Silver thiosulfate (STS) is the most reliable method for producing feminized cannabis seeds at home. The process involves spraying a female plant with STS solution to force it to grow male pollen sacs, then using that pollen to fertilize another female plant. Because both parents are female, the resulting seeds carry only female genetics, producing an all-female crop. Here’s how to do it from start to finish.
Why STS Works
Female cannabis plants rely on the hormone ethylene to develop female flowers. STS blocks that signal. Silver ions from the solution outcompete the copper that ethylene receptors need to function properly. Without working ethylene receptors, the plant’s hormonal signaling shifts, and it begins producing male flowers (pollen sacs) instead of female ones.
The plant’s genetics haven’t changed. It’s still genetically female. The pollen it produces contains only female chromosomes (XX), so when that pollen lands on a normal female flower, the seeds that form are almost exclusively female.
Chemicals You Need
STS requires two chemicals, both available from lab supply companies or some garden suppliers:
- Silver nitrate (AgNO₃), 99% purity
- Sodium thiosulfate (Na₂S₂O₃), 99% purity, anhydrous form
You also need distilled water, a scale accurate to 0.01 grams, two separate containers for stock solutions, a dark bottle for storage, and a spray bottle for application. Wear nitrile gloves throughout. Silver nitrate stains skin brown on contact, and the discoloration can take days to fade.
Mixing the Stock Solutions
You’ll prepare two separate stock solutions first, then combine them on the day you spray.
Stock Solution A: Silver Nitrate
Dissolve 1.7 g of silver nitrate into 100 ml of distilled water. This creates a 0.1 M solution. Store it in a dark bottle or wrap the container in foil, because silver nitrate breaks down when exposed to light. Ideally, mix this fresh the same day you plan to use it.
Stock Solution B: Sodium Thiosulfate
Dissolve 1.58 g of sodium thiosulfate into 100 ml of distilled water. This also creates a 0.1 M solution. Sodium thiosulfate is more stable than silver nitrate, but storing it in a cool, dark place is still good practice.
Combining Into Working STS
The critical ratio is 1 part silver nitrate to 4 parts sodium thiosulfate. On application day, slowly pour 20 ml of Stock Solution A into 80 ml of Stock Solution B. Pour slowly and stir continuously. The order matters: always add the silver nitrate into the sodium thiosulfate, not the other way around. This produces 100 ml of 0.02 M STS working solution, enough for multiple applications on one or two plants.
Mix only what you plan to use that day. Once combined, STS gradually loses potency. A fresh batch every application day gives the best results.
Choosing and Preparing Your Plants
You need two female plants of the same strain (or different strains if you’re crossing). Designate one as the “donor,” which you’ll spray to produce pollen. The other is the “recipient,” which will be pollinated and carry the seeds.
Select a healthy, vigorous donor plant. Stressed or unhealthy plants may respond inconsistently to the reversal. Keep the donor completely isolated from the recipient throughout the spraying and pollen development period, either in a separate tent or a closed-off room with its own ventilation. Even a small amount of stray pollen can seed your recipient prematurely.
Spray Schedule and Technique
The first application should happen the day you flip your donor plant to a 12/12 light cycle, or up to a few days before the flip. Spray the entire plant thoroughly: tops and undersides of leaves, stems, and especially the branch nodes where flowers will form. You want full, dripping coverage.
Apply a second spray 10 to 14 days later. Some growers add a third application 7 to 10 days after the second for stubborn strains. Spray with the lights off and let the plant dry in the dark, since silver compounds degrade in light.
Within 3 to 4 weeks of the first application, you should see pollen sacs forming at the nodes where female flowers would normally appear. These sacs look like small clusters of tiny balls. They typically take 4 to 5 weeks total from the light flip to open and release pollen.
Collecting and Storing Pollen
Reversed female plants generally produce less pollen than true males, so careful collection matters. Keep humidity below 45% in your collection area. Moisture destroys pollen viability by triggering premature germination of the grains.
When the sacs begin to split open, place the plant over a clean, flat surface (a dark-colored table makes the pale pollen easier to see). Position a fine mesh screen between the plant and the table. Gently shake the branches so pollen falls through the screen while plant debris stays trapped above. Use tweezers to remove any stray flower parts that get through.
Let the collected pollen dry under a dehumidifier for at least 12 hours before storing it.
Short-Term Storage (Up to 6 Weeks)
Pour the dried pollen into small airtight test tubes or vials and place them in the refrigerator. This keeps pollen viable for roughly 3 to 6 weeks.
Long-Term Storage (Months to Years)
For longer storage, place the pollen in a small paper pouch inside an airtight glass jar with a couple of silica gel packets. After 48 hours of desiccation, mix the pollen 1:1 with dry cooking flour (the flour absorbs residual moisture and helps distribute the pollen later). Transfer the mixture into small sealed vials and store in the freezer. When you’re ready to use it, let the vials warm to room temperature before opening. Opening a cold vial draws in warm, moist air that can kill the pollen instantly. Stored properly, frozen pollen can remain viable for several years.
Pollinating the Recipient Plant
Your recipient female should be about 2 to 3 weeks into flowering when you pollinate. At this stage, the pistils (white hairs) are abundant and receptive. Turn off all fans in the grow space to prevent pollen from drifting where you don’t want it.
Use a small brush to dab pollen directly onto the female flowers you want to seed. If you only want to seed a few branches, isolate them with a plastic bag, shake some pollen inside, seal the bag for an hour or two, then remove it. Mist the surrounding area with plain water afterward to neutralize any stray pollen, since water renders pollen non-viable.
Seeds typically mature 4 to 6 weeks after pollination. You’ll know they’re ready when the calyxes swell and the seeds inside are dark, firm, and tiger-striped. Pale or green seeds are immature and unlikely to germinate well.
Common Reasons for Failure
The most frequent cause of failed reversal is weak or degraded STS. If your silver nitrate stock was exposed to light, or if you mixed the working solution hours before spraying, the silver ions may have already broken down. Always mix fresh and apply the same day.
Incomplete coverage is another common issue. STS works locally on the tissue it contacts. If you miss branches or don’t saturate the undersides of leaves, those areas may still produce female flowers while sprayed areas develop pollen sacs, resulting in a partially reversed plant that produces very little usable pollen.
Timing matters too. Starting sprays after the plant has already begun forming female flowers makes reversal far more difficult. The plant has already committed hormonally. Begin on or before the day you switch to 12/12 lighting.
Some strains are simply more resistant to reversal than others. If a strain doesn’t respond well to two applications, a third spray and a higher concentration may be needed. Genetics with a natural tendency to produce occasional male flowers (hermaphrodite-prone strains) tend to reverse more easily, though growers generally avoid these for breeding since the hermaphrodite trait can carry into the offspring.
Safety and Disposal
Silver nitrate is classified as a non-food-use compound. While it is not toxic to humans at the concentrations used in STS, the treated donor plant should never be consumed, smoked, or processed for extracts. Only the recipient plant and its seeds are safe. The donor plant is a tool, not a crop.
Wear gloves during every step of mixing and spraying. Silver nitrate stains skin and can irritate eyes. Work in a ventilated area and avoid inhaling the spray mist. Dispose of leftover solution responsibly: don’t pour it down the drain. Small amounts can be neutralized with a salt solution to precipitate silver chloride, which is less environmentally harmful. Check your local regulations for chemical disposal guidelines.