Each part of a columnar cactus such as San Pedro has a job, and each job explains part of its care.
The short version
- The stem does the work of leaves. A thin green layer just under the skin makes the plant's food, and the paler tissue inside it stores water.
- The ribs open and close like an accordion, so the stem swells after watering and slims down in drought.
- A slim stem with deep grooves is short of water, or its roots have stopped working. Push a finger or wooden stick 2 to 3 inches into the soil. Dry: give a full soak. Damp: do not water; the roots may have rotted.
- An areole, the small felted pad the spines grow from, is a bud. Spines, flowers and new branches all start there.
- Spines are modified leaves. They are dead tissue that shades and protects the stem, and light affects how many grow.
- The skin lasts as long as the stem stays green. On blue columns a powdery wax bloom covers it and does not come back where rubbed off. Low on an old stem the skin turns to cork, a tough brown layer that builds up into bark. This is normal aging.
- Roots spread wide and shallow and take up water fast after a soak. A San Pedro cutting usually roots from its cut end.
- A cactus opens its pores to take in air at night, when less water escapes. Water by how dry the soil is, not by the time of day.
A stem that does a leaf's job
Columnar cacti grow no working leaves, so the stem makes the plant's food. Just under the skin lies a thin layer of dark green tissue that turns light, air and water into sugars, the process called photosynthesis. In Cereus repandus, another column cactus, that layer is about 2 mm (under 1/8 inch) deep. Inside it, paler tissue holds the plant's water.
Because the food-making layer is at the surface, sudden strong sun can burn it. Columnar cacti burn easily when they are moved into direct sun they are not used to. The plant usually survives, but the scar is permanent. Build up sun over a few weeks, as set out in cactus sun acclimation.
A cut opens the moist storage tissue to the air. Leave a cutting somewhere dry and airy, in bright shade, until the whole cut face is dry, firm and sealed, with no soft or shiny spots when you press it lightly.
Ribs
The ribs are the raised ridges that run the length of the stem. They let the stem take in or give up a lot of water while its surface area stays the same. As the storage tissue fills, each rib widens where it meets the stem. As the plant uses water, the 2 sides of each rib close in toward each other and the stem slims. The skin folds with the ribs, so it never has to stretch or tear.
San Pedro is classed as a variety of a broader species. The botanical description of that species gives about 6 to 8 ribs, broad and rounded rather than sharp, each about 1 inch wide near the base of a mature stem.
A well-watered column looks plump, with shallow grooves between rounded ribs. A column that has drawn down its store looks slimmer, with deeper grooves and narrower ribs. Roots that have stopped working give the same look, so first push a finger or wooden stick 2 to 3 inches into the soil.
- Dry at that depth: the plant is thirsty. Soak it until water runs out of the drainage hole, then wait until the soil is dry 2 to 3 inches down again before the next soak. Watering cactus adjusts this for season and climate.
- Still damp: do not water. Roots that sit in wet soil may rot and stop taking up water. To check the roots and decide what to do, see overwatering or underwatering.
Areoles
An areole is a small felted pad set along the crest of a rib, and every spine on the plant grows from one. In the botanical description of San Pedro's species, the areoles are round, about 1/4 inch across and about 1 inch apart.
Each areole is a bud. Inside it sits a growing tip, the point where new stem tissue forms, which stays at rest until the plant needs it.
- Spines. On San Pedro, older areoles often carry few spines, 3 to 7, or none at all.
- Flowers. Most of San Pedro's flowers open from areoles a little below the top of the stem, and few open at the tip itself. Timing is covered in cactus flowering.
- Branches. When the main tip is damaged or cut off, a resting tip in an areole can take over, and new branches start from areoles. Pups and branching are covered in the cactus life cycle.
On San Pedro, a small crease often runs across the rib just above each areole. It is clearer on some plants than others. Because it sits on the side toward the top of the stem, it shows which way a cutting grew. Which way up to plant a cactus cutting shows how to read it.
Spines
Spines are modified leaves. Once grown, a spine has no living cells. It is tough fiber inside a hard shell, without pores or veins, and it cannot make food.
Spines shade the stem. They block part of the sunlight, which keeps the stem cooler and protects the green layer from strong light. Stiff, sharp spines also protect the stem from being eaten. Handle spiny stems with thorn-proof cactus gloves.
Light and other growing conditions change how many spines grow and how long they get. On a single branch of a cultivated plant of San Pedro's species, one areole may be bare and another may carry up to 10 spines. A plant moved into more sun may grow a spinier stretch above a smoother one, so spines alone can be a poor guide to which plant you have.
