Reversal with Silver Thiosulfate (STS)

Growing, buying or possessing cannabis and seeds is legal only in some places. Check the law where you live before acting on this information.

What it is

Silver thiosulfate (STS) is a chemical complex formed by mixing silver nitrate and sodium thiosulfate. When applied as a foliar spray to a female cannabis plant during the transition to flower or early flowering, it disrupts ethylene signaling—a plant hormone that normally directs female flower development Strong evidence^[1][2].

Without normal ethylene signaling, the female plant's tissues begin producing male (staminate) flowers containing viable pollen. Because cannabis females have XX chromosomes, their pollen carries only X chromosomes. When this pollen pollinates another XX female, the resulting seeds are genetically XX Strong evidence^[1][2].

Unlike stress-induced late hermaphroditism (rodelization) or gibberellic acid treatment, STS produces structurally normal male flowers with high pollen viability Strong evidence^[1].

Important note on genetics: While XX seeds are predicted under this chromosome model, not every plant from an XX × XX cross will develop exclusively female flowers in practice. Intersex expression, environmental stress, and genetic background can produce flowers showing both male and female traits. Seed stock from STS reversal typically produces a high proportion of female plants (often 95–99%), but absolute 100% female germination is not guaranteed Weak / limited^[3].

Why growers use it

Growers use STS reversal for three main purposes:

Feminized seed production: Reverse a keeper female with STS, collect its pollen, and cross it with another female. All resulting seeds are XX and expected to produce predominantly female plants. This is faster and more cost-effective than repeatedly cloning desirable plants and is the standard method in commercial breeding Strong evidence^[1][2].

S1 production (selfing): Spray a clone of your desired plant, collect its pollen, and pollinate another clone of the same plant. The offspring are S1 seeds—genetically very similar to the parent but expressing hidden recessive traits. This allows you to preserve a clone-only cultivar in seed form and explore its genetic variation Strong evidence^[3].

Genetic preservation: Seeds store reliably for years in a freezer; mother plants require constant care, grow space, nutrients, and are vulnerable to pests, equipment failure, and (in prohibitionist regions) legal seizure. Seeds reduce this maintenance burden Strong evidence^[3].

STS is widely preferred over colloidal silver (a cheaper alternative) because STS produces predictable, abundant male flowers with fewer applications. Gibberellic acid produces inconsistent results in cannabis and is rarely used for this purpose Weak / limited^[4].

Timing and planning

For successful reversal and pollination, coordinate the flowering schedules of the donor (reversed) plant and the receiver (seeded) plant.

When to start spraying: Begin applications on the day you switch to 12/12 photoperiod, or up to one week before the switch. The goal is for the reversed donor to produce mature, pollen-shedding male flowers during weeks 4–6 of its own flower cycle, when the receiver plant will have abundant receptive pistils (typically weeks 3–4 of the receiver's flowering period).

Donor plant: Allow 3–6 weeks from first spray to mature pollen production, depending on cultivar and environmental conditions.

Receiver plant: Stagger its flower switch so it enters flower 1–3 weeks after the donor enters flower. This ensures the receiver has developed enough flower structure to catch and retain pollen effectively before seeding.

When to stop spraying: Cease applications as soon as you can visually confirm that stamens (male flowers) are forming on the reversed plant. Continued spraying after male flower development is visible wastes material and stresses tissues. The plant will continue maturing pollen without further STS applications.

How to do it

Safety precautions

Silver compounds pose health hazards with repeated skin contact, eye exposure, or inhalation. Wear nitrile gloves, eye protection, and at minimum an N95 mask (P100 respirator strongly preferred). Spray only in a well-ventilated space, never in an occupied room. Always use a dedicated spray bottle labeled "STS — DO NOT USE FOR ANYTHING ELSE" to prevent cross-contamination with other sprays.

Isolate the reversed plant completely from your main flowering area once it produces pollen. Cannabis pollen is fine and airborne; any contact with other flowering plants will seed them with hundreds of unwanted seeds.

Do not consume any plant material from an STS-treated plant. Silver residues may persist in buds. Additionally, the STS-reversed plant is hormonally redirected toward male reproduction, not cannabinoid production.

Disposal: Do not compost an STS-treated plant if your compost is used in edible gardens or will be returned to soil. Silver compounds can persist in soil and potentially harm soil microbes and aquatic organisms if runoff occurs. Consider disposing of the reversed plant via yard waste or secured trash if local regulations permit. Consult local environmental guidance for silver-compound disposal.

Preparing STS

STS must be mixed fresh from silver nitrate and sodium thiosulfate components. Distilled water is required; tap water contains chlorine and minerals that can interfere with complex formation.

