Last updated on August 25, 2026 · Originally published May 24, 2022
Cannabis is usually dioecious: XX plants generally bear cannabinoid-rich female inflorescences, while XY plants bear pollen-producing male flowers. Male plants are not devoid of cannabinoids, but female inflorescences carry far denser glandular trichomes, which is why growers want all-female crops. Breeders can redirect that floral expression without changing the plant’s genetic sex. There are two ways to do it, they run in opposite directions, and they exist for different reasons.
Making a female plant produce male flowers. Silver compounds are sprayed on a genetically female (XX) plant so that it grows male flowers and sheds pollen. Because that pollen carries no Y chromosome, crossing it onto another female produces seed that is genetically female. This is how feminized seed is made, and it is the method seed companies use.
Making a male plant produce female flowers. Ethephon is sprayed on a genetically male (XY) plant so that it grows female flowers and sets seed. This does not produce feminized seed. It allows a breeder to self a selected male, expose and fix traits in its offspring, and develop an inbred line — something otherwise difficult in a plant with separate sexes.
Both are breeding tools, not cultivation techniques — a point returned to below.
Sex reversal is useful to breeders precisely because cannabis makes breeding awkward. A plant’s sex is often not visually apparent until preflowers develop, female plants can produce male or mixed-sex flowers on their own, and the separate sexes make it hard to self a plant you like. There is considerable interest in silver thiosulfate, colloidal silver, silver nitrate, and ethephon as ways around those constraints, and the question of how cannabis determines sex in the first place sits underneath all of them.
Feminized seed is produced by taking a genetically female plant, inducing it to produce pollen, and using that pollen on a female plant — a process called “selfing” when the pollen is returned to the same plant or a clone of it. [3] Because an XX pollen donor contributes no Y chromosome, the resulting seeds are genetically female, although their eventual sex expression can still be influenced by genetics and growing conditions.
Silver for Cannabis Sex Reversal
External substances such as silver thiosulfate, gibberellic acid, and colloidal silver are known to influence sex expression in cannabis plants. [1] These substances can be used to induce male flowers on female plants. In the study “Production of Feminized Seeds of High CBD Cannabis sativa L. by Manipulation of Sex Expression and Its Application to Breeding,” researchers observed that spraying whole plants with silver thiosulfate is an efficient method of producing male flowers on female plants. They also suggest that spraying whole plants with colloidal silver works effectively; gibberellic acid did not perform well in their study.
The approach is not new. Silver’s ability to induce fertile male flowers on genetically female cannabis was demonstrated in 1982, when researchers treated plants with silver nitrate and with the silver thiosulfate anionic complex and obtained pollen-bearing flowers. [9] Foliar application was examined systematically in hemp in 2018, when researchers tested 0.3 and 3 mM STS across four strains and found 3 mM the most consistently effective concentration. [8]
Ethephon for Cannabis Sex Reversal
The plant growth regulator ethephon is also of interest for cannabis breeders. In the research paper “Effect of Timing of Ethephon Treatment on the Formation of Female Flowers and Seeds from Male Plant of Hemp (Cannabis sativa L.),” ethephon was utilized to develop a self-fertilization method by forming female flowers (and seeds) from male plants of dioecious hemp.
Researchers sprayed genetically male plants with 500 mg per liter of ethephon immediately, seven days, or fourteen days after male primordia appeared. [4] Female flowers developed on 100% of plants treated immediately, 40% of those treated after seven days, and none of those treated after fourteen days. The induced flowers had five stigmas rather than the usual two.
Plants treated immediately produced 960 seeds per plant, compared with 1,404 on untreated female controls. Their seeds were smaller and germinated at 53%, compared with 80% for the controls. The experiment therefore demonstrated that ethephon could enable self-fertilization of a selected male genotype, but not that it improved seed yield. In this experiment, timing was decisive: treatment fourteen days after primordia appeared produced no female flowers.
Does a Small Grower Need This?
Usually, no. These are breeding techniques, not cultivation techniques. A person growing a few plants for flower can buy feminized seed, take cuttings from a known female, or grow regular seed and remove the males before their pollen sacs open. With regular seed, expect approximately half the plants to be female over large numbers, although a small batch can fall heavily either way; where local plant limits permit, growers commonly start extra seeds and cull the males at preflower. Sex reversal becomes relevant only when the goal is deliberately producing seed.