Skin, wax and bark
The skin stays on the stem for as long as the stem is green. It is dotted with stomata, the tiny pores the plant takes in air through. Every stem has a cuticle, a thin waterproof film of wax and cutin, a water-repelling material, that the skin cells lay over their outer surface to limit water loss. On blue-gray columns such as some San Pedro and Peruvian torch, a powdery coat of wax called the bloom, or farina, sits on top of the cuticle. It makes the skin glaucous, meaning pale blue-gray, and reflects some sunlight. 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, and hold them by parts you do not mind marking, such as the base.
Low on an old stem, the skin cells start dividing again and lay down cork, which builds up into bark. This is corking, a normal part of aging that starts at the base. To tell it from sunburn, frost marks and rot, see corking, sunburn or rot.
Roots
Cactus roots spread wide and stay shallow. They can take up a lot of water quickly after a rain that wets only the top of the soil. A full soak, followed by a wait until the soil has dried, suits roots built this way.
A San Pedro cutting usually roots from its cut end. Give an unrooted cutting no water until it has rooted. Rooting takes 4 to 8 weeks. The method is in how to root a cactus cutting.
Breathing at night
Night breathing is about air, not water, so it is no reason to water at night. Water when the soil has dried, at whatever hour suits you.
A plant takes in carbon dioxide through its stomata and loses water through the same pores. Most plants open them by day. A cactus opens them at night, when the air is cooler and less water escapes, and holds the carbon dioxide overnight as an acid in its cells. By day, with the pores shut, it uses sunlight to turn that store into sugars. With its pores opened the same amount, a plant that works this way loses only about a fifth to a third as much water as a plant that breathes by day.
Cool nights suit this in prickly pear (Opuntia, the cactus with flat pads), which takes in the most carbon dioxide on nights of about 59 F.
Sources
Cactus Spines. James D. Mauseth, Mauseth research on cacti, University of Texas at Austin. https://web.biosci.utexas.edu/mauseth/researchoncacti/Spines.htm
Modifications to Cactus Leaves. James D. Mauseth, Mauseth research on cacti, University of Texas at Austin. https://web.biosci.utexas.edu/mauseth/researchoncacti/Leaves.htm
Modifications to Shoot Apical Meristems. James D. Mauseth, Mauseth research on cacti, University of Texas at Austin. https://web.biosci.utexas.edu/mauseth/researchoncacti/Apical%20meristems.htm
Modifications to the Cactus Cortex. James D. Mauseth, Mauseth research on cacti, University of Texas at Austin. https://web.biosci.utexas.edu/mauseth/researchoncacti/cortex%20text.htm
Modifications to Cactus Epidermis. James D. Mauseth, Mauseth research on cacti, University of Texas at Austin. https://web.biosci.utexas.edu/mauseth/researchoncacti/epidermis%20text.htm
Plant tissues. Protection. Atlas of plant and animal histology. Department of Functional Biology and Health Sciences, Faculty of Biology, University of Vigo, updated 2025. https://mmegias.webs2.uvigo.es/02-english/1-vegetal/guiada_v_proteccion-c.php
Spectral properties of heavily glaucous and non-glaucous leaves of a succulent rosette-plant. T. W. Mulroy, Oecologia 38: 349-357, Springer, 1979. https://link.springer.com/doi/10.1007/BF00345193
Cacti / Desert Succulents. Guadalupe Mountains National Park, U.S. National Park Service. https://www.nps.gov/gumo/learn/nature/cacti.htm
Botanical Nerd Word: Areola. Toronto Botanical Garden, 2016. https://torontobotanicalgarden.ca/blog/word-of-the-week/botanical-nerd-word-areola/
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
Problems and Pests of Agave, Aloe, Cactus and Yucca (AZ1399). Jack Kelly and Mary W. Olsen, University of Arizona Cooperative Extension, revised 2011. https://extension.arizona.edu/sites/extension.arizona.edu/files/pubs/az1399.pdf
Ecophysiology of Opuntia ficus-indica. Park S. Nobel, Food and Agriculture Organization of the United Nations. https://www.fao.org/4/Y2808E/y2808e06.htm
Propagation of Cactus and Agave Made Easy. Jack Kelly, Pima County Cooperative Extension, University of Arizona, Combined Proceedings of the International Plant Propagators' Society 59: 328-330, 2009 (listed in Forest Nursery Notes, USDA Forest Service, 2011). https://npn.rngr.net/publications/fnn/2011-winter/new-nursery-literature/propagation-of-cactus-and-agave-made-easy/at_download/file