Regular seed gives roughly half males, and every male is a plant you fed, lit and watered before culling it. Some growing guides say you can tilt that ratio with humidity, temperature, light spectrum, photoperiod and the nitrogen-to-potassium balance in the first two weeks. This article sets out those claims as the source gives them and explains what is known about sex determination in cannabis. It is direct about which claims are statistical tendencies at best and which have no support at all. Topic 192 covers how to tell males from females once they show; this article deals with the earlier question.
1. What the source claims
Ryan Riley’s Growing Elite Marijuana lists eight environmental factors said to influence which sex a seedling becomes:
| Factor | Claimed to favour female | Claimed to favour male |
|---|---|---|
| Humidity | High ambient humidity; high substrate moisture | Low ambient humidity; low substrate moisture |
| Temperature | Lower | Higher |
| Light spectrum | More blue-spectrum energy | Red spectrum |
| Photoperiod | Shorter days (14 h) | Longer days (18 h+) |
| Nitrogen | Higher N | Lower N |
| Potassium | Lower K | Higher K |
| Environmental stress | Absence of stress | Any environmental stress increases male development from seed |
| Seed soak | Soaking seed in gibberellic acid | — |
From this the book recommends higher nitrogen and lower potassium in the first two weeks to encourage female development. It also cites Russian research claiming that seedlings whose taproot has at least about 10 cm (4 in) to grow tend to be female.
2. What is actually established about sex determination in cannabis
Cannabis is normally dioecious, meaning each plant is either male or female. Sex is chromosomally determined by an XY system and is set at fertilisation. Nobody disputes this.
Sex expression is not the same as sex determination. Under enough stress or chemical treatment, a genetically female plant can produce male flowers. Every feminized-seed technique relies on this, including Rodelization and silver thiosulfate. The chromosomes stay the same. Only the type of flower the plant produces changes.
Hormonal and nutritional effects on sex expression are documented in the older hemp literature. That is where the practices in the table come from. These treatments act on plants whose chromosomes are already fixed. The plants that respond most strongly tend to be the ones most prone to intersex expression, and you do not want to select for that trait.
3. Assessing the eight factors honestly
We could not find modern controlled trials in cannabis that support these specific factors and directions.
Gibberellic acid: the table has this one backwards. In hemp, gibberellins have been shown to induce male flowers on female plants (Mohan Ram & Jaiswal, Planta 1972). Do not act on this row.
Environmental stress increasing male development: stress is well established as a cause of intersex expression. We have no support for the claim that it changes the ratio of chromosomally male to chromosomally female seedlings. The practical advice holds either way: avoid stressing seedlings.
High nitrogen and low potassium, high humidity, low temperature, blue spectrum and short photoperiod: these echo mid-20th-century hormonal work on hemp sex expression (e.g. Heslop-Harrison, Physiologia Plantarum 1956, on auxin and sexuality in Cannabis sativa). As far as we can establish, nobody has tested whether they have a measurable effect in a modern indoor grow. Treat as unverified.
The 10 cm taproot claim: we could not find the study the book cites. Unverified.
4. Why the numbers should make you sceptical anyway
Sex is chromosomally determined at fertilisation, so environment has zero effect on genotype. Humidity cannot turn an XY seedling into an XX one. Phenotype is a different matter. In the hemp literature, the ethylene-releasing compound ethephon (2-chloroethanephosphonic acid) induced female flowers on genetically male plants (Mohan Ram & Jaiswal, Experientia 1970). So the share of plants that look female can shift even though no plant’s genotype changes. That was a hormone treatment, not a humidity setting, and the treated plants were still genetically male.
Chance also explains most grower reports. Suppose someone applies all eight treatments and gets 6 females in a batch of 10. With a true 50% rate, that result happens about a fifth of the time (≈20.5%).
This is the core practical point. To detect a shift from 50% to 60% with reasonable confidence (α 0.05, 80% power), you need roughly 200 seedlings, or about 400 per group if you use a control. Hobby growers almost never work at that scale, which is why these claims have circulated for decades without being settled.
5. What actually reduces wasted effort on males
Feminized seed. If you do not want males, use feminized seed. Bear in mind that stress-based feminization methods can select for stress-sensitivity over generations.
Clones from a known female mother. A clone is genetically identical to its mother, so it is female with certainty.
Early sexing, and culling early. Pre-flowers at the nodes usually show from about week 4–6 from germination, or within 1–2 weeks after switching to 12/12. The earlier you remove males, the less light, water, nutrient and space you spend on them. This controls cost; it does not shift the ratio.
Germinate more than you need. The book advises germinating roughly double what you need, because about half will be male. This plans around the 50% rate instead of fighting it.
The exception: if you are breeding, males are not waste. Good males are scarce and worth keeping. “Get rid of the males” only applies to a sinsemilla crop.
6. Where the eight factors do belong
Most of the conditions in the table are good seedling practice whether or not they affect sex:
- Stable, unstressed conditions produce better seedlings.
- Adequate humidity (the book recommends 50–60% RH for most of the cycle) protects small root systems.
- Blue-dominant spectrum and nitrogen-forward feeding in veg are standard practice for growth.
- Moderate temperature: the book’s stated ideal is 21–26.6 °C with lights on.
Run these conditions because they produce stronger seedlings, not because you expect them to change the sex ratio.
7. Regional note
In Southeast Asia, high humidity is the default rather than something you have to create. After the seedling stage it becomes a botrytis and mildew risk. Holding 21–26 °C in an unconditioned room needs cooling, which is expensive. The book recommends staying below 32.2 °C. Treat that as one author’s conservative recommendation, not a physiological limit. How much heat stress a plant suffers depends on how long the heat lasts, on VPD and on genetics, and outdoor and greenhouse crops in this region regularly go above 32 °C. See topics 1073 and 1046 for climate planning.
8. Summary
- Sex in cannabis is chromosomally determined at fertilisation. Environment and hormones can change sex expression (phenotype), not genotype.
- The eight factors are unverified in modern controlled work, and the gibberellic acid entry is backwards.
- Testing a 50% → 60% shift takes roughly 200 seedlings, or about 400 per group with a control. Most grower reports come from far smaller batches.
- If you want females, use feminized seed or clones. With regular seed, germinate double and sex early.
- Run the recommended conditions anyway, because they are good seedling practice.
- In this region, climate control matters far more than any of this. Treat the book’s 32.2 °C figure as a conservative guide, not a hard ceiling.
Has anyone here run regular seed at real numbers, around 200 seedlings or a few hundred per group with a control, and counted the ratio? If you have data, even partial, please post it in this thread.
Related reading on this forum: topic 192 (plant sex and DNA testing), topic 388 (regular vs feminized seed), topic 1273 (seed myths), and topics 1302 and 1304.
This article is technical horticultural information for readers operating legally in their jurisdiction. Cannabis cultivation, propagation and seed production are regulated differently in every country. In Thailand, cultivation is controlled under the Notification of the Ministry of Public Health Re: Controlled Herbs (Cannabis) B.E. 2568 and related ministerial regulations; a permit is required — check current rules. Nothing here is legal advice.