Cannabis Plant Sex: Telling Males, Females and Hermaphrodites Apart, DNA Testing, and How to Prevent Herms

ChillTokin · July 24, 2026, 7:20am

Cannabis (Cannabis sativa L.) is a dioecious plant. This means male and female flowers normally grow on separate plants. Some plants, however, grow both kinds of flower. These hermaphrodites (“herms”) are fairly common. If you miss one, its pollen can put seeds through your crop and cut the value of the harvest.

This article covers four things: how a plant’s sex is set, how to identify it early, why plants turn herm, and how to prevent herms. Paragraphs marked “Translator’s note” add conditions specific to Thailand and Southeast Asia.

Five things to do this season

Check your flowering room for light leaks before every cycle. A leak during the dark period is the most common cause of stress herms.
Inspect every plant daily during the first two weeks of flowering. The goal is to find male flowers before they open and release pollen.
Keep the flowering room at 20–26 °C and never above 28 °C. Plants that stay above 30 °C for long periods in flower are more likely to turn herm.
Remove any plant that is genetically herm. Never take cuttings or seed from it, because the trait passes to the next generation.
Keep a simple record of herms per strain and per seed batch. After a few cycles, the record shows you which genetics are stable.

1. How cannabis sex is determined

Three kinds of plant

Female (): forms the buds (flower clusters) that growers harvest. Female flowers have the highest cannabinoid content. Cannabinoids are the active compounds, such as THC and CBD.
Male (): forms pollen sacs instead of buds. A single male can pollinate an entire grow room. In one study, fertilized flowers had about 75% less total cannabinoid content than unpollinated flowers in a THC-rich plant, and about 60% less in a CBD-rich plant (Feder et al., 2021).
Hermaphrodite: one plant carries both male and female flowers. There are two types. A true hermaphrodite carries the trait in its genes. A stress-induced hermaphrodite is reacting to its environment. Section 3 covers both types.

The XX/XY system

Cannabis has sex chromosomes, as humans do. XX plants are female and XY plants are male. A large part of the X chromosome does not swap DNA with the Y. This “non-recombining region” has let the two chromosomes grow very different over time.

Prentout et al. (2020) suggest the cannabis XY pair may be the oldest plant sex-chromosome system documented so far. A follow-up study (Prentout et al., 2021) found that hops (Humulus) share the same pair. In practice, this matters because maleness is carried on the Y chromosome. A lab can look for a piece of DNA found only on the Y and identify males at the seedling stage (section 2).

Monoecy1: a sex-related region on the X chromosome (2025)

Some cultivars are monoecious, meaning each plant normally makes both male and female flowers. Others are dioecious, with separate male and female plants. A team from University College Dublin (Ireland), working with CNRS and Université Paris-Saclay (France), searched for the genes behind this difference. Their study (Toscani et al.) came out as a preprint in July 2025 and was published in New Phytologist in 2026 (doi:10.1111/nph.71185).

The team used QTL mapping. In this method, researchers cross plants, record a trait in the offspring, and find which stretch of chromosome is inherited along with it. They named the region they found Monoecy1:

It sits on the X chromosome, in the oldest and most diverged part of the sex chromosomes.
It holds three candidate genes within about 60,000 DNA letters: CsREM16, lncREM16 and CsKAN4.
CsREM16 and CsKAN4 are related to known transcription-factor genes, which switch other genes on or off. lncREM16 is a long non-coding RNA gene, which makes RNA but no protein.
The authors propose that the three genes act together to control both male-or-female and monoecious-or-dioecious sex. The paper’s title says the genes “potentially” control sex. Their role has not yet been proven by experiment.

So researchers have found the region, but they have not yet proved which gene is responsible. That means no seed can be guaranteed herm-free on the basis of this research. Your own herm records for each strain are still the best guide to which genetics are stable.

Ethylene and the CsETR1 receptor

Ethylene is a plant hormone that the plant gives off as a gas. High ethylene suppresses male flowers, so the plant stays female. CsETR1 is the gene for the ethylene receptor, the protein that lets plant cells sense ethylene.

Silver thiosulfate (STS) blocks this receptor. A female (XX) plant treated with STS then forms male flowers. That pollen carries only X chromosomes, so the seeds it produces are female. These are called feminized seeds. Section 4 compares the two common methods for making them.

2. How to identify a plant’s sex

Reading preflowers

Preflowers are the first small flowers a plant makes before full flowering. They appear at the nodes, where leaf stalks join the main stem, usually in weeks 4–6 of vegetative growth. A female preflower shows a pistil, which looks like one or two fine white hairs. A male preflower is a small round pollen sac on a short stalk.

