Cannabis is a dioecious plant, meaning that it has separate male and female plants. However, only the female plants produce desirable flowers that are rich in cannabinoids. That means cannabis growers face a big challenge to produce all female crops.
When it comes to cannabis breeding, there are also many challenges because cannabis plants don’t really appear male or female until they flower, and because female plants can turn hermaphroditic and produce pollen sacs for a variety of reasons. Inbreeding can also become problematic when trying to preserve or create cannabis varieties.
For both growers and breeders, there is a lot of interest in colloidal silver, silver nitrate, and ethephon for reversing the sex of cannabis plants between male and female. The ability to manipulate plant gender is key to producing feminized seeds and unique cannabis varieties. Feminized cannabis seeds can be produced by self-fertilized feminized males (or females induced to produce pollen), a process that is called “selfing.”  Within one cycle of selfing, feminized seeds can be produced.
Silver for Cannabis Gender Reversal
External substances such as silver thiosulfate, gibberellic acid, and colloidal silver are known to influence sex expression in cannabis plants.  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.
Ethephon for Cannabis Gender 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. The researchers sprayed plants with 500 mg per liter of ethephon one to two weeks after primordia formation.  This resulted in the formation of female flowers with five stigmas (versus the normal two) on male plants. The resulting seeds tended to be smaller and have a lower germination ratio than regular female plants, however the number of seeds harvested was greater than normal. The research team recommends that hemp plants should be treated with ethephon soon after primordia formation to induce the formation of more female flowers on the male plants.
Ethylene in Sex Determination
One key reason that silver nitrate, colloidal silver, and ethephon work to reverse cannabis gender is because they influence the amount of ethylene in the plant. Ethylene has a feminization effect on the sex expression of hemp.  However, it is also a growth regulator, so ethephon treatment has been shown to shorten the length of plant stems. Despite the shortening effect, ethephon has been observed to increase tetrahydrocannabinol (THC) content of leaves in male and female plants, and in male flowers. Ethephon does not have that effect in female flowers though. 
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. doi:10.3389/fpls.2021.718092 [journal impact factor = 4.298; times cited = 2]
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 [journal impact factor = 5.645; times cited = 8]
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. doi:10.3389/fpls.2020.00718 [journal impact factor = 4.298; times cited = 11]
4- Moon YH, Lee Y. 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 [journal impact factor = N/A; times cited = 0]
Image: https://pixabay.com/photos/restroom-sign-restroom-sign-symbol-983390/ Image by Kerplode via Pixabay.com