After the light is cut to 12 hours a day (12/12), most photoperiod cannabis plants grow much taller over 1–3 weeks, roughly 50–300% by grower reports. Growers call this the flowering stretch. Besides training, you can slow it down with light colour and with the gap between day and night temperature. This part covers both methods and explains which growth regulators are dangerous and which natural ones the original guide lists as safe. It ends with a checklist for weeks 1–3 of flowering.
Stretch is driven mainly by gibberellin, a plant hormone that makes stem cells grow longer. Its level at the growing tip falls gradually after the switch to 12/12, but stems grow fastest in roughly the first 10 days. So the methods below work best in the first two weeks. The translator’s notes explain what still works in Thai heat, during power cuts and in rainy-season humidity.
This is Part 2. Part 1: Stretch: Controlling Height During the Flowering Transition — Complete Guide
Method 5: more blue light in weeks 1–2
Blue light (400–500 nm wavelength) activates cryptochrome, a light-sensing pigment in the plant. Cryptochrome slows down cell lengthening, so internodes (the gaps between leaf pairs) stay shorter. You can only use this method if you can change your lamp’s light spectrum, meaning its mix of colours.
- Weeks 1–2 of flowering: raise blue to 30–40% of total light, if your LED lets you adjust it.
- From week 3: lower blue and raise red (620–660 nm).
- HPS lamps (high-pressure sodium) cannot change their spectrum. This is where adjustable LEDs have a clear advantage.
Translator’s note: many LEDs have a fixed spectrum, with no separate control for blue. Check the manual for your model, or ask the official distributor in Thailand whether the colour channels can be dimmed separately. If they cannot, skip this method and rely on training and temperature. Try those first before buying a new lamp just for stretch control.
Method 6: the day–night temperature difference
Growers call this DIF: daytime temperature minus night-time temperature. Indoors, “day” means the lights-on period. When days are much warmer than nights, stems stretch more. When the difference is zero or negative, stretch is reduced.
| Strategy | Day | Night | Effect |
|---|---|---|---|
| Positive difference (+6 °C) | 28 °C | 22 °C | Strong stretch. Avoid. |
| No difference (0) | 24 °C | 24 °C | Clearly less stretch |
| Negative difference (−2 °C) | 22 °C | 24 °C | Strongest stretch control |
| Cool morning pulse | Guides suggest about 18–20 °C for the first 2–3 hours of lights-on | 22 °C | Reported by guides. No published cannabis trial was found. |
Translator’s note: these temperatures come from rooms in cooler climates. In Thailand, outside air is often 30–35 °C. Keeping a room at 22–24 °C day and night, or at 18–20 °C in the morning, needs air conditioning running hard and costs a lot of electricity. When the power goes off, the cooling stops too. These steps are more realistic:
- Start lights-on in the coolest hours, late at night or before dawn. The lamps’ heat then comes when cooling is easiest. The room is also coolest at the start of lights-on, which is the cheapest way to get close to a morning pulse.
- Avoid a large positive difference first. Spend your cooling capacity on the lights-on period. Do not chill the dark period more than you need to, because that widens the gap.
- Check that your equipment can hold the targets. See air-conditioner and dehumidifier sizing for a Thai flowering room.
- Check the timer after every power cut. A timer that loses its time shifts your 12/12 schedule without you noticing.
- Outdoors and in greenhouses, you cannot control this difference. Rely on training (Part 1).
Place a min–max thermometer at canopy height. For the first 2 weeks after the flip, record the highest day temperature and the lowest night temperature every day. This shows your real day–night difference and tells you whether to move the light schedule or adjust the air conditioning.
Growth regulators: what to avoid and what the original lists as safe
Plant growth regulators (PGRs) are substances that change how a plant grows. Some block gibberellin production. Products containing them are sold to keep plants short and buds dense.
Synthetic growth regulators: do not use
The EPA in the table is the US Environmental Protection Agency, the US government body that assesses pesticide risks.
| Compound | How it works | Risk |
|---|---|---|
| Paclobutrazol (PBZ) | Blocks gibberellin production | The EPA lists liver toxicity. |
| Daminozide (Alar) | Blocks gibberellin production | The EPA classes UDMH, its breakdown product, as a “probable human carcinogen”. All its uses on food crops in the United States were cancelled in 1989. |
| Chlormequat (CCC) | Blocks gibberellin production | Toxic to the nervous system at high doses |
Warning: never use synthetic growth regulators on cannabis meant to be smoked, eaten or drunk.
Natural growth regulators the original guide lists as safe
| Compound | Source | What it does and how it is used |
|---|---|---|
| Triacontanol | Beeswax, sugarcane, alfalfa | Boosts photosynthesis. Considered food-safe. |
| Chitosan | Shrimp and crab shells | Switches on the plant’s immune defences and thickens cell walls |
| Kelp / seaweed | Ascophyllum nodosum | Contains natural cytokinins, hormones that work against gibberellin. Spray on leaves 1–2 times a week. |
| Silica (SiO₂) | Potassium silicate | Strengthens cell walls, so stems become thicker and sturdier |
Kelp products are usually sold as biostimulants, products that support plant growth without being fertilisers in the usual sense.
