Short answer. Spray a genetic female (or one branch of it) with silver thiosulfate or colloidal silver before and into early flower; it grows male flowers that shed fertile pollen. That pollen carries only X chromosomes, so seeds it makes are nearly all female [2][4]. Lubell and Brand found 3 mM STS, sprayed to runoff three times a week apart, the most effective treatment on four hemp cultivars [1]. Treat silver compounds as hazardous chemicals. Feminised seed is only as stable as the donor you reverse: careless feminising is one of the main ways herm-prone lines reach growers.
Breeder's standard. Reverse only a female that has run fully female for at least two cycles under normal conditions, using controlled reversal, never pollen from a plant that hermed by itself. Grow out a sample of every batch, cull and record intersex plants, disclose the herm rate, and don't release a batch from a line that herms. See diagnosing hermaphrodites.
Legal first. Making seed may need a licence where you live, and chemical use on plants can be regulated. Check local rules.
What's happening
Ethylene promotes female flower development in cannabis. Silver ions (Ag⁺) block ethylene receptors, so new flowers develop as male [3]. Silver thiosulfate (STS) carries silver into the plant more effectively than silver nitrate, which is why STS outperformed silver nitrate in early work [3]. Gibberellic acid can also induce male flowers but is less consistent (Supported) [2].
The plant is still genetically XX. Its pollen is X-bearing, so any seed is XX (female), barring rare exceptions [4].
Method comparison
| Method | Typical use | Effectiveness | Notes |
|---|---|---|---|
| STS foliar spray | 3 mM silver, 1:4 silver nitrate:sodium thiosulfate (molar) [1] | High; 3 mM best for all four cultivars tested, 0.3 mM variable [1] | Industry standard; hazardous chemicals |
| Colloidal silver spray | Repeated sprays, often daily | Effective; up to 379 male flowers per plant in [2] | Less potent per spray, more applications; still a silver hazard |
| Gibberellic acid (GA3) | Spray | Lower and inconsistent [2] | Not recommended as primary method |
| Stress-induced "rodelization" (letting a female self late in flower) | None | Unreliable | Selects directly for intersex tendency; never use for seed lines |
Preparing STS (illustrative, matches the published ratio)
To make 1 L of approximately 3 mM silver STS at a 1:4 molar ratio [1][5]:
| Component | Amount | Notes |
|---|---|---|
| Silver nitrate (AgNO₃, 169.87 g/mol) | 0.51 g (3 mmol) | Dissolve in 500 mL distilled water |
| Sodium thiosulfate pentahydrate (Na₂S₂O₃·5H₂O, 248.18 g/mol) | 2.98 g (12 mmol) | Dissolve in 500 mL distilled water. If anhydrous (158.11 g/mol): 1.90 g |
| Mixing | Pour the silver nitrate solution slowly into the thiosulfate solution while stirring | Reverse order can precipitate silver; a clear solution is correct |
| Wetting agent | A few drops of non-ionic surfactant per litre | Optional |
Step by step
- Choose the donor female. Stable across at least two runs: no intersex flowers. Take a clone and reverse the clone (or one branch), not your only mother.
- Separate it. Move it to a separate space once treated; it will shed pollen.
- Spray to runoff (about 20 mL per small plant in [1]) on three occasions, 7 days apart. Growers commonly start a few days before or at the flip to 12/12 (Grower practice)being reviewed. Cover the medium to limit runoff into the pot.
- Wait. Male flowers usually appear about 2-3 weeks after flip and open about 4-6 weeks after flip (Grower practice).
- Collect pollen on paper under still air, sieve and use or store (see pollen collection).
- Pollinate an untreated female (for a feminised cross) or a clone of the same plant (for an S1).
- Destroy the treated plant after pollen collection.
- Record the batch as feminised in the GB-BAT and note the reversal method.
Safety (read this before buying anything)
- Silver nitrate is corrosive and an oxidiser. It causes serious eye damage and stains skin black-grey. Wear nitrile gloves, wrap-around goggles and long sleeves. Work away from food. Keep away from children and pets.
- Spraying makes an inhalable mist. Wear at least a particulate respirator (FFP2/N95 or better) and spray in a ventilated area away from other plants.
- Never consume flowers, leaves or extracts from a treated plant. Silver accumulates.
- Disposal. Silver compounds are toxic to aquatic life. Do not pour leftovers or runoff down the drain or onto soil. Collect waste in a labelled container and take it to a hazardous waste facility, following local rules.
- First aid. Eyes: rinse with water for at least 15 minutes and seek medical help. Skin: wash with soap and water.
- Read and keep the supplier's safety data sheet (SDS).
Common mistakes
- Reversing a plant that already hermaphrodites. You'll sell that tendency in every pack.
- Collecting "free" pollen from a stress herm instead of a controlled reversal. That is the pollen-chucker route to unstable feminised lines.
- Releasing a feminised batch without a grow-out and a recorded herm rate.
- Spraying too late in flower. Existing female flowers won't convert.
- Tap water in the mix: cloudy solution, weak effect.
- Labelling feminised seed as regular, or vice versa. DNA sex markers can check a sample of seedlings [6].
Evidence
- STS induction of fertile male flowers, 3 mM most effective: Established (controlled studies) [1][3].
- Colloidal silver and STS for feminised seed, fertile pollen confirmed: Established [2].
- Hermaphrodite and reversed-female pollen gives female progeny: Established [2][4].
- Spray timing details: Grower practice.
- Herm-prone feminised seed comes mainly from careless feminising (unproven donors, stress-herm pollen, no grow-out): GenetixBay house position (breeder experience). Well-selected, controlled-reversal feminised lines need not herm more than regular seed; controlled cannabis comparisons are limited.
