Making Your Own Feminized Cannabis Seeds: STS, S1, and Genetics Explained

Feminized seeds are not created by treating finished seeds afterwards. The decisive step takes place beforehand: a genetically female cannabis plant develops male flowers and provides the pollen for the next generation.

Silver thiosulfate, or STS for short, can trigger this change in flower development. However, anyone wishing to understand the process should know more than just a mixing ratio. Which plant provides the pollen and which one carries the seeds determines whether a feminized cross or a selfing results.

This guide explains the differences and describes preparing an STS solution from anhydrous sodium thiosulfate and silver nitrate. For the application, two published study protocols are compared. They provide comprehensible reference points, but not a treatment plan guaranteed to be optimal for every genetics.

What does feminization mean?

In ordinary dioecious cannabis plants, genetically female plants have two X chromosomes. Genetically male plants normally have one X and one Y chromosome.

In a classic cross, the female plant provides an egg cell with an X chromosome. The male germ cells can contribute either X or Y. In the simplified model, this produces XX or XY offspring.

By contrast, if an XX plant is induced to develop male flowers, it does not thereby receive a Y chromosome. In the normal inheritance model, its pollen also provides an X contribution. Together with the egg cell of an XX plant, this means XX offspring are expected.

The male form of a flower therefore does not automatically mean that the entire plant is genetically male. At the same time, an XX composition is no guarantee that a plant will develop exclusively female flowers under all conditions. Genetic sex and visible flower expression must be considered separately. Frontiers

Regular, feminized and S1: What is the difference?

The three terms do not describe the same characteristic.

For regular seeds, the simple XX/XY model gives an expected value of approximately 50 percent female and 50 percent male offspring. This is not a fixed distribution for every individual seed pack.

Feminized describes the intended sex distribution. S1 describes the ancestry.

When does an S1 occur?

In selfing, both parental contributions come from the same original genotype. This can occur within a single plant. From a genetic perspective, however, pollen from a treated clone can also be transferred to an untreated clone of the same original plant.

By contrast, two plants with the same variety name are not necessarily the same genotype. If they were grown from different seeds, their connection is not a selfing solely because they share a name.

As simplified examples:

Original genotype A × the same original genotype A: Selfing. The first offspring generation is S1.

Original genotype A × another original genotype B: Cross. With two XX parental contributions, it can be feminized, but it is not an S1.

Why S1 does not mean a copy of the original plant

Selfing is sexual reproduction. Existing gene variants are divided and recombined.

A simple calculation example: An original plant carries the variants A and a at one gene position. With equal transmission probability, selfing theoretically produces 25 percent AA, 50 percent Aa and 25 percent aa.

All offspring originate from the same original genotype. Nevertheless, they are not identical at this position.

A 2025 study used exactly this variation within the offspring of a single self-pollinated hemp plant. Selfing can split traits and make genetic differences visible. It should therefore not be equated with clonal propagation. PubMed

What STS does in the plant

STS stands for Silver Thiosulfate, or Silberthiosulfat in German. Here, the term refers to a solution in which silver is complexed by thiosulfate.

The plant-physiological connection concerns, among other things, the action of the plant hormone ethylene. Silver can interfere with its normal signalling effect. The explanation that “STS switches off ethylene production” would therefore be too imprecise.

In cannabis, treatment can trigger the development of male flower organs on genetically female plants. Studies of gene activity show changes in developmental programmes associated, among other things, with anthers, pollen and hormonal signalling pathways.

The treatment changes flower expression. It does not add a Y chromosome. Altered activity of existing genes is also not the same as a targeted change to their DNA sequence. Springer Nature Link

Before preparing the solution: Chemical safety

Silver nitrate is not a harmless garden chemical. The manufacturer’s labelling lists, among other things, oxidising properties, severe skin and eye damage, high aquatic toxicity and a reproductive toxicity classification.

Work requires safety glasses, suitable chemical-protection gloves and an appropriately equipped workspace. The safety data sheets for the products actually used are authoritative. Dust and spray mist must not be inhaled. Children, animals and food must be kept out of the work area.

Do not smoke, vaporise or consume treated plant parts. The studies described here do not provide a reliable general waiting period after which such plant parts could be considered safe.

