In a graft, one cactus provides the roots and another grows on top of it. The lower plant is the stock, also called the rootstock. The piece set on top is the scion. San Pedro (a Trichocereus) is a widely used stock and a common choice for a rare Trichocereus. The stock supplies the scion with water and nutrients, so the choice of stock shapes how the graft grows and how long it lasts.
The short version
- A good stock grows strongly enough to feed the scion, is thick enough to carry it, is easy to hold and cut, and stays healthy for a long time with a plant on top.
- San Pedro does all 4 well. It grows vigorously, has short spines, can carry a wide scion and tends to outlast other stocks.
- How long a San Pedro stock lasts varies. It generally outlasts other stocks, and every stock wears out in time. If the stock below the join stays shriveled after watering, yellows or softens, it may be failing: move the scion to a fresh stock or root it.
- Trichocereus spachianus is spinier, harder to handle and tends to wear out sooner. Blue myrtle is another established stock.
- Use a firm, plump stock at least as thick as the scion. About 8 inches long is a common starting point; a small, soft scion goes on a shorter stock, down to 4 to 5 inches. Graft in spring or summer, when both plants are growing. Water a rooted stock a few days before grafting.
- A rooted stock generally gives the scion more push. An unrooted cutting is easier to handle, which makes it the simpler start for a first graft, but it gets no water until it has rooted, 4 to 8 weeks.
- A tip cutting (the top of a stem) cuts cleanly and suits a first graft. A mid-section (a length of stem cut at both ends) works too. On either, the scion goes on the end that pointed up on the parent plant.
What a stock has to do
A graft takes when the 2 plants grow together across the cut. Inside every cactus stem is the vascular system, the tissue that carries water and food, which shows as a ring in the cut surface. The scion's ring has to sit on the stock's ring for the 2 to join.
A columnar stem and its ring are narrower near the growing tip and wider lower down. The stock is cut far enough down that its ring is about as wide as the scion's, so choose a stock at least as wide as the scion's base, with no part of the scion hanging over the edge. The cut itself is part of grafting onto San Pedro.
San Pedro as stock
San Pedro grows vigorously and can support and feed a wide scion. Its spines are short, so you can hold the stem and cut it without clearing them first. It also tends to break down more slowly under a scion than other stocks do.
Growers also value San Pedro for fast growth, for resisting pests and disease, and for forming pups (side shoots) readily. On a graft, pups that sprout from the stock compete with the scion, so remove them as they appear. Caring for grafted cacti gives the method.
A scion on a Trichocereus stock may take a little longer to start growing, then grow steadily, so a slow start alone does not mean the graft has failed.
How long a San Pedro stock lasts
Over time a scion can wear out the plant beneath it, and the stock declines.
Look at the stock below the join every week or two, and each time you water. A stock that is failing may shrivel and stay shriveled after watering, including the first waterings in spring after a dry winter. It may fade from green toward yellow, or go soft. Yellowing can also be sunburn, and soft or wet patches can be rot, so check how to tell corking, sunburn and rot apart. Once the stock is declining, act while the scion is still firm: graft it onto a fresh stock, or cut it off and root it. Caring for grafted cacti covers both.
Other stocks
Trichocereus spachianus. This is another Trichocereus and the next most common stock after San Pedro. It is spinier and harder to handle. Brown patches often form on its stem, hiding the green tissue, and a stock with them does not last as long.
Blue myrtle. Myrtillocactus geometrizans is another established stock. For a Trichocereus scion, San Pedro is the more common pick.
Collectors mostly use Hylocereus, the green base under most moon cacti, and Pereskiopsis for seedlings and small round cacti rather than Trichocereus scions.
Size and condition
Length. Length means the stock from its base to its cut top. While a graft heals, it is often held with rubber bands looped over the top of the scion and under the base of the stock. The longer the stock, the further the bands stretch and the harder they press on the scion, so a small, soft scion goes on a shorter stock. Against that, growers usually find that a longer, heavier stock gives the scion more to draw on. Some growers treat about 8 inches as a working minimum, and find that a 4 to 5 inch stock works with less push. Use the shorter length for a small or soft scion, such as a seedling.
Condition. Use a stock that is firm and plump, with no wrinkles and no brown or discolored tissue in the cut. Give a rooted stock a full watering a few days before grafting, or the day before at the latest, so it is full when you cut it. Choose an unrooted cutting that is already firm and plump.
Timing. Graft when stock and scion are both actively growing, in spring or summer. Fresh growth at the stem tips is a good sign that a plant is actively growing.
Rooted or unrooted, tip or mid-section
Rooted or unrooted. A rooted stock is already growing in a pot and can feed the scion from the start, and it tends to give the scion more push. An unrooted cutting is easier to handle while you graft, because it stands on its flat base on the table. For a first graft, an unrooted cutting is the simpler choice.
A San Pedro cutting stores enough water and food to keep both plants going while its roots form. If you cut your own stock, take the cutting about a week before grafting and lay it in shade, with moving air, so the cut end calluses (dries and seals over). A bought cutting that arrives with its base already dry and sealed is ready to graft. Once the graft has taken, pot the stock and root it as you would any cactus cutting. Give an unrooted cutting no water until it has rooted. Rooting takes 4 to 8 weeks. If you are unsure whether it has rooted, wait the full 8 weeks.
Tip or mid-section. A tip cutting is the top of a stem, with its growing point. A mid-section is a length of stem cut at both ends. Either can be a stock, because the top of a tip cutting is cut off when the graft is made. A tip has soft, young tissue at the top that cuts cleanly, and it is a common pick for a first graft. A mid-section can take a scion on a fresh cut at its upper end, or be left to grow a new tip that is grafted once it has formed; collectors do both.
Pot either one the same way up as it grew, with the end that was lower on the parent plant in the soil. The scion goes on the upper end. If you cannot tell which end of a mid-section was the top, see which way up a cactus cutting goes.
For San Pedro mid-sections sold as stock, see San Pedro mid-section grafting stock.
Sources
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
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
Does Grafting Time Affect the Cactus Performance? Neda Bayat, Roohangiz Naderi and Ardeshir Rahimi Maidani, Journal of Biological and Environmental Sciences 9(27):143-149, 2015. https://dergipark.org.tr/en/pub/jbes/article/1674713
Gymnocalycium mihanovichii (Chin Cactus, Moon cactus, Plaid Cactus). North Carolina Extension Gardener Plant Toolbox, NC State Extension. https://plants.ces.ncsu.edu/plants/gymnocalycium-mihanovichii/
Grafting Types, Cactus Grafting. University of Florida IFAS, University of Kentucky and Texas A&M University, 2023. https://propg.ifas.ufl.edu/06-grafting/02-graftingtypes/05-grafting-cactus.html
Cutting Terminology, Polarity. University of Florida IFAS, University of Kentucky and Texas A&M University, 2023. https://propg.ifas.ufl.edu/05-cuttings/01-terminology/08-cuttingterms-polarity.html