STS Spray Timing

What STS is

Silver thiosulfate (STS) is a complex of silver ions stabilized by sodium thiosulfate. In plants, silver ions block ethylene receptors ^[1]. Ethylene is the hormone that pushes cannabis toward female sex expression, so blocking it during the sex-determination window causes a genetically female plant to develop staminate (male) flowers instead of pistillate ones Strong evidence^[2]. Those male flowers produce pollen that carries only X chromosomes, so any seed made with that pollen is female ("feminized") Strong evidence^[2].

STS is the industry standard for feminized seed production, having largely replaced colloidal silver and gibberellic acid because it works with a single application window and gives dense, viable pollen sacs ^[3].

Why growers use it

Two reasons:

1. To make feminized seeds from a single prized female. If you have a keeper cut and no male, STS lets you self-pollinate it (S1 seeds) or cross it to another female. All resulting seeds are female Strong evidence^[2].
2. To preserve a clone-only line in seed form. Reversing a female and dusting another female of the same or different genetics locks the line into storable seed rather than perpetual cuttings.

STS does not increase yield, potency, or resin. It is purely a breeding tool.

When to start spraying

Timing is the whole game. Cannabis commits to a sex expression pattern in the first 1–2 weeks after the photoperiod flips to 12/12 Weak / limited^[3]. You want silver in the tissue during that commitment window.

Standard protocol:

• First spray: 3–7 days before flipping to 12/12, or on the day of the flip. This front-loads silver before pistils would normally form.
• Second spray (optional but common): 7 days after the first, still within the first week of 12/12.
• Third spray (rare): Only if the first two produced weak sac development by day 14 of flower.

After the plant has already put out visible pistils for a week or more, STS becomes progressively less effective. You can still get some sacs on a plant sprayed at day 14 of flower, but coverage will be patchy Anecdote.

Pollen typically matures and starts releasing around days 3–5 of flowering after the last spray, with peak shed around weeks 3–5 of the reversed plant's flower cycle Anecdote.

How to do it — step by step

Safety first. Silver nitrate is corrosive and stains skin and surfaces permanently black. Wear nitrile gloves, eye protection, and a respirator. Mix and spray in a ventilated area away from other plants and pets.

Mixing 0.02 M STS (standard concentration):

1. In distilled water, dissolve 1.7 g silver nitrate (AgNO₃) per 100 mL — this is Solution A (0.1 M).
2. Separately, dissolve 7.9 g sodium thiosulfate pentahydrate (Na₂S₂O₃·5H₂O) per 100 mL distilled water — Solution B (0.32 M, roughly 4× molar excess).
3. Slowly pour Solution A into Solution B while stirring. Never reverse the order — reversing precipitates silver sulfide.
4. The mix should be clear or faintly yellow. Any brown/black cloudiness means it's ruined.
5. This is your stock. Dilute 1:9 with distilled water for the working spray (roughly 0.002 M) ^[1][3].

Application:

1. Isolate the donor plant in a separate space with its own light and ventilation. STS-treated plants and their eventual pollen must never contact your flowering crop unless you want seeded bud.
2. Spray to runoff on all leaves and growing tips, top and bottom of leaves. Coverage matters more than volume.
3. Do it at lights-off or just before, so the solution doesn't evaporate instantly and doesn't cause light-refraction burn.
4. Rinse the sprayer thoroughly. Store leftover working solution in the fridge, in the dark, for up to about a week; stock keeps 1–2 months refrigerated Weak / limited.
5. Flip the room to 12/12 within a few days of the first spray (or immediately, depending on protocol).
6. Watch for staminate flower clusters forming where buds would normally develop, usually by day 10–14 of 12/12.
7. Collect pollen once anthers dehisce. Dust receptive females (any female of any age past week 2 of flower) to make seeds.

Common mistakes

• Spraying too late. Waiting until week 2–3 of flower means most sites are already committed to female expression. Front-load it.
• Spraying the whole garden. STS residue can drift. Reverse in isolation.
• Mixing solutions in the wrong order. Silver nitrate into sodium thiosulfate, not the reverse.
• Using tap water. Chlorine and minerals can precipitate the silver complex. Distilled only.
• Reusing the sprayed plant for consumption. Silver-treated tissue is not for smoking. Cull it after pollen collection.
• Assuming S1 seeds are clones. Selfing a female produces genetic recombination, not identical copies. S1 populations vary Strong evidence^[4]. Expect variation and phenotype hunt accordingly.
• Believing "stress reversals" (light leaks, extreme heat) are equivalent. Stress hermies produce unstable offspring with higher hermaphrodite rates. STS reversal is chemically targeted and gives cleaner results Weak / limited^[3].

Related techniques

• Colloidal silver (CS): Older method, weaker and less reliable. Needs daily spraying for 2–3 weeks. Mostly superseded by STS Anecdote.
• Gibberellic acid (GA₃): Also induces male flowers in females but produces less viable pollen and is finicky Weak / limited^[5].
• Rodelization: Letting a female self-hermie from prolonged flowering. Cheap, natural, but produces fewer seeds and selects for hermaphroditic tendency in offspring Disputed.
• Selfing (S1) vs. FF crosses: Selfing gives S1 seeds; crossing two feminized females gives an F1 that is still 100% female but with broader genetic variation.

See also: Feminized Seeds, Selfing and S1 Lines, Pollen Collection and Storage.

Sources

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