Cacti

notes

llm

plants

science

Building beautiful cactus gardens.

Author

Stephen J. Mildenhall

Published

2025-08-19

Modified

2025-08-20

My first two cactus gardens, August 2025.

cactus (n) pl. cacti: Any of numerous succulent plants constituting the family Cactaceae, native to arid regions of the Americas, typically lacking leaves and having thick fleshy stems, spines, and large, brightly coloured flowers. Frequently with distinguishing word.

Linnaeus included all cacti in a single genus, Cactus. This is now divided into over 120 genera; cf. Melocactus n., Opuntia n., etc. Many species are cultivated as garden plants or houseplants.

Cactus Gardens

Coming soon.

About Cacti

Appearance -> Genus

Visual appearance	Likely genera
Flat pads (segmented)	Opuntia
Flattened leaf-like stems, hanging	Schlumbergera
Columnar, hairy “old man” look	Oreocereus
Columnar, tall, spiny “toothpick”	Stetsonia
Small globular, tuberculate (warts instead of ribs)	Mammillaria, Rebutia
Small globular, ribbed	Gymnocalycium, Parodia, Astrophytum, Echinopsis
Star-shaped ribs with white speckles	Astrophytum
Rounded/short columnar, ribbed, plain green	Echinopsis
Small clustering globes, brilliant blooms	Rebutia, Parodia

Genus -> Appearance

Genus	Visual appearance
Astrophytum	Star-shaped ribs, smooth, often white-speckled
Echinopsis	Rounded to short columnar, ribbed, plain green
Gymnocalycium	Small globes, ribbed with curved spines, neat crown flowers
Mammillaria	Tiny spheres/cylinders, tubercles, flower rings
Opuntia	Flat pads or segments, often with glochids
Oreocereus	Columnar, shaggy white hair (“old man”)
Parodia	Small globes, ribbed, bright crown flowers
Rebutia	Tiny clustering balls, covered with bright blooms
Schlumbergera	Flattened, jointed stems, hanging habit
Stetsonia	Tall columnar, heavy spines (“toothpick”)

CACTACEAE - Cactus family

These perennial evergreen trees, shrubs and vines can be diverse in shape or form. Some are leafy, but more frequently they are simple or branched leafless stem-succulents. These plants have stems that are columnar or globular and are solitary or in clumps, erect to scrambling or climbing. They are terrestrial, epipetric or epiphytic and are in the last case often pendent. Their roots are diffuse, sometimes with taproots or tubers. Stems remain green or become brown and woody with age and can be unsegmented or segmented, and segments often fall and make roots. Shoots or segments are spherical or club- shaped to cylindrical, sometimes flattened into cladodes. They can be smooth or tuberculate, the tubercles distinct and nipple-shaped or ridge-like protuberances or fused into vertical ribs. These ridges or ribs, when present, two to 30 or more; short shoots (called ‘areoles’) are positioned on the crests of these ribs or tubercles, usually bearing persistent spines and often also minute, barbed, deciduous spines (called ‘glochids’). The spirally arranged, alternate leaves are deciduous to persistent, vestigial or absent; when present they are usually sessile, but can be petiolate in some leafy genera such as Pereskia. They are flat or terete and lack stipules. Spines can be flexible and hair- or bristle-like to rigid and needle- or nail-like, terete to angled or flat. The bisexual (rarely unisexual), actinomorphic or sometimes zygomorphic flowers are formed one (to several) per areole, arranged in some leafy genera into panicles or cymes. The receptacle of the flower encloses and protrudes beyond the ovary, forming a floral tube (hypanthium). The five to 50 (or more) tepals intergrade gradually from bract-like or sepal-like outer tepals to petal-like inner tepals. Stamens are usually numerous (more than 50), fused with the inner surface of the flower tube. Anthers are bilocular and open by longitudinal slits. The inferior (or half- inferior) ovary can be covered with tubercles, areoles, spines and glochids and often has scales that are derived from tepals. The ovary is composed of up to 20 fused carpels forming a unilocular pistil with a single style and with a nectary around the base of the style which carries a lobed stigma (each lobe representing a carpel). Fruits are usually succulent many-seeded berries, but some can split.

