Silver Thiosulfate Helps Produce Feminized Hemp Seed, Not Directly Increase CBD
Female hemp plants are prized for their cannabinoid-rich flowers, but producing crops made up almost entirely of female plants requires a reliable source of genetically female pollen. A 2018 study by University of Connecticut researchers Jessica D. Lubell and Mark H. Brand found that silver thiosulfate could induce female hemp plants to form male flowers, creating a practical route to feminized seed production.
The research, published in HortTechnology, examined four low-THC hemp varieties: three CBD-producing strains and one industrial hemp strain. The researchers applied silver thiosulfate, or STS, as a foliar spray at concentrations of 0.3 or 3 millimolar on three occasions, each one week apart, during short-day flowering conditions.
The stronger treatment produced the most consistent results. At 3 millimolar, most inflorescences developed entirely male flowers, while terminal inflorescences showed at least 95 percent conversion to male flowers. The lower concentration produced only partial conversion, and its effectiveness varied among the tested varieties.
These induced male flowers were still produced by genetically female plants. As a result, their pollen was expected to carry only X chromosomes. When used to pollinate female plants, that pollen can theoretically produce seed that is almost entirely XX, or female. The UConn researchers reported that pollen from the treated plants produced good all-female seed, although the masculinized flowers generated less pollen than flowers from genetically male plants.
The finding is important for breeding and crop management, but it does not show that STS directly increases the CBD concentration of a treated plant. Instead, the technique is designed to help breeders produce feminized seed, reducing the need to identify and remove male plants from commercial fields. Female flowers generally contain much higher cannabinoid concentrations than male flowers, making control of plant sex valuable for CBD-oriented production.
Breeding programs would still need to manage pollen carefully. Unwanted pollen can fertilize female flowers, leading to seed formation and reducing the quality and yield of cannabinoid-focused flower crops. Controlled-environment facilities may therefore be useful when maintaining genetic purity or producing breeding seed.
Results may also differ among cultivars. The UConn researchers noted that genetic and phenotypic differences can affect how strongly plants respond to STS. Subsequent research has examined pollen quality and other methods of inducing male flowers, while a separate 2021 study reported improved and more uniform CBD-to-THC ratios in progeny from a high-CBD breeding population. That later result came from a specific breeding experiment, however, and should not be interpreted as evidence that STS universally raises CBD levels in every hemp crop. Further testing is needed to evaluate the method across varieties and production systems.
In practical terms, the study’s main contribution was not a demonstrated CBD boost, but a breeding tool: converting genetically female hemp plants into pollen producers that can help generate feminized seed and more predictable cannabinoid crops.
