Short answer
Cannabis plants usually show their sex about 4–6 weeks from seed under long days, as single preflowers at the nodes. Look with a 30–60x loupe. Males often show a little earlier. Supported by research on preflower timing; the week range is Grower practice.
Why
Preflowers come with age. Cannabis seedlings pass from a juvenile to an adult stage on their own, then form single flowers at the nodes whatever the light schedule; in one cultivar the first pair formed at the 7th node [1]. Solitary flowers are age-dependent and not induced by photoperiod [2].
Sex is genetic. Cannabis is dioecious with an XY system: females XX, males XY [4]. A preflower simply shows which one the plant is.
DNA tests read the chromosomes directly. Validated Y-linked markers identify males from seedling leaves [3], weeks before any flower shows.
The numbers
| Method |
When you get an answer |
Cost |
| Watch preflowers |
~4–6 weeks from seed (from ~7th node) |
Free, plus a loupe |
| Flower a labelled cutting on 12/12 |
~1–2 weeks after the cut goes into long nights |
Space and time |
| PCR or PACE DNA test |
From the first true leaves |
Lab fee |
| Autoflower |
~week 3–5, as it starts flowering |
Free |
| Sign |
Female |
Male |
| Shape |
Teardrop bract |
Round ball on a short stalk |
| Hairs |
Two stigmas |
None |
What to do
- From week 4, check the upper nodes every 2–3 days with a loupe.
- Look where the leaf stalk meets the stem, behind the stipule.
- Wait for hairs before calling a female; a ball on a stalk with no hairs is male.
- Label every plant and keep the record in your journal.
- Not making seed? Remove males before the sacs open, which often happens within 1–3 weeks of flowering.
Common mistakes
- Mistaking stipules for preflowers.
- Calling sex too early from one ambiguous node.
- Flipping the whole room just to sex plants, when a cutting would do.
- Keeping a male "to see" in the same room as females.
The first individual flowers that form at stem nodes during late vegetative growth, letting a grower tell male from female before full flowering.A small handheld magnifier, typically 30-60x for cannabis use, used to inspect trichome heads and spot pests and male flowers.A seedling is a young cannabis plant from emergence of the cotyledons until it has several sets of true leaves, typically the first 2-3 weeks.The daily length of light versus dark; photoperiod cannabis flowers when nights grow long enough, rather than at a fixed age.Having male and female flowers on separate individual plants; drug-type cannabis is naturally dioecious with an XY sex-chromosome system.Cannabis uses an XY sex determination system: females carry two X chromosomes (XX) and males an X and a Y (XY), so regular seed is roughly half male.A chromosome is one long DNA molecule packaged with proteins that carries many genes; cannabis normally has 10 pairs, nine autosome pairs plus an X/Y sex pair.12/12 is the classic flowering schedule for photoperiod cannabis: 12 hours of light and 12 hours of uninterrupted darkness, which triggers and sustains flowering.Genotyping is reading a plant's DNA at selected markers to identify it, compare it with other plants, check sex or chemotype, or estimate its ancestry.The first leaves produced by the shoot tip after the cotyledons, with the plant's characteristic serrated leaflets.A day-neutral cannabis plant that starts flowering by age, usually a few weeks after germination, regardless of light schedule; the trait is mainly recessive.In cannabis, the small leaf-like sheath (perigonal bract) that wraps each female flower's ovary and carries the densest capitate-stalked trichomes.The female reproductive organ of a flower (ovary, style and stigmas); in cannabis growers loosely call the visible white stigmas "pistils".The sticky, hair-like tip of the female pistil that catches pollen; each cannabis female flower has two, white when fresh and darkening with age.The stalk that attaches a leaf to the stem at a node; in cannabis it carries all the leaflets of a palmate leaf.