Mix according to established ratios, stirring thoroughly until the solution is clear and colorless. If the mixture turns cloudy or brown, the complex formation has failed; discard and restart Strong evidence^[1]. Many growers purchase pre-mixed STS concentrate from breeding supply vendors to avoid chemistry—this is a practical alternative if available and legal in your region.

Application overview

Apply the working solution to the donor plant via fine-mist spray, ensuring coverage of all developing bud sites, leaf undersides, and stems until solution drips. Timing, frequency, and concentration vary by cultivar, growing conditions, and water quality; consult the product instructions if using commercial STS, or reference peer-reviewed protocols ^[1][2] if mixing your own.

Repeat applications on a weekly schedule for 3–4 total applications. Discontinue spraying once stamens are clearly visible forming on the reversed plant.

Pollen collection and storage

After spraying stops, allow male flowers to mature for 1–2 weeks. When anthers (pollen sacs) swell and their tips split open, gently tap branches over a clean sheet of wax paper to collect pollen. Alternatively, place the entire branch or plant in a clean plastic bag and shake gently.

Spread collected pollen thinly on wax paper and dry it in a cool, dark location with a desiccant (silica gel) for 24 hours. Once thoroughly dry, transfer pollen to an airtight container with fresh desiccant and store in a freezer at −18°C or colder. Viability depends on moisture control, temperature stability, and cultivar; pollen stored under ideal conditions can remain viable for at least 1–2 years, though longer-term viability data are limited Weak / limited^[4].

Pollination

When the receiver plant has been flowering for 2–4 weeks and displays abundant white pistils, apply collected pollen using a small clean brush, carefully touching pollen to pistil clusters. Seeds will mature over approximately 4–6 weeks. Avoid foliar sprays, misting, or overhead watering on the receiver during this period to prevent washing off pollen.

Common mistakes

Failing to isolate the donor plant: Cannabis pollen is airborne and will contaminate other flowering plants in the same space, producing hundreds of unwanted seeds. Complete physical separation (different room, sealed tent, or container) is essential.

Insufficient spraying: A single application rarely induces reliable male flower production. Plan on at least 3–4 applications over several weeks. Most reported protocols use weekly applications.

Storing mixed STS too long: Concentrated STS begins to degrade after several weeks if stored at room temperature. Working solution degrades faster; use it the same day if possible, or within a few days at most. Store all STS in dark glass bottles in a cool location.

Confusing S1 seeds with clones: S1 seeds (from self-pollination) are not genetic clones of the parent plant. Selfing reveals recessive traits, and offspring phenotypes range from identical to the parent to noticeably different. Some offspring may be superior, others inferior Strong evidence^[3]. Expect and plan for phenotypic variation.

Neglecting personal protective equipment: Chronic inhalation or skin contact with silver compounds can cause argyria (permanent gray-blue skin discoloration) and other systemic effects. Use gloves, eye protection, and respiratory protection every application.

Smoking or consuming reversed plant tissue: Beyond the risk of residual silver in buds, the reversed plant is hormonally directed toward pollen production, not cannabinoid accumulation. The buds lack the normal cannabinoid profile of untreated female plants and should be discarded.

Overapplying STS: Excessive or overly concentrated applications can cause leaf burn, necrosis, or tissue damage without increasing male flower production. Follow established protocols and discontinue once male flowers are visible.

Related techniques and alternatives

Colloidal silver (CS): An alternative silver-based reversing agent that is cheaper and easier to source than STS components. However, colloidal silver typically produces fewer or more delayed male flowers and requires more frequent applications (often daily over 2–3 weeks) compared to STS weekly protocols Weak / limited^[4]. Many professional breeders prefer STS for reliability and lower labor.

Rodelization: Extreme late-flowering stress can occasionally trigger hermaphroditic pollen production in female plants, allowing self-pollination to produce feminized seeds. However, this method is less predictable than STS, and there is disputed evidence regarding whether offspring from rodelized pollen carry higher rates of hermaphroditism Disputed. Rodelization is more commonly used as an emergency rescue technique than a planned breeding method.

Gibberellic acid (GA₃): This plant hormone induces male flowers in some species (notably hops) but shows inconsistent results in cannabis and is seldom used for practical breeding Weak / limited.

Traditional male-female crossing: Breeding a true genetic male plant with a female remains the foundational method for line development and new trait combinations. STS-reversed females produce XX × XX crosses, generating feminized seeds but not new stable lines. For creating novel cultivars or combining multiple male lines, crossing genetic males with females is still the standard approach.

Sources

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