Distribution

The family is widespread across the New World, from Canada throughout the USA and Mexico, Central America, the West Indies, the Galápagos Islands and South America. A single species, Rhipsalis baccifera occurs in tropical Africa, Madagascar, the Mascarenes, the Seychelles and Sri Lanka. Cactaceae are widely naturalised in other parts of the Old World tropics and subtropics.

Phylogeny and evolution

Cactaceae sit within the order Caryophyllales, alongside carnations, amaranths, and beets—an odd group, but united by molecular evidence. Succulence itself evolved convergently in many plant families (aloes in Africa, euphorbias, agaves), but “true” cactuses are strictly American.

Cactaceae are most closely related to parts of polyphyletic Portulacaceae s.l., and the latter have been dismembered to prevent the two from being merged. They share several characters, such as axillary hairs and indefinite numbers of stamens in bunches, that unite the group. The family might perhaps better be expanded to include the similar and closely related Anacampserotaceae, Portulacaceae and Talinaceae, although the botanical community appears to have little appetite for an expansion of this charismatic family. Traditionally, Cactaceae have been subdivided into four subfamilies, but at least Pereskioideae have been found to be polyphyletic, the genus Rhodocactus alone being sister to all other Cactaceae. Radiation of Cactaceae took place predominantly during the Miocene and Pliocene, and was most significant c. 8.5 million years ago. Generic delimitation is not always clear, especially as hybridisation is common in some groups. Their generic taxonomy is inflated due to familiarity in horticulture resulting in an emphasis on only a few minor characteristics.

They are a relatively young plant family in evolutionary terms, arising around 30–35 million years ago after the uplift of the Andes and the spread of more arid habitats. Cactuses represent an extreme form of adaptation to drought: their leaves are reduced to spines, photosynthesis is carried out in the stems (CAM photosynthesis, which minimizes water loss), and they store water in succulent tissues. They range from tiny button-sized plants to tree-like giants (Carnegiea gigantea, the saguaro).

CAM photosynthesis

CAM stands for Crassulacean Acid Metabolism, first described in the Crassula family of succulents but now known across many drought-adapted plants, especially cactuses. Unlike normal C3 photosynthesis, where stomata open during the day to take in CO₂, CAM plants open their stomata at night when it is cooler and more humid. The CO₂ is stored as malic acid in cell vacuoles and then released internally during the day when the stomata remain closed, allowing photosynthesis to proceed without excessive water loss.

This adaptation makes CAM plants much more water-efficient, though slower-growing, which explains the success of cactuses in deserts. The pathway is also found in agaves, orchids, bromeliads, and euphorbias, having evolved multiple times independently in different plant groups.

Genera and species

This is a family of c. 94 genera and approximately 1,150 species.

Acanthocereus (1)
Ariocarpus (6)
Armatocereus (6)
Arrojadoa (4)
Arthrocereus (4)
Astrophytum (6)
Austrocactus (3)
Aztekium (2)
Bergerocactus (1)
Blossfeldia (1)
Brachycereus (1)
Brasilicereus (2)
Browningia (8)
Calymmanthium (1)
Carnegiea (1)
Cephalocereus (3)
Cereus (20)
Cipocereus (5)
Cleistocactus (32)
Coleocephalocereus (6)
Copiapoa (19)
Corryocactus (5)
Coryphantha (48)
Denmoza (1)
Disocactus (10)
Disocactus (11)
Echinocactus (5)
Echinocereus (c. 55)
Echinopsis (c. 65)
Epiphyllum (12)
Epithelantha (2)
Eriosyce (30)

Escontria (1)
Espostoa (9)
Espostoopsis (1)
Eulychnia (4)
Facheiroa (3)
Ferocactus (25)
Frailea (9)
Gymnocalycium (43)
Haageocereus (19)
Harrisia (7)
Hatiora (5)
Hylocereus (13)
Jasminocereus (1)
Leocereus (1)
Lepismium (15)
Leptocereus (4)
Leuchtenbergia (1)
Lophophora (3)
Maihuenia (2)
Mammillaria (135)
Mammilloydia (1)
Melocactus (33)
Micranthocereus (9)
Mila (1)
Myrtillocactus (4)
Neobuxbaumia (8)
Neolloydia (2)
Neoraimondia (2)
Neowerdermannia (2)
Obregonia (1)
Opuntia (c. 110)
Oreocereus (6)