Do not dispose of silver-containing liquids or contaminated materials via drains, garden soil or compost. Residues, including collected spray liquid and rinse water, must be collected and handed over in accordance with local requirements. Merck

Preparing STS: Which starting materials are required?

The following calculation uses:

Silver nitrate: AgNO₃

Anhydrous sodium thiosulfate: Na₂S₂O₃

The specifications expressly apply not unchanged to sodium thiosulfate pentahydrate, Na₂S₂O₃·5H₂O. Water of crystallisation changes the mass required for the same amount of substance.

Use clearly labelled chemicals with documented purity. The following weighings are nominal values for high-purity starting materials. Technical mixtures or materials of unknown composition are not a reliable basis for this.

You will also need deionised or distilled water, separate clean measuring vessels, suitable volume-measuring equipment, a stirring rod and a verified precision balance. For the small batch, the balance should be able to resolve quantities in the milligram range meaningfully. A display with three decimal places alone does not guarantee the corresponding accuracy.

Understanding the ratio correctly

The preparation ratio used here is:

Silver to thiosulfate = 1:4 on a molar basis.

This is not a weight ratio of 1:4.

The Merck/Sigma-Aldrich protocol uses two equally concentrated stock solutions for this purpose. One volume part of the silver nitrate solution is combined with four volume parts of the sodium thiosulfate solution. In the mixture, the silver is predominantly present as a thiosulfate complex.

Step 1: Prepare two separate stock solutions

The following scaled-down preparation produces approximately 100 ml of 20 mM STS concentrate after combining the solutions. The concentration here refers to the amount of silver substance used.

The quantities are calculated proportionally from the manufacturer’s preparation and produce stock solutions of approximately 0.1 molar each.

First dissolve each powder in part of the specified water. Then make up to the stated final volume in the respective measuring vessel.

“Make up to 20 ml” does not mean “add 20 ml of water to the powder”. It refers to the final solution volume.

Step 2: Combine the solutions

Add the silver nitrate solution A slowly to the sodium thiosulfate solution B while stirring.

Do not mix the dry powders together. First prepare the two separate solutions. Then combine them.

The result is the STS concentrate. It is not yet a ready-to-use spray solution.

Merck/Sigma-Aldrich recommends preparing it freshly before use. Although the manufacturer’s protocol mentions possible refrigerated storage for up to one month, this should not be interpreted as a blanket shelf-life guarantee for any containers and working conditions. Merck

Clearly label the vessels with their contents, concentration and preparation date. Do not store chemicals together with food. If unexpected precipitation or contamination occurs, the solution should not be “rescued” with improvised additives.

Step 3: Dilute the concentrate to the application concentration

The two study protocols described below use 0,3 mM and 3,0 mM STS, respectively.

These concentrations differ by a factor of ten. They must not be confused.

Based on the nominal 20 mM concentrate, the following quantities result mathematically:

In each case, make up the measured amount of concentrate with water to the desired final volume.

For one litre at 0,3 mM, for example, use 15 ml of concentrate and then make up to a total of 1.000 ml.

The calculation follows the dilution equation:

Starting concentration × concentrate volume = target concentration × final volume.

For the example:

20 mM × 15 ml = 0,3 mM × 1.000 ml.

This table applies exclusively to the 20 mM concentrate described here. If a different starting concentration is used, the quantities must be recalculated.

Fertiliser, pH adjustment agents, rinsing agents or other additives do not automatically belong in this preparation. They would constitute an additional modification of the described process.

When and how often is STS applied?

An important distinction is necessary here: Published application schedules exist. However, they do not establish a single scientifically validated standard for every cannabis genetics.

The following comparison describes two documented variants for plants whose flowering is controlled by day length.

Day 0 here refers to the beginning of the switch to twelve hours of light and twelve hours of darkness. It does not mean the day of germination or the first visible pistil.

Flajšman and colleagues investigated a single treatment with 0,3 mM in a CBD-rich breeding population. DiMatteo and colleagues used 3,0 mM three times at seven-day intervals. In both cases, the first treatment took place at the beginning of the flowering transition. Frontiers

The two variants are not successive escalation stages. They do not imply that any number of additional sprays should be applied after a single treatment.

The combination “one week before, at the transition and one week afterwards” also does not correspond to the protocols shown here. This does not make other schedules fundamentally ineffective, but they would require separate evidence.

What is sprayed?

The plant that is intended to provide pollen later is treated. Not automatically the plant on which the seeds are intended to mature.

