The week before and after switching to 12/12 — veg ends, flower begins
"Flip day" is when you change the timer from 18 hours of light / 6 hours of dark to 12/12. That single change kicks off the most dramatic two weeks of the entire grow: branches stretch 50–200% taller, sex finally becomes obvious, and the plant pivots from making leaves to making flowers.
Most first-time grows go wrong in the first 14 days post-flip — canopy hits the light, environmentals weren't right, a defoliation got skipped, or a single light leak triggered a stress response. This page is your week-by-week prep manual so the flip is boring, not chaotic.
For the underlying biology — why 12/12 triggers flowering, why darkness matters, why nutrients shift — see Plant Physiology.
On this page
🔄 What 12/12 actually does
Cannabis genetically "decides" whether to flower based on the length of the dark period, not the day length. When uninterrupted darkness exceeds about 11 hours, a pigment called phytochrome stays in its inactive state long enough that the genetic program for flowering switches on. In nature this happens at the autumn equinox; indoors, you trigger it by moving your timer from 18/6 to 12/12.
Within roughly 7–14 days of the flip you'll see two big changes:
- Vertical explosion — the "flip stretch". Internodes elongate; some plants double in height. The plant is racing to put bud sites high in the canopy before flowering finishes them.
- Pre-flowers appear at nodes — tiny structures revealing sex. Pistils (white hairs) = female, keeper. Pollen sacs (small grape clusters) = male, remove immediately.
🌱 The flip stretch — first 14 days
Same plant on Day 0, Day 7, and Day 14 of flower. Most plants double in height during the stretch — plan your tent height accordingly.
How much stretch you get depends on:
- Genetics — sativas can triple, indicas often only 1.5x. Your three: Gorilla Glue (1.5–2x), Do-Si-Do (1.3–1.5x), Purple Haze (2–3x — most stretch).
- Light intensity — weak light = aggressive reach (lots of stretch). Strong light = compact stretch.
- Distance to light — closer (without bleaching) = less stretch.
- Training before flip — topped + LST'd plants stretch more evenly, not less. Untrained plants stretch one dominant stem skyward.
📆 Day-by-day timeline (Day −7 → Day +14)
✅ Pre-flip checklist
Tick these off in the 7 days before flip. Persists in your browser until cleared.
🌡️ Environmental shift — flower targets
| Parameter | Veg target | Flower target | Why it changes |
|---|---|---|---|
| Day temp | 72–82°F | 68–78°F | Cooler nights enhance terpene + anthocyanin (purple) expression late in flower. |
| Night temp | 65–72°F | 62–70°F | Slightly higher day-to-night delta (Δ8–10°F) is ideal. Don't go below 60°F. |
| Humidity | 55–70% | 50–55% → 40–45% | Drops as flower progresses. Late flower <50% RH protects against bud rot. |
| VPD | 0.8–1.2 kPa | 1.0–1.5 kPa | Drier air pulls more water through the plant, denser buds. |
| PPFD | 400–600 | 600–900 | Bud-bulking needs more light energy. Bring the panel down or dim up. |
| Photoperiod | 18 / 6 | 12 / 12 | Triggers + sustains the flowering program. |
🧪 Nutrient transition
The N-P-K ratio shifts dramatically. Veg wants lots of nitrogen (N) for leaves; flower wants lots of phosphorus (P) for bud development and potassium (K) for ripening + sugar transport.
| Stage | N–P–K shape | Fox Farm products | Add-ons |
|---|---|---|---|
| Late veg (last 2 wk) | High N, moderate P, moderate K | Grow Big (6-4-4) | CalMag, Recharge |
| Flip → Day +14 (stretch) | Drop N, raise P, hold K | Big Bloom (0.01-0.3-0.7) + Tiger Bloom (2-8-4) at half strength | CalMag, Hydroguard, Silica |
| Mid flower (Day +14–+45) | Low N, high P, high K | Tiger Bloom full strength + Big Bloom | CalMag, Recharge bi-weekly |
| Late flower (Day +45–+60) | Very low N, moderate P, high K | Tiger Bloom + Big Bloom; reduce Tiger 25% | Drop CalMag, drop Recharge |
| Flush (last 7–14 days) | Plain pH'd water only | None | None — let the plant burn stored stocks |
🔍 Sex confirmation — reading pre-flowers
Around Day +7 to +14, look at upper nodes (4 and above). Pre-flowers form in the "armpit" where a petiole meets the stem — right next to the stipules.
- Female (keeper): a small teardrop-shaped calyx with 1–2 white hairs (pistils) emerging from the tip. Hairs may be pale or pinkish.
- Male (cull): small round/oval sacs in clusters of 3–5 with no white hairs. They look like tiny green grapes on short stalks. Remove the entire plant from the tent immediately — do not let them open. A single male can pollinate every female nearby and seed your buds.
- Hermaphrodite: female plant showing both pistils AND male sacs (often called "bananas" for their shape). Caused by stress, light leaks, or genetics. If only a few sacs, pluck and isolate. If widespread, cull.
Your three plants are feminized seeds (Gorilla Glue, Do-Si-Do, Purple Haze), so the odds of any male are very low — but a 1–2 minute check at Day +10 with a phone macro clip is cheap insurance.
⚠️ Top mistakes to avoid
- Flipping a too-tall plant. Stretch will double it. Half tent height is the rule.
- Topping in the last week of veg. Plant needs to recover apical dominance to produce a strong central cola, not heal a wound.
- Leaving light leaks unaddressed. A single porch light, an LED indicator, or a cracked tent zipper can hermie the entire grow.
- Hammering bloom nutrients on Day 0. Plant doesn't switch metabolism that fast. Half strength, ramp up.
- Not lowering humidity. Keeping veg's 65% RH into late flower is how new growers get bud rot. Each week down 5–10%.
- Forgetting the dark period is sacred. Don't peek into the tent during lights-off. Don't open the zipper. If you must, use a green LED — phytochrome doesn't respond to it.
- Defoliating during the stretch. Wait for stretch to end (~Day +14). Stripping leaves while the plant is racing upward stalls it.
- Missing the male check. Especially on regular (not feminized) seeds. A single missed male in the tent for a week can ruin the whole grow.