Ortegocactus (1)
Pachycereus (12)
Parodia (c. 50)
Pediocactus (6)
Pelecyphora (2)
Peniocereus (15)
Pereskia (17)
Pereskiopsis (6)
Polaskia (2)
Pseudoacanthocereus (2)
Pseudorhipsalis (6)
Pterocactus (9)
Quiabentia (2)
Rebutia (26)
Rhipsalis (35)
Rhodocactus (1)
Samaipaticereus (1)
Schlumbergera (6)
Sclerocactus (17)
Selenicereus (11)
Stenocactus (7)
Stenocereus (22)
Stephanocereus (2)
Stetsonia (1)
Strombocactus (1)
Tacinga (6)
Thelocactus (11)
Uebelmannia (3)
Weberbauerocereus (5)
Weberocereus (8)

Uses

The berries of many species are edible, although some are covered in spines that have to be removed before consumption. Economically, the most important is Opuntia ficus-indica, of which the fruits (prickly pear or cactus fruit) are commonly found in fruit stalls around the world. Dragon fruit or pitaya (Hylocereus undatus), although of Mexican origin, is now mostly cultivated in Southeast Asia and exported to European and North American fruit stalls. It is used as an offering in Chinese temples and has a sweet but bland taste. Barbados gooseberry (Pereskia acutifolia) is cultivated for its edible fruits and leaves. The flat stems of nopal (spineless forms of O. ficus-indica and other species) are considered a delicacy in Mexico and are eaten cooked or pickled as a vegetable (nopalitos). Similarly, the despined young stems of cadushi (Cereus repandus) are eaten in Curaçao. The pulp of the stem of some Echinocactus species is used in confectionery. Cochineal is produced from dried mealy bugs that live on Opuntia cochenillifera; they are used to make carmine, a scarlet dye that is now synthetically produced. The jumping cholla (Opuntia fulgida) is known in the southwestern USA and northern Mexico for its hard to remove stems (which the plant uses for vegetative reproduction); it is used to produce cholla gum, which has similar applications as gum arabic (Senegalia, Fabaceae). Some species of Cactaceae contain mescaline, which has hallucinogenic properties; these include Echinopsis pachanoi and especially Lophophora williamsii (peyote), the use of which can lead to colourful visions and a feeling of weightlessness. Many cactus species are cultivated as ornamentals, some specialists growing nothing else. Some species are popular houseplants; especially common are Christmas and Easter cacti (cultivars of Schlumbergera ×buckleyi), and ‘epiphyllums’ (Disocactus ×hybridus). Cactaceae are valuable garden plants for arid regions around the world, although some naturalise easily and can cause problems in nature conservation. International trade is regulated by CITES. Turks Island of Turks and Caicos received its name because of the abundant Melocactus intortus stands there that resemble Turk’s caps. Becoming 20 m tall, the saguaro (Carnegiea gigantea) is the largest member of the family. One of the smallest is Blossfeldia liliputana, a resurrection plant that is only a few millimetres across.

Etymology

Cactus is the Latinised form of the Greek κάκτος (kaktos), a name originally used for the cardoon (Cynara cardunculus, Asteraceae), a vegetable commonly eaten in the eastern Mediterranean. Linnaeus placed all known cacti in one genus, Cactus, complicating the nomenclature of Cactaceae because he even included a leafless orchid (Cactus parasiticus = Dendrophylax funalis, Orchidaceae). For nomenclatural stability, the generic name Cactus was rejected against Mammillaria and is now considered its synonym.

Source: Plants of the World, Kew Publishing (2017) plus some GPT5.

Common Cactus Genera for Houseplants

Astrophytum (star cacti)

Overview: Distinctive, geometric ribbed bodies often with white speckles.
Size: 5–15 cm diameter.
Flowers: Large, silky yellow flowers.
Range: Mexico and Texas.
Watering: Very sensitive to overwatering; thrives with heat and strong light.
Notes: Iconic “bishop’s cap” and “star cactus” species are sought-after.