Reading preflowers by eye is about 85–90% accurate, so roughly one call in ten may be wrong. The method costs nothing and suits small growers. A magnifying loupe makes the small parts much easier to see.

Translator’s note: Outdoors in Thailand, day length stays fairly close to 12 hours all year. Photoperiod plants can therefore start flowering earlier than guides from temperate countries suggest. See why photoperiod plants flower early at Thai latitudes. Start checking the nodes of outdoor plants before the 4–6 week mark, so early preflowers do not catch you out.

Flowering from seed on 12 hours of light

In this method, seedlings get 12 hours of light and 12 hours of darkness from day one (“12/12 from seed”). This forces early flowering, and sex shows at around days 10–14. Accuracy is about 95%. It costs no money, but it stresses the plants and costs yield. It suits growers who need to screen large numbers of plants.

Laboratory DNA test: the most accurate option

PCR (polymerase chain reaction) is a lab method that copies a short piece of DNA many times, until there is enough to detect. For sexing, the lab looks for male markers on the Y chromosome, such as the SCAR and MADC2 markers. A SCAR (sequence-characterized amplified region) is one type of DNA marker. If the marker is found, the plant is male.

Accuracy: above 98.5% for a qPCR test of the MADC2 marker, checked on about 8,200 leaf samples (Torres et al., 2022).
Timing: from day 5–7, at the cotyledon stage (when the seedling has only its first round seed leaves).
Cost: USD 5–15 per sample.
Best for: commercial growers and breeders.

	Preflower check	12/12 from seed	Lab DNA test (PCR)
Accuracy	85–90%	about 95%	above 98.5%
When you know	Weeks 4–6 of vegetative growth	Days 10–14	Days 5–7
Cost per plant	Free	Free (but yield is lost)	USD 5–15
Best for	Small growers	Screening many plants	Commercial growers and breeders

Translator’s note: We could not confirm which laboratories in Thailand offer cannabis sex testing by PCR, or their prices. Ask a lab directly about the price per sample, the sample type it needs and the turnaround time. Each lab sets its own rules, so ask how to collect, store and ship samples before you take them. Give every seedling a number and label its sample with the same number. Without this, you cannot match a result to the right plant. You can ask other growers about labs in B2B Services: Laboratories, Logistics and Contract Work.

Translator’s note: If a lab test is not practical for you, there are two other ways to avoid males. You can buy feminized seed from a vendor you have checked. You can also root cuttings from a female plant you already own (mother plants and cloning). Be careful with clones bought from other farms. They can bring pests, hop latent viroid (HLVd) or the wrong strain into your grow.

3. Hermaphrodites and nanners

A herm usually looks like a female plant that also forms male flowers. These can be full pollen sacs at the nodes, like those on a male. They can also be “nanners” (short for “bananas”). Nanners are small yellow or pale green male parts that poke straight out of the buds. Like pollen sacs, they can release pollen onto the same plant and onto plants nearby.

Genetic hermaphrodites

The original article says genetic herms come from a mutated Mo1 allele (gene variant) or from unstable genetics. In one study of three commercial strains, herm flowers appeared on 5–10% of the plants examined (Punja & Holmes, 2020). The trait passes to the next generation. Remove these plants at once and never use them as mothers or for breeding.

Section 1 shows that the gene behind Monoecy1 has not yet been confirmed by experiment. We found no study that uses the name “Mo1” or links a variant of it to herms in drug-type cannabis. Treat “a mutated Mo1 allele” as a working idea rather than a proven cause.

Stress-induced hermaphrodites: five causes

Light leaks during the dark period. This is the number-one cause.
Temperatures above 30 °C for long stretches during flowering.
Severe potassium (K) deficiency.
Harvesting too late. Buds left too long start to grow nanners.
pH swings of more than ±1.0 in the root zone.

How hormones flip the switch

Hormones control which kind of flower a plant makes. High auxin (a growth hormone) favours female flowers, and high gibberellin (another growth hormone) favours male flowers. Ethylene works as the “female hormone”. When a plant is stressed, ethylene falls, gibberellin rises, and male flowers appear.

The original article describes this as a survival mechanism. A stressed plant makes its own pollen so it can still produce seed. This explains why all five causes above come down to stress. When you remove the stress, you remove most of the trigger.

