A large cactus flower can open every year without ever making fruit. Whether it sets seed depends on 2 things: an animal or a person moving pollen, and a second plant that is genetically different.
The short version
- A cactus flower grows from an areole, the spot on the stem that also carries the spines. On San Pedro (a Trichocereus), flowers form mostly near the top of a stem.
- San Pedro's flowers are white, and like most of its group they are large and open at night. Each flower generally lasts 1 to 3 days.
- In the wild, San Pedro's relatives are visited by moths at night and by bees and birds during the day.
- Several close relatives cannot make seed with their own pollen. Growers find that San Pedro behaves the same way.
- Cuttings and pups of one plant are all the same plant. For seed, you need 2 plants that are not copies of each other, in flower at the same time.
- To hand pollinate, use a fine paintbrush at night or early the next morning. Pick up pollen from a flower on one plant and brush it onto the stigma of a flower on the other.
- A pollinated flower generally stays on the stem and its base swells into a fruit. A flower that was not pollinated generally falls off.
The parts you need to find
To pollinate a flower by hand, you need to find 2 parts: the anthers, which hold the pollen, and the stigma, which receives it. Both sit inside a long tube that opens into a funnel or bell of petals.
On San Pedro and its relatives the bud is covered in hairs, and the open flower is large. In this group, flowers generally run about 6 to 12 inches long. Most are white, some are yellow, and a few are red. Small scales and hairs cover the outside of the tube.
- Petals. Dozens of them, overlapping around the opening.
- Stamens. Many thin stalks attached to the inside of the tube. Each ends in an anther, a small head that holds the pollen, the fine dust that carries the male cells.
- Style and stigma. The style is the single stalk in the center of the flower. The stigma is the cluster of small lobes at its tip.
- Ovary. The swollen base of the flower, below the petals. The seeds form here, and after pollination it grows into the fruit.
Nectar, the sugary liquid that draws visitors, collects at the base of the tube. An animal reaching down for it brushes past the anthers and the stigma and carries pollen to the next flower it visits.
Night flowers and day flowers
Most Trichocereus flowers, San Pedro's included, open at night, so evening and early morning are the times to watch them and to pollinate.
Night flowers tend to be long and white and are often scented. They suit hawk moths, large night-flying moths that feed while hovering. Day flowers in related groups tend to be shorter, and many suit bees. A few related plants have flowers suited to hummingbirds.
A night flower does not always close at dawn. In some of San Pedro's relatives, flowers that open at dusk stay open into the next day, so moths reach them at night and bees and birds in the morning. To see how your own plant behaves, check an opening bud at dusk and again after sunrise. A flower that opens at dusk is a night flower. If it is still open in the morning, day visitors can reach it too, and you have a second chance to pollinate.
Why one plant is often not enough
Some cacti are self-incompatible: a flower cannot make seed with pollen from its own plant. The pollen lands on the stigma, but the plant stops it from growing down the style to the ovary, and no fruit forms. Several close relatives of San Pedro behave this way, among them the peanut cactus (Echinopsis chamaecereus), a low, mat-forming cactus. Growers find the same in San Pedro.
A cutting or a pup (a small offset that grows from the side or base of a stem) is genetically the same plant as the one it came from. All the plants grown from one original plant this way make up a clone. In a self-incompatible plant, pollen from one piece of a clone does nothing on another piece.
So for seed, you need 2 plants that are not copies of each other, such as 2 plants each raised from seed, or plants from 2 different clones, with flowers open on both at the same time. San Pedro is often sold under the label PC (see names on the label), and a label alone does not tell you whether 2 plants came from the same original plant. If you do not know where your plants came from, try the cross and watch the result.
Even 2 different plants may fail to make seed together, because some pairs are also incompatible with each other. If 2 plants still set no fruit after you have hand pollinated them over several flowerings, pair one of them with a different plant.
Hand pollinating
You need 2 compatible plants in flower and a fine paintbrush.
- Pick the night. Wait until a flower is open on each plant on the same evening.
- Work while the flowers are open. Pollinate after dark or early the next morning, before the flowers close.
- Collect pollen. Brush the anthers of the flower on the first plant until the bristles come away dusted with pollen. If nothing comes off, the anthers are not shedding yet; try again later that night or in the morning.
- Apply it. Brush the pollen across the stigma lobes of the flower on the second plant. For seed on both plants, repeat the other way round with a second brush, so each flower gets only the pollen you meant it to.
- Keep a note. Write down which flower received which plant's pollen, so you know the parents of any seed.
Then watch the flowers over the following days. A pollinated flower generally withers but stays on the stem, and its base starts to swell. A flower that was not pollinated generally falls off. The fruit tends to take a few months to ripen.