Echinopsis (hedgehog or sea-urchin cacti)

Overview: Ribbed, rounded to columnar cacti.
Size: Small forms 8–12 cm; some species grow larger.
Flowers: Very large, spectacular white or colored blooms, often night-opening and strongly scented.
Range: South America, especially Argentina and Bolivia.
Watering: Standard cactus care; needs full sun for flowering.
Notes: Hybridized extensively, producing stunning flowers.

Gymnocalycium (chin cactuses)

Overview: Globular cacti with distinctive ribbed bodies and curved spines.
Size: Usually 5–12 cm across, stays compact.
Flowers: Funnel-shaped, pink, white, or red, appearing at the crown.
Range: Argentina, Paraguay, Bolivia.
Watering: More tolerant of regular watering than many cacti; prefers some shade.
Notes: Famous for brightly colored mutants grafted onto Hylocereus rootstock.

Mammillaria

Overview: One of the largest cactus genera (200+ species), known for small, spherical or cylindrical plants with tubercles instead of ribs.
Size: Typically 5–15 cm across, ideal for pots.
Flowers: Rings of small pink, yellow, or white flowers; often prolific in spring.
Range: Mexico and the southwest USA.
Watering: Needs dry winters, moderate summer watering; very rot-prone.
Notes: Extremely popular as they flower freely even when small.

Opuntia (prickly pears, bunny ears)

Overview: Segmented, pad-forming cacti.
Size: Can be large, but small species (O. microdasys) are pot-suitable.
Flowers: Yellow, red, or orange; followed by edible fruits.
Range: Broad—from Canada to Patagonia.
Watering: Drought tolerant; needs very sharp drainage.
Notes: Glochids (tiny barbed hairs) can be very irritating.

Oreocereus

Overview: Columnar, slow-growing cacti often covered with long white hairs, giving a “old man of the Andes” look.
Size: Can reach 2–3 m in habitat; in pots usually under 50 cm.
Flowers: Tubular, red to orange, appearing near the stem tips.
Range: High Andes of Peru, Bolivia, Chile, and Argentina.
Watering: Needs very free drainage and a dry, cool winter rest; tolerates some cold if kept dry.
Notes: Attractive for their shaggy appearance rather than abundant flowering.

Parodia (fka Notocactus)

Overview: Globular, ribbed cacti with cheerful blooms.
Size: 5–12 cm, stays manageable.
Flowers: Yellow, orange, or red, usually at the crown.
Range: Brazil, Uruguay, Argentina.
Watering: Fairly forgiving; tolerate more humidity than most cacti.
Notes: Excellent beginner’s genus. Originally, Notocactus included the South American globular species with cheerful yellow or red flowers. Taxonomists later concluded that they were not distinct enough from Parodia, so the genera were united, with Parodia having priority as the older name.

Rebutia (including Sulcorebutia)

Overview: Small, clustering globular cacti.
Size: Individual heads 2–5 cm; form dense clumps.
Flowers: Brilliant red, orange, or yellow flowers, often covering the plant in spring.
Range: Andes of Bolivia and Argentina, high altitude.
Watering: Like cool nights, regular summer water, dry winters.
Notes: Easy to flower, very popular in collections.

Schlumbergera (Christmas/Easter cacti)

Overview: Epiphytic forest cacti with flattened leaf-like stems.
Size: Hanging/branching habit, great in pots or baskets.
Flowers: Tubular, brightly colored, in winter (Christmas cactus) or spring (Easter cactus).
Range: Coastal Brazil, in rainforests.
Watering: Unlike desert cacti, need more regular moisture and humidity; never allow to bone-dry for long.
Notes: Among the few true “indoor cacti.”

Stetsonia

Overview: Tall, branching columnar cactus known as the “toothpick cactus,” with strong spines.
Size: In habitat up to 10 m tall; in pots usually stays under 1 m.
Flowers: Large white nocturnal flowers, sometimes followed by edible fruits.
Range: Dry Chaco of Argentina, Bolivia, Paraguay.
Watering: Standard desert cactus care; water moderately in summer, dry in winter.
Notes: More suited to greenhouse or outdoor planting in warm climates; impressive but less common as a houseplant.

