How a Day of Irrigation Is Shaped
The P0–P3 phase model is the shared skeleton behind every fertigation program — learn shot-size-as-%VWC, 'transpiration before irrigation,' and how the curve encodes veg vs generative intent.
Irrigation is a shape, not a schedule
A commercial irrigation program is not "water three times a day." It is a shape drawn across the 24-hour photoperiod — a rise to field capacity, a flat maintenance band, and a controlled overnight decline. That shape is the same whether you run coco or rockwool; only the numbers move. Master the shape first, and both media tracks become the same skill applied at two different tolerances.
The industry-standard framework (Agrowtek/AROYA/Grodan all converge here) breaks the day into four phases — P0, P1, P2, P3 — running over the lights-on period starting at lights-on. Critically, shot size is expressed as %VWC: a "3% shot" is however much runtime raises volumetric water content by 3 percentage points, not a fixed volume. This is what makes a recipe portable across pot sizes.
| Phase | When | What happens | Veg shot / dryback | Generative shot / dryback |
|---|---|---|---|---|
| P0 — Activation | Lights-on, 1–3 h, no irrigation | Substrate dries; wait for transpiration to trigger | 2–4% VWC dryback | 4–6% VWC dryback |
| P1 — Ramp / Saturation | Reach field capacity + runoff within 1–3 h | Stack shots to hit FC and first drain | 3% shots, 1% dryback between | 4–8% shots, 2–3% dryback between |
| P2 — Maintenance | Midday, bulk of the day | Hold VWC steady with repeating shots | 1–3% shots, 1–3% dryback | 4–8% shots, shot ≈ dryback → low runoff |
| P3 — Overnight dryback | Begins 1–2 h before lights-off | No/little water; substrate declines overnight | 10–15% of FC | 20–30% of FC |
The golden rule of the day is "transpiration before irrigation." P0 deliberately withholds water at lights-on so the plant wakes up, opens stomata, and begins pulling water through the transpiration stream before you re-saturate. Irrigating at lights-on — the coldest, least-active moment — waterlogs roots and drowns the P0 dryback signal you steer on. The first drain of the day (end of P1) should land during the active transpiration window, not at the light flip.
The single most important idea: the curve encodes steering intent. Small frequent shots with small drybacks and runoff every event = vegetative (bushy growth, canopy build). Fewer larger shots, big overnight drybacks, and minimal runoff that lets salts stack = generative (flower initiation, density, resin). Environment (VPD, leaf temp, light) is the throttle; irrigation EC and dryback are the fine-tune.
You don't need a $2,000 controller to run P0–P3. Hand-water in the same shape: skip the first hour after lights-on (P0), water to a little runoff mid-morning (P1), give one or two small maintenance drinks midday (P2), and stop watering a couple hours before lights-off so the pot dries back overnight (P3). Lift the pot — a lighter pot in the morning means your dryback is working.
The environment is the throttle; irrigation EC and dryback are the fine-tune.