Reversal with Silver Thiosulfate (STS)

What it is

Silver thiosulfate (STS) is a complex formed by reacting silver nitrate (AgNO₃) with sodium thiosulfate (Na₂S₂O₃). When sprayed on a female cannabis plant entering flower, it blocks ethylene signaling in the plant's tissues. Ethylene normally promotes female flower development; without it, the plant defaults to producing male flowers (staminate flowers) that release viable pollen Strong evidence^[1][2].

The pollen from an STS-reversed female carries only X chromosomes (cannabis sex determination is XY, with females XX). Cross that pollen onto another female and every resulting seed is XX — feminized Strong evidence^[2]. This is the foundation of nearly all commercial feminized seed production today.

Why growers use it

Three main reasons:

1. Feminized seed production. Cross reversed Plant A's pollen onto Plant B and get feminized seeds of the A×B cross.
2. S1 lines (selfing). Spray a clone of your keeper plant, collect its pollen, and pollinate another clone of the same plant. The offspring are S1 seeds — a way to preserve a clone-only cut in seed form and explore its genetic variation Strong evidence^[3].
3. Preserving genetics without keeping mothers. Seeds store for years; mother plants need constant care and can be lost to pests, power outages, or law enforcement.

STS is preferred over colloidal silver (CS) because it is more potent, more consistent, and requires fewer applications. CS works but often produces sparse, late-arriving male flowers Weak / limited^[4]. Gibberellic acid also induces some male flowers but is generally less reliable for cannabis Weak / limited.

When to start

Start spraying either the day you flip to 12/12, or up to a week before. The goal is for male flowers to be mature and shedding pollen around weeks 4–6 of flower, which is when your receiving female plant will have plenty of receptive pistils.

Time your two plants so they're on compatible schedules:

• Donor (reversed plant): flipped first or simultaneously; needs ~3–6 weeks from spray to pollen drop.
• Receiver (seeded plant): should have abundant white pistils when pollen is ready, typically week 3–4 of its own flower.

Stop spraying after 3–4 weekly applications, or once you can clearly see stamens (banana-shaped male flowers) forming. Spraying past that point is wasteful and stresses the plant unnecessarily.

How to do it (step-by-step)

Safety first. STS is mildly toxic and should not contact your skin, eyes, or lungs. Wear nitrile gloves, eye protection, and at minimum an N95 (P100 is better). Spray in a ventilated area, never in your living space. The reversed plant and its pollen are for breeding only — do not smoke buds from a sprayed plant.

Stock solutions (mix in glass, use distilled water only):

• Solution A: 0.5 g silver nitrate in 500 mL distilled water.
• Solution B: 2.5 g sodium thiosulfate (anhydrous) in 500 mL distilled water.

Combining: Slowly pour Solution A into Solution B while stirring. The result is your concentrated STS. It should be clear; if it turns cloudy or brown, you mixed wrong or used tap water — discard and restart Strong evidence^[1].

Working solution: Dilute the concentrate 1:9 with distilled water (e.g., 100 mL concentrate + 900 mL water). This is what you spray.

Application:

1. Label a dedicated spray bottle "STS — DO NOT USE FOR ANYTHING ELSE."
2. Spray the donor plant to runoff, coating all bud sites, undersides of leaves, and stems.
3. Do this just before lights-off so the solution doesn't photodegrade or burn leaves under intense light.
4. Isolate the plant from your main flower room — once it starts producing pollen, that pollen will travel.
5. Repeat once a week for 2–3 more weeks, or until stamens are clearly forming.
6. Stop spraying. Wait for the male flowers to mature (roughly week 4–6 of flower from the flip).

Pollen collection: When stamens swell and split, tap branches over wax paper, or bag the entire plant briefly. Dry pollen 24 hours over silica gel, then store in the freezer in airtight containers with desiccant.

Pollination: Apply pollen to the receiver plant in week 2–4 of flower using a small paintbrush. Seeds mature in 4–6 weeks.

Common mistakes

• Using tap water. Chlorine and minerals destabilize STS. Distilled only.
• Mixing A into B in the wrong order. Pour silver nitrate solution into sodium thiosulfate solution, not the reverse.
• Storing mixed STS for weeks. Concentrate keeps a few weeks refrigerated in a dark bottle; working solution should be used same-day or within a few days Weak / limited.
• Spraying once and quitting. One spray is rarely enough. Plan on 2–4 applications.
• Not isolating the donor. Cannabis pollen is fine and travels on air currents and clothing. A reversed plant in your main flower room will accidentally seed your entire crop.
• Smoking the reversed plant. The buds are chemically contaminated and the plant has been redirected toward male flower production anyway. Compost it after pollen collection.
• Assuming S1 seeds are clones. They aren't. Selfing reveals recessive traits and produces a range of phenotypes, some better and some worse than the parent Strong evidence^[3].
• Skipping PPE. Silver compounds can cause argyria (skin discoloration) with chronic exposure. Don't be casual about it.

Related techniques

• Colloidal silver (CS): Cheaper, easier to source, but less reliable. Requires daily spraying for 2–3 weeks Weak / limited^[4].
• Rodelization: Stressing a female into late-flower hermaphroditism. Produces feminized seeds but with much higher rates of hermaphrodite offspring — generally considered inferior Disputed.
• Gibberellic acid (GA₃): Used in hops and some other species; works inconsistently in cannabis Weak / limited.
• Traditional male-female breeding: Still the gold standard for creating new stable lines. Feminized seeds are best for production, not foundational breeding work Anecdote.

For the next step in the workflow, see Pollen Collection and Storage and Making Feminized Seeds.

Sources

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