Colloidal silver does not keep a plant female. It does the opposite: it makes an XX plant produce male flowers and pollen. A treated plant is therefore a pollen donor, not a smoking plant, and it can pollinate nearby females if it is not isolated.
Why Ethylene Sits at the Center of Both Methods
Ethylene is the hormone both approaches are aimed at, but they work on it from opposite ends, and they produce opposite results.
Ethylene generally promotes female flower expression in cannabis. Ethephon releases ethylene, and therefore pushes expression toward female flowers — which is why it is applied to male plants.
Silver acts in the opposite direction. Silver ions are understood primarily to antagonize ethylene perception rather than simply stopping ethylene production, so the plant cannot fully respond to the ethylene it already has. [6] With that signal interrupted, female plants produce male flowers. This is why silver thiosulfate, silver nitrate, and colloidal silver are grouped together as anti-ethylene treatments, and why they are applied to female plants rather than male ones.
The downstream molecular network remains incompletely understood. Transcriptome work on silver-treated female plants has identified a broad set of genes spanning several hormone pathways, and the authors of that study note that the molecular basis of silver’s effect on floral sex is still unresolved. [7]
Ethephon has other physiological effects as well. In one 2013 study, it shortened stems and increased tetrahydrocannabinol (THC) concentrations in leaves from male and female plants and in male flowers, but not in female flowers. [2]
What Silver Thiosulfate Actually Does to the Plant
Detailed anatomical work published in 2026 gives a clearer picture of what happens after treatment, and the picture is more partial than the phrase “sex reversal” suggests. The study followed a single CBD cultivar, ‘Pasionaria S’, so the proportions below describe that cultivar rather than cannabis generally. [5]
Researchers examining STS-treated female plants found that the first male floral meristems appeared about seven days after treatment. Female flowers that had already formed kept developing; what changed was the sex of flowers formed afterward. In other words, the treatment does not convert existing female flowers into male ones. It redirects the flowers that have not committed yet.
The plant’s overall architecture also stayed female. Treated plants kept the compact inflorescence structure typical of female cannabis rather than the loose, open shape of a genetically male plant, producing what the researchers describe as a mosaic: a female-patterned inflorescence carrying newly induced male flowers.
Within those inflorescences, three outcomes appeared. Some flowers developed normally and released viable pollen. Others formed anthers that never opened, with pollen development arrested and collapsed inside. A few were mixed, retaining an ovary and stigmas alongside a single reduced anther. Only about a fifth to a quarter of mature flowers fell into the first category. Pollen viability in the flowers that did open reached 54%, and when treated plants were left to pollinate untreated females, mature seeds formed — confirming the pollen works, but also confirming that a plant covered in male-looking flowers is not the same thing as a plant producing usable pollen.
That 54% figure should not be compared directly with the much lower germination percentages in the next section. Viability assays test whether pollen appears biologically intact; germination assays test whether it actually produces a pollen tube. They are different measurements from different experiments.
The Pollen Is the Weak Point
That gap between appearance and function shows up in breeding trials as well. A 2025 study compared silver thiosulfate against three other ethylene-inhibiting compounds and found STS the most effective at masculinizing female plants — but the pollen it produced germinated at rates between 2.2% and 5.8%, far below what genetically male plants yield. [6] The same study found pollen kept its viability for about three weeks when stored cold, and lost it within a week at room temperature.
The alternatives fared worse. Aminoethoxyvinylglycine produced pollen but caused visible chemical burn on the plants. Cobalt nitrate and 1-methylcyclopropene failed to masculinize plants at all. Gibberellic acid, added as a possible synergist, was associated with substantial plant damage: of the thirteen plants that collapsed and died during the field trials, twelve had received it.
Handling and Practical Cautions
Silver treatments carry costs beyond the plant. Silver nitrate is among the more toxic silver compounds, and although silver complexed by thiosulfate has relatively lower environmental toxicity, researchers working on this problem consistently flag silver’s environmental persistence as a reason to look for alternatives. [6] The search for a less hazardous substitute remains an open research question rather than a solved one.
