The moment you switch your light cycle from 18/6 (or 20/4) to 12/12, a cascade of hormonal changes begins inside the plant. The first visible result is dramatic: the plant stretches. In 10–14 days, most cannabis strains will grow 50–200% taller than they were at the flip.
This is the transition period — not quite veg, not yet full flower. Managing it well sets up the rest of your bloom. Managing it poorly leaves you with stretched plants hitting the light, an uneven canopy, and nutrient problems that haunt you for weeks.
What happens during the stretch
The hormonal shift
When cannabis receives 12 hours of uninterrupted darkness, it triggers the production of florigen — a flowering hormone. But the plant doesn't immediately stop growing vegetatively. Instead, it enters a growth sprint:
- Days 1–3: no visible change. The hormonal shift is happening internally
- Days 4–7: rapid vertical growth begins. Internodes elongate. The plant may grow 2–4 inches per day
- Days 8–14: stretch peaks, then slows. Pre-flowers (white pistils at the nodes) become visible
- Days 15–21: stretch ends. The plant's energy shifts fully to bud production
How much stretch to expect
| Genetics | Typical stretch | Example strains |
|---|---|---|
| Indica-dominant | 50–75% height increase | Northern Lights, Granddaddy Purple, Bubba Kush |
| Hybrid | 75–150% height increase | Blue Dream, Wedding Cake, Gelato |
| Sativa-dominant | 150–200%+ height increase | Haze strains, Durban Poison, Super Silver Haze |
Example: a plant that's 12 inches tall at the flip may finish the stretch at 18 inches (indica) to 36 inches (sativa). Plan your flip height accordingly.
When to flip
The height rule
Flip when the plant is 50% of the maximum height your space allows. If your light can be 24 inches from the canopy and your tent ceiling is 60 inches, your maximum plant height is 36 inches. Flip at 18 inches.
Adjust for genetics:
- Indica-dominant: flip at 50–60% of target height
- Sativa-dominant: flip at 33–40% of target height (they stretch more)
- First time growing a strain? flip conservative (earlier). You can always veg longer next time
When the canopy is ready
The flip isn't just about height. Before flipping:
- Training should be complete — topping, manifolding, and major LST should be done at least 7 days before the flip. The plant needs time to recover before the stretch
- The canopy should be level — an uneven canopy means uneven stretch, and the tallest colas will be too close to the light
- Defoliation — do a light defoliation 2–3 days before the flip to open up the canopy. The plant will fill back in during the stretch
Managing the stretch
Keep training during weeks 1–2
The stretch is your last opportunity for significant canopy management. During this period:
- Continue LST: bend stretching branches outward to maintain an even canopy. Check ties daily — growth is fast enough to snap tight ties
- Supercrop tall branches: if one cola is racing ahead of the others, a gentle supercrop (bending until the inner fibers crack) will slow it and redirect growth hormones to the lower branches. Do this before week 2 ends — after that, stems harden and become brittle
- Adjust the light height: raise the light to maintain the recommended distance as the plant grows toward it. Falling behind on light height during the stretch causes light burn on the tallest colas
Week 1 defoliation
Many growers do a strategic defoliation at the start of flower:
- Remove large fan leaves that block light to interior bud sites
- Remove any leaves touching the soil or pot (moisture trap, pest harbor)
- Don't strip the plant. Remove 15–20% of fan leaves max. The plant needs those leaves to fuel the stretch
A second, heavier defoliation at the end of the stretch (day 14–21) is common and effective. See our defoliation guide.
Nutrient transition
When to switch to bloom nutrients
Don't switch immediately at the flip. The plant is still growing vegetatively during the stretch — it needs nitrogen to fuel that growth.
Recommended transition:
| Days after flip | Feed |
|---|---|
| 1–5 | Full veg nutrients (or a 50/50 veg/bloom mix) |
| 6–10 | 50/50 veg and bloom nutrients |
| 11–14 | Full bloom nutrients |
Common transition mistakes
- Switching to bloom nutrients on day 1: the plant yellows during the stretch because it's not getting enough nitrogen for the rapid growth. Bloom nutrients are low-N
- Continuing full veg nutrients through week 3: the plant gets nitrogen toxic as it enters actual flowering. The shift should be gradual, not abrupt or delayed
Cal-mag during transition
Maintain your cal-mag regimen through the stretch — calcium demand increases during rapid growth. This is a common deficiency point. See our cal-mag guide.
Environmental adjustments
Temperature
Keep flower temps at 75–80°F during the day, 68–72°F at night. The stretch benefits from slightly warmer daytime temps (more growth hormone activity). After the stretch ends, you can begin cooling toward the 72–76°F range that's optimal for mid-to-late flower.
Humidity
Begin lowering humidity during weeks 1–2:
- End of veg: 55–65% RH
- Week 1 of flower: 50–60% RH
- Week 2 of flower: 45–55% RH
- Week 3+: 45–50% RH
The reduction is gradual. Don't crash humidity from 60% to 40% on flip day. See our VPD guide.
Light intensity
You can increase light intensity during the stretch — the plant can handle more light as it grows larger. Many growers run their LED at 75% during veg and ramp to 100% during the first two weeks of flower.
Monitor for light stress: if leaf tips curl upward (taco) or bleaching appears on the top leaves, dim slightly or raise the light.
Watching for problems
Pre-flowers and sex identification
By the end of week 2, you should see pre-flowers — small pistils (white hairs) emerging from the nodes. This confirms the plant is female and flowering.
Check for male traits: round, smooth pollen sacs (no pistils) at the nodes indicate a male or hermaphrodite plant. Remove immediately. See our hermaphrodite prevention guide.
Light leaks
The first two weeks are the most sensitive period for light leaks. The plant is transitioning its hormonal state, and any light during the dark period can:
- Delay or prevent flowering
- Cause hermaphroditism
- Create an uneven stretch (confused growth patterns)
Check your tent for light leaks during the dark period. Cover indicator LEDs, seal zipper gaps, and never open the tent during lights-off.
Common mistakes
- Not accounting for stretch height — the plant you flipped at 24 inches is now 48 inches and touching the light. Flip earlier than you think you need to
- Stopping training at the flip — the stretch is the last window for canopy management. Use it. After week 2, the stems harden and you can't bend them
- Switching nutrients too abruptly — the transition from veg to bloom should take 7–14 days, not happen overnight
- Panicking about the growth rate — the stretch is normal and expected. A plant doubling in height in two weeks isn't wrong. That's what cannabis does
- Not raising the light — the plant is growing toward the light at 2+ inches per day. If you don't adjust the light height daily, the top colas will bleach
For late flower management, see our late flower and ripening guide. For defoliation strategy during flower, see our defoliation guide. For full flower feeding, see our flower feeding strategy.