# Stretchy, leggy cannabis: a differential

> Long internodes and thin stems mean the plant is reaching for light: PPFD too low, light too far away, a low red-to-far-red ratio (shade or crowding), or warm days relative to nights. Some stretch in the first 2–3 weeks after the switch to 12/12 is normal and cultivar dependent. Measure PPFD and day/night temperatures before blaming genetics.

- Canonical: https://genetixbay.com/knowledge/diagnosis/stretching.html
- Category: Diagnosis · Level: Beginner
- Evidence: Supported · Status: Draft: awaiting expert review (some figures are being checked)
- Stage: Seedling, Veg, Transition · Systems: Any system
- 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)"

## Questions and answers

**Why is my cannabis [seedling](https://genetixbay.com/knowledge/glossary/seedling.md) so stretched?** A stretched cannabis seedling, tall, thin, floppy and pale, is usually getting too little light because the fixture is too weak or too far away. Grower practice targets about 150 to 300 [µmol/m²/s](https://genetixbay.com/knowledge/glossary/ppfd.md) and a DLI of 10 to 20 mol/m²/day for seedlings. A hot [humidity dome](https://genetixbay.com/knowledge/glossary/cloning-dome.md) or high temperature adds to stretch. At [transplant](https://genetixbay.com/knowledge/glossary/transplanting.md), bury the stretched stem deeper.

**Why is my cannabis plant leggy with long gaps between nodes?** A leggy cannabis plant with long internodes is reaching for light. The usual causes are low PPFD, shading from crowded neighbours or a far-red-rich spectrum that lowers the [red to far-red ratio](https://genetixbay.com/knowledge/glossary/red-far-red-ratio.md), and warm days with cool nights. Shade avoidance through [phytochrome](https://genetixbay.com/knowledge/glossary/phytochrome.md) is established plant biology. Measure PPFD and day and night temperatures before blaming genetics.

**How much do cannabis plants stretch in flower?** Cannabis plants commonly stretch about 1.5 to 3 times their height in the first 2 to 3 weeks after the switch to [12/12](https://genetixbay.com/knowledge/glossary/12-12.md), according to grower practice, though no controlled cannabis data exist and the amount depends heavily on cultivar. Plan for it by flipping at about half the final height your space allows, and use a trellis, [SCROG](https://genetixbay.com/knowledge/glossary/scrog.md) or supercropping to manage it.

**Does far-red light cause cannabis stretching?** Far-red light can cause cannabis stretching. A low red to far-red ratio triggers the shade-avoidance response, lengthening stems and reducing branching, which is established in plant biology and shown for far-red in many species, though cannabis-specific data are grower practice. If you run far-red supplements and stretch is unwanted, reduce them or confine them to end of day.

**How can I stop cannabis from stretching?** Stop cannabis stretching by giving more light, keeping temperatures even and spacing plants. Raise PPFD to about 400 to 700 µmol/m²/s in veg, narrow the day and night temperature gap to 3 °C (5 °F) or less, which is extrapolated from ornamentals, and avoid crowding. Topping and low-stress training control height, and gentle air movement strengthens stems.

**Short answer.** A stretched plant is a plant looking for more light. The usual causes are low PPFD (light too weak or too far away), shade from neighbours or a light spectrum rich in far-red, and warm days with cool nights. Seedlings stretch fastest. Some stretch after the switch to 12/12 is normal: many cultivars grow substantially taller in the first 2–3 weeks of flower. Measure light and temperature before blaming genetics.

## What's happening

Plants sense shade through phytochrome: a low ratio of red to far-red light (from neighbouring leaves or far-red-rich sources) triggers stem elongation and less branching, the "shade avoidance" response (*Established* in plant biology; far-red promotes elongation in many species [3]). Low overall photon flux does the same. Temperature matters too: in many ornamental crops, a warmer day than night ([positive DIF](https://genetixbay.com/knowledge/glossary/dif.md)) increases internode length, and the day–night difference has more effect than the average (*Supported* outside cannabis [3]). [Blue light](https://genetixbay.com/knowledge/glossary/blue-light.md) generally restrains elongation; Westmoreland et al. tested blue fractions in cannabis and found fixture efficacy mattered more for yield than spectrum [2].

Post-flip stretch is a developmental phase. Its size depends heavily on cultivar (*Grower practice*: roughly 1.5× to 3× height gain; no controlled cannabis data located).