Translator’s note: Here is how these causes show up in Thailand and Southeast Asia, and what to do about each one:

Light at night outdoors and in greenhouses. Street lights, security lamps and lights from nearby houses can reach plants in flower. Shield the plants from these lights, or grow them somewhere else. A torn blackout cover in a light-deprivation greenhouse lets light in, so check covers after every storm.
Power cuts. A mechanical timer stops during a power cut and runs late after it. The lights may then come on during the plants’ night. Check the timer clock after every outage, which matters most in the rainy season when storms are frequent. A digital timer with battery backup keeps the correct time.
Heat. When it is 30–35 °C outside, holding a room at 20–26 °C takes planned climate control. Size cooling for the real heat load (air-conditioner and dehumidifier sizing for a Thai flowering room). Running the lights at night, when it is cooler outside, puts less load on the air conditioner. A min/max thermometer shows you how hot the room got during a power cut while you were away.
Genetics. If you cannot keep the room below 30 °C, choose genetics that have done well in heat (choosing genetics for rooms above 30 °C).

Screening for herms, and what to do when you find one

Check every plant daily during the first two weeks of flowering. Check the buds again in the last weeks before harvest, because late harvest brings nanners.
Before you move a plant with male flowers, cover it with a plastic bag. This keeps pollen from shaking loose onto other plants.
For every herm, write down the date, the strain and the room conditions: temperatures, light schedule and any power cuts. This tells you what kind of herm it is. If one plant herms while others of the same strain in the same room stay clean, genetics are the likely cause. If many plants of different strains herm after a power cut or heat wave, fix the stress first.
Some breeders stress plants on purpose to see which ones herm before keeping a mother. The original article gives no method for this, and we found no published, tested protocol. Rely on your own records over several cycles.

4. Making feminized seed: silver thiosulfate or colloidal silver

Both methods use silver to block the ethylene receptor on a female plant, so that it makes pollen (section 1). Colloidal silver is very fine silver particles suspended in water.

Silver thiosulfate (STS)

Feminization rate: 99.7–99.9%.
Spray 2–3 times, 5–7 days apart.
Never consume a plant that has been sprayed.

Colloidal silver

Feminization rate: 95–99%.
Spray daily for 14–21 days.
Easy to make at home.

	STS	Colloidal silver
Feminization rate	99.7–99.9%	95–99%
Spraying	2–3 times	Daily for 14–21 days
Best for	Professional breeders	Home growers

The forum topic on STS linked in section 1 explains both methods step by step.

Translator’s note: Read the safety data sheet (SDS) for every chemical you use. Wear gloves, goggles and a mask while you mix and spray. Spray treated plants in a separate space, so the spray cannot drift onto your flowering crop and make it produce pollen. Never consume, sell or supply flower from treated plants. Do not pour leftover solution into drains or waterways; ask the supplier how to dispose of it.

Translator’s note: STS is made from silver nitrate and sodium thiosulfate. Ask the supplier which Thai rules apply to buying, storing and using them. To find a supplier, ask an official laboratory-chemical distributor in Thailand or ask growers in Breeders Lab. Start with one female plant or a single branch. That way you learn the method before you risk a whole batch.

5. Matching sex management to your scale

Scale	Seed type	How to check sex	Note
Personal (1–10 plants)	Feminized	Preflower check	Inspect daily in the first week of flowering
Medium (10–50 plants)	Feminized + regular	Lab DNA test + preflowers	Use the lab test for regular seed
Commercial (50+ plants)	Feminized (STS-made)	Lab DNA test is mandatory	One male can cost the whole crop
Breeding	Regular	Lab DNA test + phenotype (the plant’s visible traits)	Outdoors, keep males far from flowering females

Translator’s note: There is no single agreed safe distance for male plants outdoors. Hemp seed-certification rules set isolation distances from a few metres to more than 1,500 m, depending on the country and the seed class. Pollen travels on the wind, and male plants on nearby farms can also pollinate yours. Indoors, keep males in a separate room with separate airflow. Change clothes before you go from the male room into the flowering room.

Herm-prevention checklist

✓ Check for light leaks before every cycle. Stand in the closed room with the lights off for a few minutes and look for any point of light. Seal every one you find, because leaks are the main cause of stress herms.
✓ Keep flowering temperature at 20–26 °C, with 28 °C as the maximum. This keeps you well below the 30 °C stress zone.
✓ Keep pH stable: within ±0.3 in hydroponics and ±0.5 in soil. These ranges are much tighter than the ±1.0 swing that triggers herms.
✓ Harvest on time. Buds left too long grow nanners.
✓ Choose stable genetics. Stable lines produce fewer genetic herms.
✓ Check daily during the first two weeks of flowering. This catches male flowers before they open. It extends the one-week daily check in the table above.