General Cultivation Notes

Light: Most desert cacti need bright, direct sun; forest cacti like Schlumbergera prefer bright shade.
Watering: “Soak and dry” rule—water thoroughly, then let the soil dry completely before watering again. Drier in winter when growth slows.
Soil: Very free-draining cactus mix (often with added grit/pumice).
Temperature: Most tolerate cool nights but not frost; ideal is 10–25 °C.
Flowering: Often triggered by seasonal cues (short days, cool nights, dry rest period).

Summary

Cactus overview by genus.
Genus	Overview / Appearance	Size (pot/habitat)	Flowers	Range	Watering & Notes
Astrophytum	Geometric, ribbed, often white-speckled “star”	5–15 cm	Large silky yellow	Mexico, Texas	Very rot-sensitive; likes heat and sun.
Echinopsis	Ribbed, rounded to columnar	8–12 cm / larger	Huge, scented, white or colored, night-opening	Argentina, Bolivia	Needs sun for flowering; widely hybridized.
Gymnocalycium	Globular, ribbed with curved spines	5–12 cm	Funnel-shaped, pink, white, red	Argentina, Paraguay, Bolivia	Tolerates more water; prefers some shade.
Mammillaria	Small, spherical/cylindrical, tuberculate	5–15 cm	Rings of small pink/yellow/white	Mexico, SW USA	Needs dry winter; flowers when small.
Opuntia	Pad-forming, segmented	Small to very large	Yellow, red, orange; edible fruits	Canada to Patagonia	Very drought-tolerant; beware glochids.
Oreocereus	Columnar, shaggy with white hairs	<50 cm / 2–3 m	Tubular, red to orange	Andes (Peru, Bolivia, Chile, Argentina)	Needs drainage, dry cool winter; grown for appearance.
Parodia	Globular, ribbed	5–12 cm	Yellow, orange, red at crown	Brazil, Uruguay, Argentina	Forgiving, good beginner genus. Formerly Notocactus.
Rebutia	Tiny, clustering globes	2–5 cm / clumps	Brilliant red/orange/yellow carpets	Andes (Bolivia, Argentina)	Likes cool nights; easy to flower.
Schlumbergera	Epiphytic, leaf-like stems	Hanging/branching	Tubular, bright (winter/spring)	Brazilian rainforests	Needs humidity; never bone-dry. True indoor cactus.
Stetsonia	Tall, branching “toothpick cactus”	<1 m / 10 m	Large white night blooms; edible fruits	Dry Chaco (Argentina, Bolivia, Paraguay)	Impressive but greenhouse-suited; less common indoors.

Glossary

actinomorphic: Radially symmetrical; a flower that can be divided into equal halves along multiple planes (like a daisy).
areoles: Specialized cushion-like structures unique to cacti, from which spines, glochids, branches, and flowers arise.
bilocular: Having two compartments (locules), usually referring to an ovary or anther.
cladodes: Flattened, photosynthetic stem segments that resemble leaves (e.g., in Opuntia).
cymes: Inflorescences in which the central (terminal) flower opens first, followed by side flowers.
epipetric: Growing on rocks.
epiphytic: Growing on other plants (usually trees) for support, but not parasitic; common in rainforest cacti like Schlumbergera.
glochids: Tiny, barbed bristles found in the areoles of some cacti (especially Opuntia), easily detached and irritating to skin.
hypanthium: A floral cup or tube formed by the fusion of the bases of sepals, petals, and stamens, often surrounding the ovary.
intergrade: To blend gradually between two forms; a population showing traits intermediate between two species or varieties.
nectary: A gland that secretes nectar, attracting pollinators.
panicles: Branched inflorescences in which the flowers are borne on multiple levels of stalks (a loose, irregular cluster).
petiolate: Having a petiole (a stalk attaching a leaf blade to the stem).
sessile: Lacking a stalk; attached directly without a petiole or pedicel.
tepals: Undifferentiated petals and sepals that look alike (as in many monocots and cacti).
terete: Circular in cross-section; cylindrical.
teretestipules: Stipules (small leaf-like structures at the base of a petiole) that are terete, i.e., slender and cylindrical.
tuberculate: Bearing tubercles—small, wart-like projections, often seen on Mammillaria.
unilocular: Having a single compartment (locule), usually in an ovary or anther.
zygomorphic: Bilaterally symmetrical; a flower divisible into mirror halves along only one plane (like an orchid).