A female cannabis plant sprayed with a silver solution can grow male flowers. Her pollen has no Y chromosome, so every seed it makes grows into a female plant. This guide explains how to do this step by step without contaminating yourself, your grow room or your crop.
All commercial feminized seed depends on this one fact about plant biology. Part 1 covers safety, why silver works and what the research measured. It also covers how to mix and spray silver thiosulfate and how the colloidal silver method compares. Part 2 covers choosing the two plants, handling pollen, pollinating, harvesting seed and the limits of feminized lines.
1. Safety first: read this before you mix anything
1.1 Two plants, and why one is destroyed
The method uses two plants. The donor is the female you spray so that she grows male flowers and makes pollen. The seed mother is the female you pollinate, and she carries the seeds.
The donor is a sacrificial plant. Every source on this technique says that sprayed plant material is not safe to consume. Silver salts get into the plant tissue, and flushing with plain water does not remove them. You grow the donor only for her pollen and then destroy her.
Never spray the seed mother. If spray drifted onto her, treat her as contaminated too.
1.2 Protective gear
- Nitrile gloves. Silver nitrate stains and burns skin.
- Sealed eye protection. Use goggles that seal around the eyes. Ordinary glasses are not enough.
- A fitted particulate mask. Spraying creates a fine mist at head height, and without a mask you breathe it in.
- A dedicated sprayer. Mark it and never use it for nutrients or water again, so no silver reaches other plants.
1.3 Set up before the first spray
- Isolate the donor in her own tent with her own exhaust fan before the first spray, not after. The spray mist, and later her pollen, must not reach other plants.
- Label everything: stock bottles (contents, strength and date), the spray bottle, the plant, the pollen jar, the pollinated branches and the seed packets. Months later, the labels will be your only record of what you made.
- Dispose of leftovers as chemical waste. Leftover solution and treated plant waste never go into compost or down a drain. Keep them away from children, pets and food.
Translator’s note: At 30–35 °C, which is normal in a Thai grow room, goggles fog up and gloves fill with sweat. Many people then take the gear off halfway through. Cool the room before you start and keep each spraying session short, so the gear can stay on. Ask the laboratory supplier who sold you the chemicals, or your local administration, how to hand over waste that contains silver. If you would rather not handle silver salts at all, the forum topic on rodelization describes a method that uses no chemicals.
Translator’s note: In Thailand, cannabis flower is a controlled herb. That status comes from the Ministry of Public Health Notification on Controlled Herbs (Cannabis), B.E. 2568, in force since 26 June 2025. Flower from a silver-sprayed donor is waste, never a product. It goes out with the rest of the treated plant.
What to do now: buy gloves, goggles, a mask and a new sprayer, and set up the separate tent. Do all of this before you buy any silver salt.
2. Why silver turns a female into a pollen donor
2.1 How sex is set in cannabis
Cannabis is dioecious, which means male and female flowers normally grow on separate plants. A pair of sex chromosomes sets each plant’s sex: XX for a female, XY for a male. The chromosomes work through plant hormones, and chemicals from outside the plant can block those hormones. This is why a spray can change which flowers a genetically female plant builds.
2.2 Ethylene and gibberellin: the two hormones in flower sex
Ethylene is a gas that the plant makes itself, and it pushes flowers toward female. The current explanation comes from studies of other plant families, done long before anyone tested cannabis. Ethylene tells the young flower to stop its stamens from developing. Stamens are the male organs that make pollen. If you block that signal, an XX plant switches on its male program and builds stamens.
Gibberellin is a second group of plant hormones. The form sold for farming is gibberellic acid, GA3, a plant growth regulator. In a 1972 paper in the journal Planta, Mohan Ram and Jaiswal reported that gibberellins, including GA3, induced male flowers on female cannabis plants. This guide gives no GA3 dose or schedule, because the studies it relies on tested only silver.
For breeding, the practical point is this: silver blocks ethylene, and silver is the method with the most published cannabis data. The rest of this guide therefore uses silver.
2.3 How silver blocks ethylene
Arabidopsis is a small weed of the mustard family and the standard plant for laboratory research. A study on it was published in the Journal of Biological Chemistry. It showed that silver acts on the ethylene receptor, the protein in the cell that picks up the ethylene signal.