6. Research to watch, 2024–2026

Editing the Monoecy1 genes with CRISPR-Cas9 to create strains that do not herm. CRISPR-Cas9 is a gene-editing tool that cuts DNA at a chosen spot. For growers, the hoped-for result is genetics that do not herm. Section 1 shows that the key gene has not yet been confirmed, so this is still a research direction. We found no published study that has edited these genes in cannabis. There is nothing to buy yet. For now, keep choosing stable genetics.
Epigenetic markers to predict “herm potential.” Epigenetic marks are chemical tags on DNA that change how genes are used without changing the DNA code. A test based on them could flag herm-prone plants before you invest time in them. We found no such test on sale; research on DNA methylation and herms is at an early stage. Your herm records will let you judge such a test if one becomes available.
Field DNA test (LAMP assay), results in about 30 minutes, minimal equipment. LAMP (loop-mediated isothermal amplification) copies DNA at one steady temperature, so it does not need a full lab machine. In one study, a LAMP test for a male marker showed a result after 30 minutes at 65 °C, but it was tried on only a small number of samples and needs more validation (Torres et al., 2022). We could not confirm that LAMP sex tests for cannabis are offered in Thailand. To start, ask your lab about it. Compare it with a standard lab test on a small batch before you rely on it.
Artificial intelligence that spots preflowers in photos. We found no published study that gives its accuracy. Use it as a second opinion and confirm sex by eye or by test.

Thai legal note

This article is for education and science only. Cannabis flower (ช่อดอกกัญชา) is a controlled herb under the Notification of the Ministry of Public Health Re: Controlled Herbs (Cannabis), B.E. 2568. The notification was issued under the Thai Traditional Medicine Protection Act B.E. 2542 (1999) and has been in force since 26 June 2025. Flower may be sold only against a PT 33 (ภ.ท. 33) prescription, for a maximum of 30 days. Advertising and online sale of flower are prohibited.

Two ministerial regulations of B.E. 2569 followed in 2026. The Ministerial Regulation on Controlled Herbs (No. 2) B.E. 2569 came into force on 30 April 2026 and tightens the licence criteria for cannabis flower. The Ministerial Regulation on Cannabis or Hemp Extracts B.E. 2569 came into force on 26 April 2026.

Commercial sale, processing and export of cannabis flower require a licence. PT 33 is the prescription form for dispensing flower; it is not a cultivation permit. Under Section 78 of the 1999 Act, unlicensed sale or processing of a controlled herb carries up to 1 year in prison and/or a fine of up to 20,000 baht. Sale of flower for recreational use is not allowed.

Share your cases

If you have had herms, post them below this topic. Include the strain, seed type, week of flowering and room conditions. Photos of nanners or preflowers help other growers learn to spot them early.

References

Prentout, D. et al. (2020). An efficient RNA-seq-based segregation analysis identifies the sex chromosomes of Cannabis sativa. Genome Research, 30(2), 164–172. doi:10.1101/gr.251207.119
Prentout, D. et al. (2021). Plant genera Cannabis and Humulus share the same pair of well-differentiated sex chromosomes. New Phytologist, 231(4), 1599–1611.
Toth, J.A. et al. (2020). Development and validation of genetic markers for sex and cannabinoid chemotype in Cannabis sativa L. GCB Bioenergy, 12(3), 213–222. doi:10.1111/gcbb.12667
Mohan Ram, H.Y. & Sett, R. (1982). Induction of fertile male flowers in genetically female Cannabis sativa plants by silver nitrate and silver thiosulphate anionic complex. Theoretical and Applied Genetics, 62, 369–375. doi:10.1007/BF00275107
Divashuk, M.G. et al. (2014). Molecular cytogenetic characterization of the dioecious Cannabis sativa with an XY chromosome sex determination system. PLoS ONE, 9(1), e85118.
Mandolino, G. et al. (1999). Identification of DNA markers linked to the male sex in dioecious hemp (Cannabis sativa L.). Theoretical and Applied Genetics, 98, 86–92. doi:10.1007/s001220051043
Toscani, M., Malik, A., Riera-Begue, A., Dowling, C., Rougemont, Q., Rodríguez de la Vega, R.C., Giraud, T., Schilling, S. & Melzer, R. (2026). Three closely linked X-chromosomal genes potentially control sex determination in Cannabis sativa. New Phytologist. doi:10.1111/nph.71185 (preprint July 2025).
Feder, C.L. et al. (2021). Fertilization following pollination predominantly decreases phytocannabinoids accumulation and alters the accumulation of terpenoids in Cannabis inflorescences. Frontiers in Plant Science, 12, 753847. doi:10.3389/fpls.2021.753847
Torres, A. et al. (2022). High-throughput methods to identify male Cannabis sativa using various genotyping methods. Journal of Cannabis Research, 4, 57.
Punja, Z.K. & Holmes, J.E. (2020). Hermaphroditism in marijuana (Cannabis sativa L.) inflorescences – impact on floral morphology, seed formation, progeny sex ratios, and genetic variation. Frontiers in Plant Science, 11, 718. doi:10.3389/fpls.2020.00718