Each seedling from a cross is a new plant that differs from both parents. This holds for torch hybrids too, the low clumping crosses grown for their flowers in many colors: their seed gives seedlings that differ from the parent plant. To keep a plant exactly as it is, grow it from cuttings instead. For how seed, cuttings and grafts compare, see how cacti reproduce.
Pollinators in the garden
San Pedro is native to southern Ecuador and Peru. Its columnar relatives (tall, upright, column-shaped cacti) show how loose the match between flower and visitor can be. One has night flowers that moths, bees and birds all visit, and the moths do most of the pollinating. In another, with flowers shaped for moths, bees and small perching birds arriving by day do more of the work. A flower shaped for moths can still be pollinated by a bee in the morning.
In a garden, a flower meets whatever animals live there. Night moths may or may not visit. Bees may find flowers that are still open in the morning, which can be enough for pollination if a compatible plant is in flower nearby.
A visitor can only move pollen between the plants you have. One plant, or one clone in several pots, generally makes no seed however many bees come. A plant that flowers indoors or under cover gets few or no visitors, so hand pollination is the reliable way to get seed there.
For when a cactus starts to flower, and why it may not have yet, see when will my cactus flower.
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 pachanoi (Britton & Rose) Friedrich & Rowley. HortFlora, Horticultural Flora of South-eastern Australia, Royal Botanic Gardens Victoria. https://hortflora.rbg.vic.gov.au/taxon/5eb287ff-8e2a-4201-8e6e-fe7d6a10f65b
Echinopsis pachanoi (Britton & Rose) H.Friedrich & G.D.Rowley. Plants of the World Online, Royal Botanic Gardens, Kew. https://powo.science.kew.org/taxon/urn:lsid:ipni.org:names:88444-2
Cactus Flowers. University of Texas at Austin. https://web.biosci.utexas.edu/mauseth/researchoncacti/Flowers.htm
Botanical Nerd Word: Areola. Toronto Botanical Garden, 2016. https://torontobotanicalgarden.ca/blog/word-of-the-week/botanical-nerd-word-areola/
Echinopsis. Nederlandse Vereniging van Botanische Tuinen (NVBT), Botanische tuin Zuidas. https://botanischetuinen.nl/en/plant_en/3007/echinopsis
Molecular phylogenetics of Echinopsis (Cactaceae): Polyphyly at all levels and convergent evolution of pollination modes and growth forms. Boris O. Schlumpberger and Susanne S. Renner, American Journal of Botany 99(8): 1335-1349, 2012. https://profiles.wustl.edu/en/publications/molecular-phylogenetics-of-echinopsis-cactaceae-polyphyly-at-all-/
Reproductive biology of Echinopsis terscheckii (Cactaceae): the role of nocturnal and diurnal pollinators. P. Ortega-Baes, M. Saravia, S. Sühring, H. Godínez-Alvarez and M. Zamar, Plant Biology 13 (Suppl. 1): 33-40, 2011. https://doi.org/10.1111/j.1438-8677.2010.00332.x
Pollination biology of Echinopsis leucantha (Cactaceae): Passerine birds and exotic bees as effective pollinators. Pablo Gorostiague and Pablo Ortega-Baes, Botany 95(1): 53-59, 2016. https://ri.conicet.gov.ar/handle/11336/75597
Physiology and genetics of self-incompatibility in Echinopsis chamaecereus (Cactaceae). T. H. Boyle and A. Idnurm, Sexual Plant Reproduction 13: 323-327, 2001. https://link.springer.com/article/10.1007/s004970100073
Echinopsis chamaecereus | peanut cactus. Royal Horticultural Society. https://www.rhs.org.uk/plants/31628/echinopsis-chamaecereus/details
Propagation (chapter 13). Extension Gardener Handbook, NC State Extension. https://content.ces.ncsu.edu/extension-gardener-handbook/13-propagation
Plant Care Information: Torch Cacti, Trichocereus hybrids. Arizona-Sonora Desert Museum. https://www.desertmuseum.org/visit/sheets/TrichoX.pdf
Pitaya (Dragonfruit) Growing in the Florida Home Landscape. Jonathan H. Crane and Carlos F. Balerdi, UF/IFAS Extension, 2020. https://ask.ifas.ufl.edu/publication/HS303
Evaluation of Different Procedures to Pollinate Self-Compatible 'Royal Red' Pitaya Under Protected Cultivation. Juan José Hueso, El Mehdi Bouzar and Julián Cuevas, Plants 14(19): 3102, 2025. https://pmc.ncbi.nlm.nih.gov/articles/PMC12526123