That receptor normally holds copper in a site that binds metal. Silver takes copper’s place. The receptor still catches ethylene, but it no longer passes the signal on.
Two researchers, Mohan Ram and Sett, were the first to use this on cannabis. In a 1982 paper in the journal Theoretical and Applied Genetics, they reported fertile male flowers on female plants. They got this result with silver nitrate and also with silver thiosulfate, a compound of silver and thiosulfate.
2.4 Why the pollen carries only X chromosomes
The spray changes which organs the plant builds, but not her chromosomes. To make pollen, the plant splits each chromosome pair through meiosis, the cell division that produces pollen and egg cells. An XX plant only has X chromosomes to pass on, so every viable pollen grain carries an X.
| Cross | Pollen carries | Offspring |
|---|---|---|
| Normal male × female | X or Y | about 50% female, 50% male |
| Silver-reversed female × female | X only | XX: all female |
No Y chromosome enters the cross, so none of the seeds can grow into males. Commercial batches are still close to all female rather than exactly 100%. A few intersex or hermaphrodite plants, with both male and female flowers, are normal. Their share rises when plants are stressed.
What to do: when you grow out a feminized batch, record every intersex plant and the conditions it grew in, such as heat or light leaks. This tells you whether the cause was the conditions or the genetics.
3. What the published research measured
Two studies in Plants, a peer-reviewed plant science journal, sprayed silver thiosulfate on cannabis leaves. They gave doses in millimolar (mM), a unit for how much silver is dissolved in a litre. You do not need to follow the chemistry, because Section 4 converts these figures into grams and millilitres.
| Study | Strength | How it was applied | Result |
|---|---|---|---|
| Plants, 2024: leaf spray at the start of flowering or 4 weeks into it | 0.3, 1.5 and 3.0 mM tested | One spray, or three sprays 7 days apart; 20 mL per plant | Male flowers scored about 6 weeks after treatment. Over 65% of the pollen took up a stain that shows which grains are alive. One variety responded at 0.3 mM; the others needed 1.5 mM or more |
| Plants, 2026: study of flower anatomy | 3.5 mM, sprayed until dripping, about 50 mL per plant | Three sprays 4 days apart, one of them before the light change | The growth points that form stamens were visible at 7 days. Pollen was released by day 31, and 54% of it was alive |
The dose that works depends on the variety, and the 2024 study measured this directly. The two Plants studies used 0.3 to 3.5 mM silver, and hobby recipes go below 0.3 mM. So no single figure is the correct dose.
What to do: the first time you reverse a variety, start at the low end and write down the strength and how the plant responded. That record gives you the dose for your next run.
4. Silver thiosulfate: preparing the solution
You cannot buy silver thiosulfate as a powder and dissolve it. It is a complex ion, Ag(S₂O₃)₂³⁻. It forms when you mix a silver nitrate solution with a sodium thiosulfate solution. So you make two separate stock solutions (concentrated solutions kept for mixing) and then combine them.
4.1 The two stock solutions
The table below gives two recipes. The first is worked out for 20 mM stocks from the molar masses of the two salts. The second comes from a tissue culture protocol (a written lab method) by Sigma-Aldrich, a laboratory chemical company.
| Recipe | Silver nitrate | Sodium thiosulfate (anhydrous) | After combining 1 part to 4 parts |
|---|---|---|---|
| 20 mM (calculated) | 3.40 g per litre | 3.16 g per litre | about 4 mM silver |
| 0.1 molar, which equals 100 mM (Sigma-Aldrich tissue-culture protocol) | 1.7 g per 100 mL | 1.58 g per 100 mL | 0.02 molar (20 mM) concentrate |
Sodium thiosulfate is sold in two forms. Anhydrous means dry, with no water in the crystals. Pentahydrate crystals contain water, so you need more by weight for the same amount of thiosulfate. You need 7.8 g of pentahydrate where a recipe calls for 5 g anhydrous.
Use distilled water only. Tap water contains chloride, which reacts with silver immediately and forms a white cloud. That cloud is silver that has dropped out of the solution, and it is lost to the spray.