In the cited experiments, entire above-ground plants were sprayed. The described wetting continued until the solution began to run off. No universal millilitre quantity per plant can be derived from this because plant size and leaf area vary. Frontiers

For implementation, this means collecting dripping liquid and avoiding its entering the substrate. The described process is not a drench treatment.

If clear tissue damage occurs, do not blindly continue treatment according to the calendar. Higher concentrations are not automatically better. Flajšman and colleagues documented leaf spotting, desiccation and leaf loss at high concentrations. The undiluted concentrate must therefore not be equated with the working solution. Frontiers

Planning an S1: Distinguishing the pollen donor from the seed plant

Two roles are useful for practical planning:

The pollen donor is induced to form male flowers.

The seed plant carries the female flowers to be pollinated.

For an S1, both roles can be performed by clones of the same original plant. One clone provides the pollen after treatment. Another remains untreated and carries the seeds.

This separation is a practical consequence of the process: If many flowers on a single plant develop as male, a sufficient number of female flowers for self-pollination is not automatically available at the same time.

Selfing within the same plant is biologically possible. However, it requires functional pollen and receptive female flowers to coincide in time. Spraying an entire plant does not guarantee this combination.

In a feminized cross, by contrast, pollen from original genotype A is transferred to female flowers of another original genotype B. The chemical treatment alone does not determine the ancestry of the seeds.

When can male flowers and pollen be expected?

The change does not occur immediately after spraying. Flower organs must first develop.

In the experiment by DiMatteo and colleagues, male flowers opened in several treated genotypes after approximately 25 days under short-day conditions. Others responded differently. Some flowers that formed did not open or produced little usable pollen.

Visible male flowers are therefore not yet proof of good pollen quality. A particular genetics may respond to STS and still be unreliable as a pollen donor. ResearchGate

Actual development is therefore decisive for planning. A specific calendar day replaces neither checking the opened flowers nor observing pollen release.

Pollination: How the parental contributions come together

In controlled pollination, released pollen is collected and transferred to receptive female flowers. In one scientific study, for example, cannabis pollen was collected on wax paper by gently tapping open flowers and then applied to female flower clusters.

The pollen can germinate on suitable stigmas. The pollen tube enables the subsequent fertilisation process. Only then does the next generation develop within the seed. Frontiers

At a minimum, document the actual pollen donor, the seed plant and the pollination date. The designation “variety A” alone is not sufficient for traceable ancestry if several different original individuals are present.

Unintended foreign pollen must be excluded. As soon as several pollen donors can reach the same female flowers, the ancestry of individual seeds is no longer unambiguous without further examination.

Allow seeds to mature and assess them

Seed set and seed maturity are different developmental stages. A swollen flower area alone does not prove that it contains a fully developed, viable seed.

The condition of the seeds is therefore decisive for harvesting. In a study on hemp genetics, the onset of seed maturity was identified, among other things, by the surrounding bracts opening and the darkening seed coat becoming visible. Another study also used drying bracts and sorting out underdeveloped seeds for assessment. PubMed Central (PMC)

A single harvest date cannot be derived from this for every genetics. Nor is colour alone a complete quality assessment.

For a traceable assessment, different results should be documented separately: How many seeds were harvested? How many were fully developed? How many germinated? And how did the resulting plants behave?

What feminization does not guarantee

The process results neither in an identical copy of the original plant nor in a fully evaluated line.

Feminized offspring can genetically split. An S1 can reveal desirable and undesirable traits. And the successful chemical induction of male flowers does not automatically answer how consistently the untreated offspring will flower as females.

Three statements should therefore be kept separate:

The treatment induced male flowers.

The resulting pollen led to the formation of viable seeds.

The offspring meet the desired quality characteristics.

These are three different outcomes. Evidence of the first is not yet evidence of either of the other two.

Conclusion

Producing feminized cannabis seeds combines plant physiology, chemistry and inheritance. STS can change the flower expression of a genetically female plant. However, which offspring subsequently result depends on the parents actually involved.

For practical implementation, clearly labelled starting materials, correctly calculated concentrations and a treatment schedule selected on a traceable basis are particularly important. Different recipes and spray schedules should not be combined without verification.

Feminization describes the goal of female offspring. S1 describes selfing. The quality of a line must be evaluated beyond this.