Hatiora. Spineless or nearly spineless, leaf-like or trailing stems growing on tree branches in tropical and subtropical forests. These are the “non-cactus-looking cacti” that often surprise people by being true members of the family.
Miniature and button cacti. Blossfeldia (under 2 cm), Frailea, Turbinicarpus, Ariocarpus, Lophophora. Tiny, slow-growing, often mimetic plants prized by specialist collectors.
Geography — An Almost Exclusively American Family
The Cactaceae are native to the Americas, from southern Canada (49°N — Opuntia fragilis) to Patagonia (50°S — Maihuenia), with centres of diversity in Mexico (the global hotspot, with over 500 species), the Andes, the Brazilian caatinga, and the arid southwestern United States.
There is exactly one exception: Rhipsalis baccifera, the mistletoe cactus, which occurs naturally in tropical Africa, Madagascar, and Sri Lanka — the only cactus native to the Old World. Its presence is attributed to ancient long-distance dispersal by birds across the Atlantic, not to a pre-continental-drift distribution.
Outside their native range, cacti have been introduced and naturalised worldwide. Opuntia species are invasive in Australia, South Africa, the Mediterranean basin, and parts of Asia. Hylocereus (dragon fruit) is cultivated commercially across Southeast Asia. Schlumbergera (Christmas cactus) is produced in the hundreds of millions as a global flowering houseplant.
Biology — Key Adaptations
CAM photosynthesis. Nearly all cacti use Crassulacean Acid Metabolism: stomata open at night to absorb CO₂ (stored as malic acid), then close during the day while photosynthesis proceeds using the stored carbon. This reduces water loss by up to 90% compared to conventional C₃ photosynthesis.
Stem succulence. Water is stored in the cortex and pith of the stem, which serves as both the photosynthetic organ and the water reservoir. The expandable rib structure allows the stem to swell when water is available and contract during drought.
Spines. Modified leaves that provide shade (reducing stem temperature), dew collection (fog drip along spines into the root zone), and herbivore deterrence. Spine morphology — shape, number, colour, density — is one of the primary identification characters.
Shallow, widespread root systems. Most cacti have extensive but shallow roots that spread far beyond the plant’s canopy, designed to capture rain from brief desert showers before it evaporates.
Economic and Cultural Importance
Edible fruits and pads. Opuntia ficus-indica (prickly pear / tuna) and Hylocereus (dragon fruit / pitahaya) are significant fruit crops. Nopal (prickly pear pads) is a staple vegetable in Mexican cuisine.
Ornamental horticulture. Cacti are a global industry: Schlumbergera (Christmas/Thanksgiving cactus), Gymnocalycium grafts (‘Hibotan’ coloured cacti), Echinocactus grusonii (golden barrel), and the entire collector market for rare species.
Cultural symbolism. The saguaro (Carnegiea gigantea) is a protected symbol of the American Southwest. The eagle-on-cactus motif appears on the Mexican national flag and coat of arms (depicting an eagle perched on an Opuntia). Peyote (Lophophora williamsii) has been used in indigenous spiritual practices for millennia.
Conservation. Approximately 31% of cactus species are threatened with extinction — one of the highest proportions of any major plant family. Illegal collection, habitat destruction, and climate change are the primary threats. All Cactaceae are listed under CITES Appendix II, with the most threatened species under Appendix I.
Frequently Asked Questions
How many species of cactus are there?
Approximately 1,400–1,800 species in roughly 155 genera, depending on the taxonomic authority. The family is under continuous revision: molecular studies have shown that many traditional genera are not monophyletic, so the exact count shifts with each new classification. Mexico is the global centre of diversity, with over 500 species.
Are all cacti from deserts?
No. While the desert-dwelling barrel and columnar cacti are the most iconic, a significant proportion of species grow in tropical and subtropical forests as epiphytes — plants that grow on tree branches, not in soil. The Christmas cactus (Schlumbergera), orchid cacti (Epiphyllum), dragon fruit (Hylocereus), and mistletoe cacti (Rhipsalis) are all forest cacti that prefer shade, humidity, and regular moisture — the opposite of desert conditions.
What is the difference between a cactus and a succulent?
All cacti are succulents (plants with thickened, water-storing tissues), but not all succulents are cacti. “Succulent” is an informal, ecological term that spans many plant families. “Cactus” refers specifically to members of the family Cactaceae, characterised by the presence of areoles — the woolly, cushion-like structures from which spines emerge. Many non-cactus succulents (particularly euphorbias) resemble cacti closely, but they lack areoles and belong to unrelated families.
Are any cacti native to Africa or Europe?
Only one species: Rhipsalis baccifera, the mistletoe cactus, occurs naturally in tropical Africa, Madagascar, and Sri Lanka — the sole cactus native to the Old World. All other cacti are native to the Americas. The spiny, cactus-like plants found in African deserts (such as Euphorbia ingens, Euphorbia horrida, or Euphorbia virosa) are euphorbias, not cacti — a spectacular example of convergent evolution between two unrelated families.
Sources and Further Reading
- Plants of the World Online (POWO), Royal Botanic Gardens, Kew — Cactaceae. powo.science.kew.org
- Majure, L.C. et al. (2025). Phylogenomics and classification of Cactaceae based on hundreds of nuclear genes. Plant Systematics and Evolution. [Angiosperms353 phylogenomic study recognising 6 subfamilies and 155 genera]
- Anderson, E.F. (2001). The Cactus Family. Timber Press, Portland. [ICSG consensus classification]
- Nyffeler, R. & Eggli, U. (2010). A farewell to dated ideas and concepts — molecular phylogenetics and a revised suprageneric classification of the family Cactaceae. Schumannia, 6: 109–149.
- Bárcenas, R.T. et al. (2011). Molecular systematics of the Cactaceae. Cladistics, 27(5): 470–489.
- Nyffeler, R. (2002). Phylogenetic relationships in the cactus family (Cactaceae) based on evidence from trnK/matK and trnL-trnF sequences. American Journal of Botany, 89(2): 312–326.
- Hunt, D., Taylor, N. & Charles, G. (2006). The New Cactus Lexicon. DH Books, Milborne Port.
- Hernández-Hernández, T. et al. (2011). Phylogenetic relationships and evolution of growth form in Cactaceae. American Journal of Botany, 98(1): 44–61.
- CITES — Convention on International Trade in Endangered Species. Cactaceae listing. cites.org