046: The Perfect Flip: Mastering the Veg-to-Flower Transition

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• Episode focus: Mastering the veg → flower transition—called the most influential ~10-day window of the cycle. Bad transitions cap yield early; you can’t “add days” later.
• Core idea: Avoid compounding stress. Moving plants is already stressful—don’t stack harsh environmental shifts, heavy pruning/topping, high VPD, and high EC all at once.
• Environment fundamentals
• Lighting / DLI matching: Match day-one flower DLI to veg by increasing intensity (PPFD) for the shorter 12-hour photoperiod.
• Typical targets: ~35–40 mol·m⁻²·day⁻¹ early, ramp to ~45–50 later.
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• VPD matching: Keep VPD close to veg on flip; veg ~0.8–1.0 kPa, then ease into ~1.2–1.4 kPa early flower rather than jumping to dry rooms (1.5–2.0 kPa is too stressful at flip).
• Use VPD strategically to moderate stretch (slightly lower VPD can curb internodal elongation).
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• CO₂: Less finicky but watch costs. Start around ~800 ppm (veg level), 800–1000 ppm in week 1–2, then up to ~1200–1400 ppm depending on style.
• Humidity & red stems
• Many flower rooms lack humidification; day-one rooms run too dry, forcing excessive transpiration and causing stress cues like red stems.
• Plants are humidifiers, but early in flower they’re too small to humidify the room—installing steam/dedicated humidifiers prevents early drought stress.
• EC balance with VPD: High EC + high VPD (e.g., 1.5–2.0 kPa) risks toxicity/lockout. If humidity is good, 3.0 EC is fine; if day-one is dry, start with lower EC until humidity improves.
• “Balance over silos” principle
• Climate (light/VPD/CO₂), root-zone, and irrigation are intrinsically linked. Don’t change one without adjusting the others (“law of minimums” thinking).
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• Canopy management
• Topping vs. no topping: Depends on planting density and cultivar. Topping can delay flower set 3–5 days—costly in a fast crop. Many short/medium cultivars in dense programs perform better untopped.
• Crop registration: Build a simple program to learn genetics (short/medium/tall, heavy/light feeders, topping response).
• Run experiments with new cultivars (e.g., vary drip stakes and leave some untopped) to see true morphology and water response.
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• Defoliation/skirting: Do light skirting in the last 5–7 veg days for airflow and access (easier emitter placement). Avoid heavy defol or topping right before flip—it causes hormonal “whiplash.”
• Trellis best practices: Usually two layers suffice (third is for extremes). Lay both layers in one visit and “hover” the second high, then slide down as plants grow.
• Aim for ~1 top per square; redistribute shoots early before