Since 1920, San Pedro (a Trichocereus) has been published under 4 botanical names. Each one reflects a different view of where the plant belongs: in Trichocereus, in Cereus, in Echinopsis, or as a variety of a related species. A genus is a group of related species, and the first word of a botanical name is the genus.
The short version
- San Pedro's names, in order: Trichocereus pachanoi (1920), Cereus pachanoi (1931), Echinopsis pachanoi (1974) and Trichocereus macrogonus var. pachanoi (2012). All 4 refer to the same plant.
- The name Trichocereus is tied to one species, Cereus macrogonus, described in 1850 from a garden plant whose description gave no flowers and no place of origin.
- Alwin Berger made Trichocereus a subgenus (a division of a genus) of Cereus in 1905. Riccobono made it a genus of its own in 1909.
- Britton and Rose gave Trichocereus 19 species in 1920 and named San Pedro from a plant collected in Ecuador in 1918.
- In 1974 Friedrich merged Trichocereus into Echinopsis on the strength of 2 flower features. That merger produced the name Echinopsis pachanoi.
- In 2012 Albesiano and Kiesling chose a wild plant collected in Peru as the reference specimen for Cereus macrogonus, and made San Pedro a variety of that species.
- Kew's Plants of the World Online lists San Pedro as Echinopsis pachanoi and puts some southern species in a third genus, Leucostele. Other botanists keep Trichocereus. For which name to use on a label, see Trichocereus vs Echinopsis names.
A plant with no address
In 1753 Linnaeus put every cactus he knew into a single genus, Cactus. In 1754 Philip Miller published Cereus, and several South American columns were first described in it. One was Cereus terscheckii in 1837, the Argentine saguaro, a tall column of Argentina and Bolivia.
In 1850 Joseph zu Salm-Reifferscheid-Dyck, known as Salm-Dyck, published a catalogue of the cacti growing in his garden at Dyck in 1849. In it he described Cereus macrogonus. The original description (the protologue) covers the stem in some detail but says nothing about the flowers or where the plant came from. Under habitat it gives only "Hab.?". The name had been in use at the Berlin botanical garden, and the plant probably reached Salm-Dyck from there.
That species later became the type of Trichocereus. A type is the specimen (or, for a genus, the species) that a name is tied to, so that later botanists can check what the name means. Because Trichocereus rests on Cereus macrogonus, deciding what Cereus macrogonus is also decides what Trichocereus means.
Berger and Riccobono make a genus
In 1905 Alwin Berger, in the annual report of the Missouri Botanical Garden, grouped 14 species as Cereus subgenus Trichocereus. He gave the origin of Cereus macrogonus as the Andes, with a question mark to show he was not sure of it.
In 1909 Riccobono, in the bulletin of the Palermo botanical garden, raised Trichocereus to a genus. He put only 2 species in it: Trichocereus macrogonus and Trichocereus spachianus.
Britton and Rose and the field collectors
In 1912 J. N. Rose left the United States National Museum for the Carnegie Institution of Washington to write a monograph (a full account of a plant group) of the world's cacti with N. L. Britton. Three partners paid for it: the Carnegie Institution, the New York Botanical Garden and the USDA. The pair had already named the saguaro's genus Carnegiea in 1908, after Andrew Carnegie.
Two of Rose's South American collections became the types of familiar plants. On 9 July 1914 he and Mrs. Rose collected a column near Matucana, Peru, at about 2,100 m (about 6,900 feet). It became Trichocereus peruvianus, the plant sold as Peruvian torch. In September 1918 Rose collected another on the outskirts of Cuenca, Ecuador, with A. Pachano and George Rose. It became Trichocereus pachanoi, San Pedro.
The Cactaceae came out in 4 volumes from 1919 to 1923. Volume 2, in 1920, gave Trichocereus 19 species and a key, a step-by-step set of choices for telling the species apart. It named Cereus macrogonus as the type species of the genus and described both San Pedro and Trichocereus peruvianus.
