Two cuttings of San Pedro (a Trichocereus) can look like different plants: one nearly smooth, the other spiny. Collectors look at shape, ribs, skin color, spines and flowers, at unusual growth forms, and at where a plant came from.
The tall columns include San Pedro (Trichocereus pachanoi), Peruvian torch (Trichocereus peruvianus) and Bolivian torch (Trichocereus bridgesii); Trichocereus species gives each its own entry. The torch hybrids are low, clumping plants sold under Trichocereus and Echinopsis names, bred mostly from Argentine species.
The short version
- Shape runs from slim columns to thick ones, a trunked tree and low clumps.
- Ribs are the long ridges down the stem. Areoles are the small felted pads that spines grow from. Rib count and the pattern of areoles help tell plants apart.
- San Pedro and Peruvian torch have glaucous skin, meaning a pale blue-gray bloom over the green. Color tends to show best on new growth and may shift when a plant is moved.
- Spine number and length change with light and growing conditions, so spines alone do not identify a plant.
- The torch hybrids are best known for large flowers in many colors. Their seedlings can differ from the parent, so a named hybrid is kept going by cuttings.
- Crested plants grow as a flat fan, monstrose plants as a knobby mass of small stems, and variegated plants with pale patches. All 3 are kept by cuttings or grafts. Crested and variegated plants can revert, meaning they go back to normal growth, and monstrose plants may too.
- A cutting is a copy of its parent. A seedling is a new plant. If you want a particular plant, buy a cutting and ask what plant it came from.
Shape and size
San Pedro has stems about 2.5 to 4.5 inches across. Peruvian torch is stouter, with stems about 6 to 8 inches across. Both grow into tall columns that often branch from the base.
At the large end, the Argentine saguaro (Trichocereus terscheckii) becomes a tree 20 to 50 feet tall, with distinct trunks and several upright branches. At the small end, the torch hybrids stay low: their stems reach about 2 feet, then lean over and spread.
Ribs and areoles
San Pedro and Peruvian torch usually have 6 to 9 broad, rounded ribs, occasionally as few as 4. Bolivian torch tends to have fewer, often 4 to 6, with sharper edges. The Argentine saguaro has 8 to 18.
A young plant often does not yet show the ribs and areoles it will have when mature, so compare plants of a similar size.
Skin color and bloom
The blue in a stem comes from the glaucous bloom. Peruvian torch is known for its glaucous stems. Rubbed-off bloom does not come back on that spot, so fingerprints stay; new growth comes in with its own. Handle blue stems as little as possible.
The bloom tends to show best on new growth. Color may shift after a plant moves to more or less light, so judge a plant's color by the new growth it makes in its new spot.
Spines, and why they change
Peruvian torch can carry 18 to 20 spines on an older areole, 3 or 4 of them stronger and about 2 inches long. San Pedro often has no spines on its older areoles, or 3 to 7 under an inch long. Bolivian torch tends to have fewer spines per areole, but long ones. Scopulicola (Trichocereus scopulicola) tends to be nearly spineless, which makes it easy to handle.
Spine number and length change with light. On a single branch of a garden plant, one areole may have no spines and another up to 10, and wild plants are spinier than cultivated ones. A plant moved from shade into a more exposed spot may grow spinier new sections. If your plant is spinier or smoother than its description, look at how much light it has had before you doubt the label.
Flowers and the torch hybrids
The tall columns can flower once mature, but the torch hybrids are best known for their flowers. They come in many bright colors, and one named hybrid, 'Flying Saucer', has flowers 8 inches across or more. A big plant can open 50 or more flowers over 2 or 3 days, then a few at a time on another 5 to 30 days of the year, depending on the hybrid. To flower, they need at least 4 to 5 hours of direct sun a day and enough size (cactus flowering).
Named torch hybrids are cultivars: plants chosen and named by a breeder, written in single quotes. Breeders have made thousands of these crosses, sold under both Echinopsis and Trichocereus names, so 2 plants can look alike without being the same hybrid. A torch cactus cutting grows into the same plant as its parent, flowers included.
Crested, monstrose and variegated forms
Crested and monstrose forms turn up in many kinds of cactus. Growers find them in many members of the Trichocereus group, among them crested and monstrose forms of San Pedro and of the Bolivian torch.
Crested. A crest forms when the growing tip stretches out into a line instead of staying a single point. The stem grows as a flat fan or a folded, brain-like mass.
Monstrose. The main growing tip stops, and many areoles start growing at once. The plant becomes a knobby mass of small stems.