Translator’s note: in Thailand, paclobutrazol is widely used in mango orchards to make trees flower out of season. Your plants can pick it up through a shared sprayer or tank, or through staff who also spray orchards. If your plot or soil came from a former mango orchard, ask how it was treated. Studies in mango orchard soil found that paclobutrazol breaks down slowly, with a half-life of about two to three months. Spray only products registered with the Department of Agriculture (กรมวิชาการเกษตร), and use them as the label says (see what you may legally spray under Thai GACP). For kelp, chitosan or silica products, check their status first (see verifying a fertiliser is legally in Thailand). Read whether GACP certification means PGR-free, and if a buyer asks, show a lab certificate of analysis.
Read the ingredient list of every product in your store room. Remove anything containing paclobutrazol, daminozide or chlormequat. Label one sprayer “cannabis only” so residues from other crops cannot reach your plants.
Checklist for weeks 1–3 of flowering
Week 1 (days 1–7 after the flip)
- ☐ Check lamp distance. Use a PPFD reading for LEDs. For HPS, use the hand test: back of your hand at canopy height for 30 seconds, and raise the lamp 5–10 cm if it is uncomfortably hot. Purpose: tops do not burn, and light stays even.
- ☐ Start low-stress training. Bend the tallest branches down to horizontal. Purpose: tops stay level before the fastest growth starts.
- ☐ Use the cool morning pulse if you can: about 18–20 °C for the first 2–3 hours after lights-on. Purpose: slow stem growth (a guide recommendation, not tested in published cannabis trials).
- ☐ Raise blue light to 30–40% if your LED allows it. Purpose: shorter internodes.
- ☐ Measure PPFD at the canopy, the amount of usable light reaching the tops. Target 600–800 μmol/m²/s. Purpose: enough light without stressing the plants.
- ☐ Switch to flowering nutrients: more phosphorus (P) and potassium (K), less nitrogen (N). Purpose: match feeding to flower formation instead of leaf growth.
Week 2 (days 8–14, fastest stretch)
- ☐ Supercrop branches that rise above the canopy. Squeeze until soft inside without breaking the skin, then bend to 90°. Purpose: each bend lowers height by 15–30 cm.
- ☐ Tuck branches into the screen-of-green net, if you use one. Purpose: fill the net evenly before the stretch ends.
- ☐ Raise lamps as fast as the plants grow. This may mean 5–10 cm a day. Purpose: tops never reach the lamp.
- ☐ Spray kelp extract on the leaves (1–2 ml/L). Purpose: natural cytokinins that work against gibberellin.
- ☐ Light defoliation: remove large fan leaves that block bud sites. Purpose: light and air reach the lower buds.
- ☐ Check VPD (vapour pressure deficit, how strongly the air draws water out of leaves). Keep it at 1.0–1.4 kPa, with humidity at 45–55%. Purpose: healthy transpiration and less risk of mould in the thickening canopy.
Translator’s note: in the Thai rainy season, outside humidity is often much higher than 45–55%. Reaching that range needs a dehumidifier sized for the room (see the sizing topic linked above). Spray kelp early in the lights-on period, so the leaves dry before dark. If you cannot bring humidity down, skip the foliar spray that week. Wet leaves in a dense canopy raise the risk of bud rot.
Week 3 (days 15–21, stretch slowing down)
- ☐ Stop supercropping. Branches are stiffening and now break instead of bending.
- ☐ Fix the lamp at your normal flowering distance. Purpose: steady light for bud growth.
- ☐ Change the spectrum: less blue, more red (620–660 nm). Purpose: blue is no longer needed to hold height.
- ☐ Return temperatures to normal: day 24–26 °C, night 20–22 °C. Purpose: end the stretch-control settings and save cooling costs.
- ☐ Look for pistils, the white hairs on young flowers. Purpose: they show the stretch is ending.
- ☐ Check the whole room. Is the canopy even? Is any branch too close to the lamp? Purpose: fix the last problems while you still can.
The point to remember
The stretch lasts only 2–3 weeks. If plants grow too tall during that time, you cannot make them shorter afterwards. So the deciding work happens before the flip. Calculate your flip height and do any topping 2–3 weeks ahead. Set up ties, net and thermometer, and test your lamp settings, all before day 1.
If you have tried these settings in a Thai room, post your temperatures, lamp type and stretch results below this topic. Questions are welcome too.
References
- Dutch Passion (2022). Cannabis Flowering Stretch Master Guide. Cannabis flowering stretch master guide
- Royal Queen Seeds (2023). How to Control and Prevent Stretching in Cannabis Plants. How to Control and Prevent Stretching in Cannabis Plants - RQS Blog
- Chilled Tech (2024). Reduce Plant Stretch by Controlling Day/Night Temperatures.
- 420 Fast Buds (2024). PGR Weed — Synthetic PGRs and Their Dangers.
- Alter H, Sade Y, Sood A, Carmeli-Weissberg M, Shaya F, Kamenetsky-Goldstein R, Bernstein N, Spitzer-Rimon B (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
This article is educational information for lawful medical and agricultural cultivation. In Thailand, cannabis flower is a controlled herb under the Notification of the Ministry of Public Health Re: Controlled Herbs (Cannabis), B.E. 2568, in force since 26 June 2025. The Ministerial Regulation on Controlled Herbs (No. 2) B.E. 2569, in force since 30 April 2026, tightens the licence criteria for cannabis flower. Growing cannabis requires a licence under the law of your country or territory.