Splitters and lumpers
Botanists who draw narrow boundaries and recognize many small groups are often called splitters, and those who draw broad ones lumpers.
Curt Backeberg split the group. In 1941 he reported finding Trichocereus macrogonus growing wild in Peru, which gave the garden plant a reported wild home. In 1949 he moved the species with colorful flowers that open by day into a new genus, Helianthocereus, and left the white, night-flowering species in Trichocereus. In 1953 he named another genus, Leucostele.
In 1974 Friedrich lumped. He merged Trichocereus and several other genera, among them Lobivia, into Echinopsis. He based the merger on 2 flower features: hairs in the axils of the scales on the flower tube (the angle where each scale meets the tube), and stamens arranged in 2 groups. He and Rowley published the new names, such as Echinopsis pachanoi.
A number of later botanists did not accept the merger. In 1978 Roberto Kiesling described the Argentine species under Trichocereus, gave a key to them, and treated Helianthocereus as no more than a subgenus. In 1981 Ritter treated Peruvian torch as a form (a minor variant) of San Pedro, after seeing wild populations that looked intermediate between the 2. Others followed Friedrich: Anderson in 2001 and Hunt and colleagues in 2006 both used the broad Echinopsis in their accounts of the cactus family.
Neotypes, DNA and today's names
In 2012 Sofía Albesiano and Roberto Kiesling fixed what Cereus macrogonus means. They chose a neotype, a specimen selected to serve as the type when no original material exists. D. Smith collected it on 1 October 1982 between Acobamba and Palca, in the department of Junín, Peru, at 2,890 m (about 9,500 feet). It agrees with Salm-Dyck's description apart from having fewer spines. In the same paper they made San Pedro and Peruvian torch 2 varieties (named forms within a species) of one species: Trichocereus macrogonus var. pachanoi and var. macrogonus.
The same year, Schlumpberger moved several southern species into Backeberg's Leucostele, among them Leucostele terscheckii, the Argentine saguaro, and the Chilean Leucostele chiloensis. In 2015 Albesiano's doctoral thesis at the Universidad Nacional de La Plata recognized 58 taxa, or named groups (38 species plus subspecies, varieties and forms), chose 52 neotypes, and used chloroplast DNA to argue that Trichocereus and Echinopsis should stay separate.
Both names remain in use. The Caryophyllales.org checklist of the cactus family, published in 2021, keeps Trichocereus as a genus. A 2025 classification of the family built from more than 300 nuclear genes (genes in the cell nucleus) keeps Echinopsis and Leucostele but not Trichocereus. Kew's Plants of the World Online follows that classification, so San Pedro is Echinopsis pachanoi there and the Argentine saguaro is Leucostele terscheckii. For each species' current name and native range, see Trichocereus species and where they grow.
Into gardens and hybrids
In Peru, Ecuador and Bolivia, San Pedro and Peruvian torch are common garden and fence plants, and planting has carried them well past the areas where they grow wild. Garden plants and wild plants can differ in look, so a garden San Pedro may not match a wild one exactly, and long cultivation makes the wild origin of garden plants hard to trace. Trichocereus spachianus, one of Riccobono's 2 original species, has spread outside its native range in the Canary Islands, Spain, Sicily, South Africa, Lesotho and northwest Mexico.
Breeders have added hybrids. The torch cacti are Trichocereus hybrids with big flowers in many colors, bred mostly from Argentine species. Across the wider Echinopsis group, hybridizers have made thousands of crosses.
Our Trichocereus are grown and sold strictly as ornamental specimens.
Sources
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Identity and neotypification of Cereus macrogonus, the type species of the genus Trichocereus (Cactaceae). Sofía Albesiano and Roberto Kiesling, Haseltonia 17: 24-34, 2012. https://cactusconservation.org/wp-content/uploads/2020/03/Albesiano_Kiesling_2012_Haseltonia_17_24-34.pdf
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About The Hybrids. ISI Schick Hybrids, The Huntington. https://media.huntington.org/Schick/abouthybrids.html