A treated plant should be regarded as a pollen donor, not part of the consumable harvest. There is no established evidence that silver-treated cannabis tissue is safe to smoke or eat, so the prudent approach is to exclude the entire plant from consumption and isolate it from flowering females.
What This Means for Breeders
For breeders, the useful takeaway is that these are two different tools pointed at the same hormone from opposite directions, and serving different ends: silver to bring male flowers out of a female plant for feminized seed, ethephon to bring female flowers out of a male plant so a chosen male genotype can be selfed. Both have worked experimentally, although silver thiosulfate has a substantially stronger practical evidence base. Neither produces a clean swap.
Cannabis sex expression turns out to be plastic enough to redirect but structured enough to resist a complete conversion, which is a fair summary of the wider picture of sex determination in this plant. For breeding programs, that argues for evaluating reproductive function — pollen germination and seed formation — rather than judging a treatment by how thoroughly the plant appears to have changed sex. The same principle applies to precision breeding approaches and to propagation methods like tissue culture.
Image credit
Hero image cropped from Figure 6 of Judkevich MD, Marassi MA, Gonzalez AM. Anatomical and ontogenetic basis of silver thiosulfate-induced masculinization in genetically female Cannabis sativa. Plants. 2026;15(14):2153. Licensed under CC BY 4.0. Panels show a treated plant 12 and 31 days after treatment; cropped and captioned, otherwise unaltered.
References
1- Flajšman M, Slapnik M, Murovec J. Production of feminized seeds of high CBD Cannabis sativa by manipulation of sex expression and its application to breeding. Frontiers in Plant Science. 2021;12:718092. doi:10.3389/fpls.2021.718092
2- Mansouri H, Salari F, Asrar Z. Ethephon application stimulates cannabinoids and plastidic terpenoids production in Cannabis sativa at flowering stage. Industrial Crops and Products. 2013;46:269-273. doi:10.1016/j.indcrop.2013.01.025
3- Punja ZK, Holmes JE. Hermaphroditism in marijuana (Cannabis sativa) inflorescences – Impact on floral morphology, seed formation, progeny sex ratios, and genetic variation. Frontiers in Plant Science. 2020;11:718. doi:10.3389/fpls.2020.00718
4- Moon YH, Lee YJ, Koo SC, Hur M, Huh YC, Chang JK, Park WT. Effect of timing of ethephon treatment on the formation of female flowers and seeds from male plant of hemp (Cannabis sativa). Korean Journal of Plant Resources. 2020;33(6):682-688. doi:10.7732/kjpr.2020.33.6.682
5- Judkevich MD, Marassi MA, Gonzalez AM. Anatomical and ontogenetic basis of silver thiosulfate-induced masculinization in genetically female Cannabis sativa. Plants. 2026;15(14):2153. doi:10.3390/plants15142153
6- Fitzgerald TM, Brown JW, Steinmaus S, Prince J, Bhandari R, Da Cunha Leme Filho JF. Chemical masculinization of female Cannabis sativa L.: impacts on pollen viability and feminized seed production. Horticulturae. 2025;11(11):1286. doi:10.3390/horticulturae11111286
7- Adal AM, Doshi K, Holbrook L, Mahmoud SS. Comparative RNA-Seq analysis reveals genes associated with masculinization in female Cannabis sativa. Planta. 2021;253:17. doi:10.1007/s00425-020-03522-y
8- Lubell JD, Brand MH. Foliar sprays of silver thiosulfate produce male flowers on female hemp plants. HortTechnology. 2018;28(6):743-747. doi:10.21273/HORTTECH04188-18
9- Mohan Ram HY, Sett R. Induction of fertile male flowers in genetically female Cannabis sativa plants by silver nitrate and silver thiosulphate anionic complex. Theoretical and Applied Genetics. 1982;62:369-375. doi:10.1007/BF00275107
Updated August 25, 2026: this article was expanded with current research on silver thiosulfate and ethephon sex reversal. The section on ethylene was corrected to distinguish the opposite mechanisms of silver compounds and ethylene-releasing treatments, and the reported results of the ethephon timing study were corrected. Reviewed by Nani Frenkel, chief editor.