Translator’s note: In Thailand, laboratory chemical suppliers sell silver nitrate and sodium thiosulfate. Ask for reagent grade, the purity grade used in laboratories. Check whether the thiosulfate is anhydrous or pentahydrate, and ask for the safety data sheet. Bottled drinking water and reverse-osmosis water are not distilled water and can still contain chloride, so buy water labelled distilled or deionised. A kitchen scale cannot weigh 1.58 g accurately, so use a scale that reads to 0.01 g.
4.2 The mixing order matters
Pour the silver nitrate slowly into the sodium thiosulfate, never the other way round. In the reverse order, the mix passes through a stage with too much silver. Silver compounds that do not dissolve then form and settle out.
- Make both stocks at the same strength, each in its own labelled bottle.
- Measure 80 mL of sodium thiosulfate stock into a clean glass container.
- Slowly pour in 20 mL of silver nitrate stock. This gives the 1:4 molar ratio used in the Sigma-Aldrich protocol: one silver particle for every four thiosulfate particles.
- Check the result. It should stay clear. If it turns cloudy or grey, the silver has settled out, so throw that batch away as chemical waste.
4.3 Working spray strength
The concentrate is too strong to spray, so dilute it with distilled water.
| Starting point | Dilution | Silver in the spray |
|---|---|---|
| 0.02 molar concentrate (Sigma-Aldrich recipe) | 1 part to about 5 parts water | about 3.5 mM, as in the 2026 study |
| 0.02 molar concentrate | 1 part to about 12 parts water | about 1.5 mM, the middle of the 2024 study’s range |
| Hobby recipe: 1 g silver nitrate per litre and 5 g thiosulfate per litre, then 100 mL + 100 mL + 600 mL water | Already dilute | about 0.7 mM |
Start low on a variety you have not reversed before. Too much silver burns the leaves and can damage the plant so badly that she never finishes making pollen. Too little only means you have to spray one more time.
What to do: for a new variety, pick one of the lower strengths in this table. Write the date and strength on the plant’s label.
4.4 Storage and shelf life
Sources disagree on how long the solutions keep. Research protocols say to keep stocks in the dark. They also say the combined mix keeps in the fridge for up to a month, but is best made just before use. Hobby guides often claim a much longer fridge life.
Everyone agrees on why the solutions lose strength. Light breaks down silver salts, and the silver thiosulfate slowly reacts with oxygen. The amount of active silver drops, but the label on the bottle stays the same.
Mix the working spray on the day you spray. Old solution of unknown age is the most likely reason a reversal did nothing.
Translator’s note: Thai rooms are much warmer than the cold storage these protocols assume, and a power cut can switch the fridge off for hours. Keep stocks in the fridge in brown glass or in bottles wrapped in foil, with the mixing date on the label. If the fridge lost power, or you do not know how old a bottle is, make a fresh batch. The chemicals cost far less than a reversal cycle that fails.
5. The spray schedule and coverage
Silver has to be inside the plant while her flower growth points are still forming as male or female. So you spray the first time shortly before the light change and spray again after it. The light change is the switch to 12 hours of light and 12 hours of dark that starts flowering. The topic on the vegetative stage and when to switch to flowering explains this step.
| Day (day 0 = light change) | What to do or what happens |
|---|---|
| Day −7 to −1 | First spray, before you switch to 12 h light / 12 h dark on day 0. Some guides keep long days for one more week. |
| Day +4 and day +8 | Second and third sprays, 4 days apart, as in the 2026 study. Three sprays is the number most often reported, and 7-day intervals also worked in the 2024 study. |
| Day +10 to +14 | The first male flower structures appear at the nodes. |
| Day +21 to +31 | The anthers (pollen sacs) open and release pollen. |
Many hobby growers spray every three to five days until male flowers appear. That is a rule of thumb, not a measured result.
Coverage. Spray until the liquid starts to drip off (“to runoff”). Cover the upper growth and the nodes, where leaves join the stem, because that is where flowers form. The 2024 study used 20 mL per plant, and the 2026 study used about 50 mL. Spray with the lights off and keep them off for one to two hours afterwards. Do not pour the solution onto the roots instead, because the published protocols use leaf sprays only.