## How to recognise it

| Pattern | Most likely | Also consider |
|---|---|---|
| Seedlings tall, thin, floppy, pale | PPFD too low / light too far | High temperature; too warm a humidity dome |
| Lower and inner plants taller and thinner than edges | Crowding/shade (low R:FR) | Uneven light footprint |
| Whole room elongated, light measured fine | Warm days vs cool nights; far-red-rich spectrum | Genetics |
| Big height gain weeks 1–3 after 12/12 | Normal flower stretch | Cultivar trait |
| Stretch with pale lower leaves | Low light plus N limitation | See [yellowing lower leaves](https://genetixbay.com/knowledge/diagnosis/yellowing-lower-leaves.md) |

## Numbers that matter

| Parameter | Target | Notes |
|---|---|---|
| PPFD, seedlings | ~150–300 µmol/m²/s | Grower practice |
| PPFD, veg | ~400–700 µmol/m²/s | Grower practice |
| PPFD, flower | ~800–1200+ µmol/m²/s | Yield increased linearly to 1800 [1] |
| DLI, seedlings | ~10–20 mol/m²/day | Grower practice |
| Day–night difference (DIF) | 0 to small positive (≤ 3 °C / 5 °F) to limit stretch | Extrapolated from ornamentals [3] |
| Air temperature | ~22–28 °C (72–82 °F) lights on | [4] for [photosynthesis](https://genetixbay.com/knowledge/glossary/photosynthesis.md) response |

## What to check first

1. **Measure PPFD** at the top of the seedling or canopy. Phone apps are rough; a quantum sensor is better.
2. **Distance** from light vs the fixture's recommended height.
3. **Day and night temperatures.** A hot day and a cold night stretch plants.
4. **Spacing.** Crowded plants shade each other.
5. **Spectrum.** Supplemental far-red, or old [fluorescent tubes](https://genetixbay.com/knowledge/glossary/t5-fluorescent.md) with low output.
6. **Calendar.** Is this the normal first 2–3 weeks of flower?

## How to fix it

- **Seedlings:** lower the light or increase intensity to ~200–300 µmol/m²/s; bury a stretched seedling's stem deeper at transplant (it will root along the buried stem in soil and coco; grower practice); add gentle air movement to strengthen stems.
- **Veg:** increase PPFD; space plants; train (top, LST) to control height.
- **Temperature:** narrow the day–night gap; a brief cool period at lights-on is a grower trick extrapolated from ornamental "DIP" practice.
- **Flower stretch:** plan for it. Flip at about half the final height you can accommodate (grower practice); use a trellis or SCROG; supercrop if needed.
- **Far-red:** if you run far-red supplements, reduce or confine them to end-of-day if stretch is unwanted.

## How to confirm it worked

- Next internodes are shorter and stems thicker within 5–7 days.
- Canopy evens out; inner plants stop outgrowing the edges.

## Common mistakes

- Keeping seedlings under a weak light "so they don't burn".
- Flipping a tall cultivar to flower with no headroom left.
- Assuming "sativa genetics" when the PPFD is 150 µmol/m²/s.

## Evidence

- Low light increasing elongation and light-limited yield: *Established* [1].
- Far-red and DIF effects: *Established* in other crops [3]; *Grower practice* for cannabis.
- Spectrum effects on cannabis yield are smaller than efficacy effects: *Supported* [2].
- Size of post-flip stretch: *Grower practice*.

## Sources

1. Rodriguez-Morrison V, Llewellyn D, Zheng Y., 2021. Frontiers in Plant Science 12:646020. https://doi.org/10.3389/fpls.2021.646020
2. Westmoreland FM, Kusuma P, Bugbee B., 2021. Cannabis lighting: Decreasing blue photon fraction increases yield but efficacy is more important for cost effective production of cannabinoids. PLOS ONE 16(3):e0248988. https://doi.org/10.1371/journal.pone.0248988
3. Horticultural Science (Prague), 2020, 47(3):169-179. Selection of cut-flower species affected in promotion of flowering and stem elongation by far-red lighting or heat. https://doi.org/10.17221/174/2018-HORTSCI
4. Chandra S, Lata H, Khan IA, ElSohly MA., 2008. Physiology and Molecular Biology of Plants 14(4):299-306. https://doi.org/10.1007/s12298-008-0027-x