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) increases internode length, and the day–night difference has more effect than the average (Supported outside cannabis [3])being reviewed. Blue light generally restrains elongation; Westmoreland et al. tested blue fractions in cannabis and found fixture efficacy mattered more for yield than spectrum [2].being reviewed
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 |
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]being reviewed |
| Air temperature | ~22–28 °C (72–82 °F) lights on | [4] for photosynthesis response |
What to check first
- Measure PPFD at the top of the seedling or canopy. Phone apps are rough; a quantum sensor is better.
- Distance from light vs the fixture's recommended height.
- Day and night temperatures. A hot day and a cold night stretch plants.
- Spacing. Crowded plants shade each other.
- Spectrum. Supplemental far-red, or old fluorescent tubes with low output.
- 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" practicebeing reviewed.
- 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.
