How do I stop my plants from turning hermaphrodite?

Q: How do I stop my plants from turning hermaphrodite?

Prevent hermaphrodites by starting with well-bred genetics first, then removing the stresses that reveal weak plants. Choose lines with a published herm rate and plant count, keep the 12-hour dark period fully light-tight, hold canopy temperature under about 30 °C (86 °F), and avoid heavy pruning or drought in flower. A herm is mainly a breeding fault: cull any plant that herms and never breed from it.

Short answer

Start with sound genetics, then take away the stresses that reveal weak plants. Buy lines that publish a herm rate with a plant count, keep the 12-hour dark period fully light-tight, and keep the canopy under about 30 °C (86 °F). House position: herms are mainly breeder error, so cull any plant that herms and never breed from it. The stress triggers are Supported.

Why

Genetics comes first. In hemp, sex expression has a genetic basis and responds to selection ^[2], and genome-wide work has mapped loci tied to it ^[3]. A line built on repeated selfing, careless feminising or untested crosses carries the tendency into every pack.

Stress is the trigger. Research links interrupted darkness, altered photoperiods, low temperatures and certain chemicals to male flowers on female plants ^[1]. Heat at the tops stresses the plant as well ^[4]. A stable plant next to the same leak stays female, which is why GenetixBay reads stress as a test the plant either passes or fails.

Herms spread their own weakness. Pollen from a herm gives almost all female seed carrying the same tendency ^[1].

The numbers

Control	Target
Dark period in flower	12 h (or 11 h on 13/11), uninterrupted, light-tight
Leak test	Nothing visible after 5–10 minutes with eyes adjusted
Canopy temperature	Under about 30 °C (86 °F)
Herm onset to watch for	Week 4 of flower onwards ^[1]
Line standard	Published herm rate; under 2% needs about 150 plants to verify

What to do

Buy from breeders who publish tester herm rates with plant counts. Avoid lines of untested crosses of famous cuts.
Seal the leaks: zipper ends, duct ports, equipment LEDs. Use a timer you trust.
Don't open the tent in the dark period.
Keep leaf temperature down at the tops, and keep watering and feeding steady through flower.
Do heavy pruning in veg, not mid-flower.
Inspect bud interiors and nodes weekly from week 3–4 with a hand lens.
If a plant herms, bag it, remove it, record it and tell the breeder.

Common mistakes

Buying "free" or untested seed and blaming the tent.
Keeping a herm clone because it smokes well.
Running far past the harvest window.
Calling late bananas normal.

Full article: Hermaphrodite cannabis (bananas and nanners)
Answers: seeds in my buds · can you smoke a hermie? · opening the tent at night
Articles: breeders vs pollen chuckers · feminised seed and sex reversal
Terms: herm threshold · herm-prone

The herm threshold is GenetixBay's two-tier herm standard: a 10-plant screening grow with zero herms to be featured, then about 150 or more community-grown plants before a line can be Verified under 2% herm rate.A plant count limit caps how many cannabis plants a person, household, patient or licence holder may have, and defines what counts as a plant.In cannabis growing, a plant (usually female) that develops both pistillate and staminate flowers or anthers; botanists call it intersex or monoecious expression.Hemp is cannabis grown for fibre, seed or other uses and, in many laws, defined as Cannabis sativa with no more than 0.3% THC by dry weight.A locus is the fixed position on a chromosome where a particular gene or DNA marker sits; the plural is loci.Selfing is pollinating a plant with its own pollen, in cannabis usually by reversing part of a female to make pollen; it is the most intense form of inbreeding.The daily length of light versus dark; photoperiod cannabis flowers when nights grow long enough, rather than at a fixed age.A male flower that carries stamens (anthers on filaments) and no functional pistil; in cannabis, five drooping sepals around five pollen-shedding anthers.Top shelf is the highest price tier of flower in a shop, used as slang for any premium-looking cannabis; it reflects pricing and appearance, not verified quality.13/11 is a flowering schedule of 13 hours light and 11 hours dark; GenetixBay endorses it, as most cultivars still flower while gaining extra daily light.Leaf temperature is the actual surface temperature of a leaf, which often differs from air temperature by 1–3 °C depending on light source and transpiration.An LED (light-emitting diode) grow light uses solid-state diodes to produce light; modern fixtures are the most efficient grow lights, at roughly 2.5–3 µmol/J.The span of days, often one to two weeks, in which a cannabis plant is ripe enough to cut, judged mainly by trichome head colour and bud development.Nanners (bananas) are exposed, banana-shaped anthers on a female flower: a hermaphrodite trait, not a normal part of late flowering, and a reason to cull.

Follow-up questions

Can I stop a herm-prone strain from herming?

You can reduce, but not reliably stop, herming in a herm-prone cannabis line. A perfect environment removes the triggers, such as light leaks, heat and drought, but the tendency is in the genetics and can still show late in flower. GenetixBay's position is that herm-prone lines are mainly breeder error: grow them out once if you must, cull any herm, and replace the line.

Do feminised seeds herm more than regular seeds?

Feminised seeds herm more only when they come from careless feminising, such as reversing an unproven female or using pollen from a stress herm. Well-made feminised seed from a female proven stable over several runs can be very stable. Ask for the herm rate and plant count from tester grows; GenetixBay disqualifies lines above 2% in tester grows.

Does stress cause hermaphrodites?

Stress triggers hermaphrodite flowers in cannabis plants that are already susceptible; it does not make a well-bred female herm. Research links interrupted darkness, altered photoperiods, low temperatures and chemicals to male flowers on females, yet the same light leak leaves a stable clone beside it untouched. Fix the stress to protect the room, and cull the plant that failed the test.

Sources

Punja ZK, Holmes JE., 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.org/10.3389/fpls.2020.00718

Faux AM, Berhin A, Dauguet N, Bertin P. 2014. Sex chromosomes and quantitative sex expression in monoecious hemp (Cannabis sativa L.). Euphytica 196(2):183-197. doi.org/10.1007/s10681-013-1023-y

Petit J, Salentijn EMJ, Paulo MJ, Denneboom C, Trindade LM. 2020. Genetic architecture of flowering time and sex determination in hemp (Cannabis sativa L.): a genome-wide association study. Frontiers in Plant Science 11:569958. doi.org/10.3389/fpls.2020.569958

Chandra S, Lata H, Khan IA, ElSohly MA., 2008. Physiology and Molecular Biology of Plants 14(4):299-306. doi.org/10.1007/s12298-008-0027-x