# Can I flower a photoperiod plant 12/12 from seed?

> Yes, a photoperiod plant can be grown 12/12 from seed, but it won't flower straight away: it first has to mature, forming its first single flowers at around the 7th node, typically after 3–5 weeks. The result is a short plant with one main cola, ready in roughly 3 months. It suits low headroom or many small plants, but per plant it yields less than a vegged plant.

- Canonical: https://genetixbay.com/knowledge/answers/12-12-from-seed.html
- Topic: Basics · Type: How-to
- Evidence: Supported · Status: Draft: awaiting expert review
- Full article: [Photoperiod and light leaks: 18/6, 12/12 and the dark period](https://genetixbay.com/knowledge/environment/photoperiod-light-leaks.md)
- Also asked as: 12/12 from seed | Flowering from seed | Can I skip veg? | Running photoperiods on 12/12 from day one
- Updated: 2026-10-31 · Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay Knowledge Base (genetixbay.com/knowledge)"

## Short answer

Yes. A [photoperiod](https://genetixbay.com/knowledge/glossary/photoperiod.md) [seedling](https://genetixbay.com/knowledge/glossary/seedling.md) on [12/12](https://genetixbay.com/knowledge/glossary/12-12.md) grows normally until it matures, then flowers straight away, giving a short plant with one [main cola](https://genetixbay.com/knowledge/glossary/cola.md) in roughly three months. It costs yield per plant, and suits low headroom or many small plants. *Supported* for the maturity step; outcomes are *Grower practice*.

## Why

Seedlings must grow up first. Cannabis seedlings pass from a juvenile to an adult, flower-ready stage on their own, whatever the day length; in one cultivar the first single flowers formed at the 7th node [1]. Before that, long nights have little to work on.

Once mature, long nights take over. Short days make the [shoot tips](https://genetixbay.com/knowledge/glossary/apical-meristem.md) branch into compact inflorescences [2]. With no veg, there's little frame to build on, so the plant makes one [dominant](https://genetixbay.com/knowledge/glossary/dominant.md) cola and a few small side buds.

Light hours cost growth. At the same intensity, 12 h gives two-thirds of the daily light of 18 h, and yield follows light [4]. Small plants also leave canopy gaps unless packed densely; higher density raises yield per area but lowers it per plant [3].

## The numbers

| | 12/12 from seed | Vegged photo (4 weeks) |
|---|---|---|
| [Veg time](https://genetixbay.com/knowledge/glossary/veg-time.md) | ~3–5 weeks while it matures | 4+ weeks at [18/6](https://genetixbay.com/knowledge/glossary/18-6.md), then flip |
| Shape | One main cola | Multi-top, trainable |
| Final height (typical) | Short | 1.5–3× height at flip |
| Seed to harvest | ~3 months | ~3.5–5 months |
| Yield per plant | Low | Higher |
| Best use | Low headroom, testing, many small plants | Filling a canopy with few plants |

## What to do

1. Use a compact cultivar and a 7–11 L (2–3 gal) pot.
2. Run 12/12 (or [13/11](https://genetixbay.com/knowledge/glossary/13-11.md)) from [sprouting](https://genetixbay.com/knowledge/glossary/germination.md), at seedling intensity first.
3. Keep the dark period truly dark from day one.
4. Grow several [plants per square metre](https://genetixbay.com/knowledge/glossary/plant-density.md) to fill the canopy, within legal limits.
5. Harvest by [trichomes](https://genetixbay.com/knowledge/glossary/trichome.md), not the calendar.

## Common mistakes

- Expecting flowers in week 1.
- Using a stretchy, long-flowering cultivar.
- Leaving one tiny plant alone under a big light.
- [Light leaks](https://genetixbay.com/knowledge/glossary/light-leak.md), which push the plant back towards veg.

## Related

- Full article: [Photoperiod and light leaks](https://genetixbay.com/knowledge/environment/photoperiod-light-leaks.md)
- Answers: [autos and photos in one tent](https://genetixbay.com/knowledge/answers/autos-and-photos-in-one-tent.md) · [when to flip](https://genetixbay.com/knowledge/answers/when-to-flip-to-flower.md) · [why flowering needs long nights](https://genetixbay.com/knowledge/answers/why-flowering-needs-darkness.md)
- Article: [Sea of green](https://genetixbay.com/knowledge/training/sog.md)
- Terms: [12/12](https://genetixbay.com/knowledge/glossary/12-12.md) · [veg time](https://genetixbay.com/knowledge/glossary/veg-time.md)

## Follow-up questions

**Why doesn't a seedling flower immediately on 12/12?** A cannabis seedling doesn't flower immediately on 12/12 because it must pass from a juvenile to an adult stage first. Research found this change happens on its own, with the first pair of single flowers forming around the 7th node in one cultivar, whatever the day length. Until then long nights have little to act on, so the seedling grows leaves and nodes.

**Is 12/12 from seed better than autoflowers?** 12/12 from seed and autoflowers both give small, fast plants, but they suit different goals. Autos run 18–20 hours of light from seed, giving about 1.5–1.7 times the daily light of 12 hours at the same intensity, so they usually grow more per day. 12/12 from seed suits a photoperiod cultivar you want to sex, test or grow small without vegging.

**Does 12/12 from seed help sex plants early?** Running seedlings on 12/12 can show sex a little sooner, because the plants flower as soon as they mature, but preflowers appear with age under long days too, usually at about 4–6 weeks. A cheaper way to sex without stunting the plant is to root a labelled cutting and flower it on 12/12, which answers in roughly 1–2 weeks.

## Sources

1. Spitzer-Rimon B, Shafran-Tomer H, Gottlieb GH, et al. 2022. Non-photoperiodic transition of female cannabis seedlings from juvenile to adult reproductive stage. Plant Reproduction 35(4):265-277. https://doi.org/10.1007/s00497-022-00449-0
2. Alter H, Sade Y, Sood A, et al. 2024. Inflorescence development in female cannabis plants is mediated by photoperiod and gibberellin. Horticulture Research 11(11):uhae245. https://doi.org/10.1093/hr/uhae245
3. Danziger N, Bernstein N, 2022. Too dense or not too dense: higher planting density reduces cannabinoid uniformity but increases yield/area in drug-type medical cannabis. Frontiers in Plant Science 13:713481. https://doi.org/10.3389/fpls.2022.713481
4. Rodriguez-Morrison V, Llewellyn D, Zheng Y, 2021. Cannabis yield, potency, and leaf photosynthesis respond differently to increasing light levels in an indoor environment. Frontiers in Plant Science 12:646020. https://doi.org/10.3389/fpls.2021.646020