Translator’s note: Outdoors in Thailand, days stay close to 12 hours long all year, so you cannot set a clear “day 0”. The topic on why photoperiod plants flower early in southern Thailand explains this. Reverse donors indoors, in a tent where a timer controls the light. Indoors, rain also cannot wash the spray off.
What to do: before the first spray, write out the full calendar and stick it on the tent door. Include the spray dates, the light-change date and the expected pollen window.
6. The colloidal silver method
Colloidal silver is a liquid with tiny silver particles floating in water. Its strength is given in ppm (parts per million; 1 ppm is 1 mg per litre).
6.1 Working strength and spray window
In a 2021 study in Frontiers in Plant Science, Flajšman and colleagues sprayed 30 ppm colloidal silver every day until male flowers appeared, and the method worked. A single spray produced almost no male flowers. Growers usually start in late vegetative growth or at the light change. The study did not test stronger solutions, and the number of days before male flowers appear varies.
No peer-reviewed cannabis trial supports these figures. They come from breeding guides and grower reports, and those sources disagree with each other about which strengths work.
6.2 Comparing the two methods
| Point | Silver thiosulfate | Colloidal silver |
|---|---|---|
| Evidence | Peer-reviewed cannabis studies, 1982 to 2026 | One peer-reviewed study (2021); otherwise grower reports |
| Number of sprays | One to three | Daily, until male flowers appear |
| Reliability | Consistent; the dose varies by variety | Inconsistent; low-strength attempts often fail |
Silver thiosulfate is more reliable and better documented. Colloidal silver is easier to get started with, but it does not work better. Concentrated silver nitrate is the more dangerous chemical to handle. If a colloidal attempt fails, the most likely cause is that you do not know how much silver the product really contains.
Translator’s note: Products sold as colloidal silver vary in strength, and you cannot check the real silver content at home. Whichever method you use, wear all the protective gear listed in Section 1.
What to do: if you need a reliable result, use silver thiosulfate. If you use colloidal silver, choose a product that states a strength of at least 30 ppm. Spray it daily until male flowers appear, and record each spray.
References (Part 1)
- Mohan Ram HY, Sett R (1982). Induction of fertile male flowers in genetically female Cannabis sativa plants by silver nitrate and silver thiosulphate anionic complex. Theoretical and Applied Genetics 62(4): 369–375. Client Challenge
- Flajšman M, Slapnik M, Murovec J (2021). Production of feminized seeds of high CBD Cannabis sativa L. by manipulation of sex expression and its application to breeding. Frontiers in Plant Science 12: 718092.
- Mohan Ram HY, Jaiswal VS (1972). Induction of male flowers on female plants of Cannabis sativa by gibberellins and its inhibition by abscisic acid. Planta 105(3): 263–266.
- McDaniel BK, Binder BM (2012). ETHYLENE RECEPTOR 1 (ETR1) is sufficient and has the predominant role in mediating inhibition of ethylene responses by silver in Arabidopsis thaliana. Journal of Biological Chemistry 287(31): 26094–26103. Redirecting
- Kim J, Kim D-G, Kim WJ, Lee Y-J, Lee SH, Ryu J, Kim JH, Kim SH (2024). Characterization of male flower induction by silver thiosulfate foliar spray in female cannabis at the middle reproductive stage for breeding. Plants 13(17): 2429. https://doi.org/10.3390/plants13172429
- Judkevich MD, Marassi MA, Gonzalez AM (2026). Anatomical and ontogenetic basis of silver thiosulfate-induced masculinization in genetically female Cannabis sativa. Plants 15(14): 2153. https://doi.org/10.3390/plants15142153
- Sigma-Aldrich (Merck). Silver Thiosulfate: plant tissue culture protocol (0.1 molar stocks). https://www.sigmaaldrich.com/technical-documents/protocol/cell-culture-and-cell-culture-analysis/plant-tissue-culture/silver-thiosulfate
If you have a question about mixing or spraying, post it below this topic. Include the strength you used and the age of your solution.
This article is part of the Asiannabis Community educational series on cannabis genetics and breeding. It is for education within jurisdictions where growing cannabis is legally permitted.
Continued in Part 2: Making Feminized Seeds with Silver: Choosing the Parents, Handling Pollen and Harvesting Seed (Part 2 of 2)