Variegated. The plant has pale patches, cream, yellow or white, with little or no chlorophyll, the green pigment a plant uses to make food from light. It usually starts as a mutation on a green plant, and seed rarely passes it on.
All 3 forms are kept going by cuttings or grafts. Crested and variegated plants can revert: a crest can send up a normal column, and a variegated plant an all-green section. Growers report that monstrose plants can send up a smooth stem too. Green growth has more chlorophyll and grows more strongly, so on a variegated plant it can take over.
- Your crest or monstrose plant grows a normal column. To keep the form and gain a second plant, cut the column off and root it (taking cuttings from your own plant). Some growers leave it one growing season first, because a reverted section sometimes crests again.
- Your variegated plant grows an all-green section. Cut it off as soon as you notice it, back to where the variegated growth starts. Root it if you want a green plant as well.
Named clones, seedlings and provenance
A clone is a group of plants made from one original by cuttings or grafts, all with the same genes as the original. San Pedro is often sold under the label PC, a clone label used by growers and sellers (names on the label). Other named clones carry trade names, not botanical names.
Every seedling from a cross is a new plant, and seed from a hybrid can give plants that look unlike the parent, so 2 plants sold under the same cross can differ. If you want a particular plant, buy a cutting of it (how cacti are propagated).
Telling similar plants apart
Since spines and color shift with conditions, growers generally rely on provenance, a record of where a plant came from, and on the flowers once it blooms. When you buy, ask whether the plant is a cutting or a seedling, and from what parent.
On San Pedro and Peruvian torch a small furrow crosses the rib just above each areole, which growers often call the V-notch. Collectors often compare the depth of the notch, the spacing of the areoles and their size and color, on mature stems grown in similar light. On scopulicola the areoles tend to be small and look sunken.
Looking after an unusual plant
Variegated plants are less vigorous because they have less chlorophyll, and crested and monstrose plants are often slower than the normal form too. Variegated columns may be thinner and more prone to rot. Water them when the mix has dried all the way through, and less often in winter (watering cactus). Mealybugs and scale can hide in the folds of a crest, so check there (cactus pests). A large crest can be top-heavy; pot it firmly, in a pot wide or heavy enough that it will not tip.
These forms are often grafted onto a vigorous column, and San Pedro is a widely used stock. A graft can speed up a slow plant, and a cactus with no green anywhere depends on a graft to survive (cactus grafting).
Our Trichocereus are grown and sold strictly as ornamental specimens.
Sources
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
Echinopsis terscheckii. University of Arizona Campus Arboretum. https://apps.cals.arizona.edu/arboretum/taxon.aspx?id=831
Plant Care Information: Torch Cacti, Trichocereus hybrids. Arizona-Sonora Desert Museum. https://www.desertmuseum.org/visit/sheets/TrichoX.pdf
About The Hybrids. ISI Schick Hybrids, The Huntington. https://media.huntington.org/Schick/abouthybrids.html
Fascinating Fasciation. Susan Mahr, Wisconsin Horticulture, University of Wisconsin-Madison Division of Extension. https://hort.extension.wisc.edu/articles/fascinating-fasciation/
Stem Fasciation in Cacti and Succulent Species: Tissue Anatomy, Protein Pattern and RAPD Polymorphisms. A. N. El-Banna, M. F. El-Nady, Y. H. Dewir and M. E. El-Mahrouk, Acta Biologica Hungarica 64(3): 305-318, 2013. https://d-nb.info/1218946474/34
Reversion in plants. Royal Horticultural Society. https://www.rhs.org.uk/problems/reversion
Plants Reverting? What's a Gardener to Do? Dawn Pettinelli, UConn Home and Garden Education Center. https://homegarden.cahnr.uconn.edu/wp-content/uploads/sites/3479/2022/08/Plant-Reversions.pdf
Propagation (chapter 13). North Carolina Extension Gardener Handbook, NC State Extension. https://content.ces.ncsu.edu/extension-gardener-handbook/13-propagation
Studies on Inter-generic Grafting Compatibility of Ornamental Cacti. R. Perumal, M. Prabhu, M. Kannan and S. Srinivasan, International Journal of Current Microbiology and Applied Sciences 9(11): 2133-2144, 2020. https://www.ijcmas.com/9-11-2020/R.%20Perumal,%20et%20al.pdf
The Double-Cut Technique for Grafting Cacti to Trichocereus pachanoi Rootstock. Dan Bach, Desert Plants 3(1), 1981 (reprinted), University of Arizona. http://hdl.handle.net/10150/556561