Crop Steering Irrigation Schedule: P0, P1, P2 and P3
A crop steering day is split into four irrigation phases between one lights-on and the next. You control the first three directly. The fourth, the overnight dry-back, is the result of when you stopped watering.
| Phase | What happens | Ends when |
|---|---|---|
| P0 Activation | No irrigation. Plants start transpiring and VWC keeps falling from the overnight low. | A set extra dry-back is reached, VWC hits a safe minimum, or a time limit runs out |
| P1 Ramp-up | A series of shots, with a short wait between each, brings the substrate back to field capacity. | Target VWC or first runoff is reached |
| P2 Maintenance | A shot each time VWC falls a set amount below target. Holds water and EC steady. | A fixed time before lights-off |
| P3 Dry-down | No irrigation, apart from small emergency shots. VWC falls overnight. | Next lights-on |
P0: wait for the plant to wake up
After lights-on, stomata open and the plant starts drawing water. P0 gives it time to do that before you irrigate. The extra VWC drop you see during P0 is sometimes called the additional dry-back.
- Vegetative: first shot 0 to 1 h after lights-on, with 1 to 4 VWC points of extra dry-back.
- Generative: first shot 1 to 3 h after lights-on (some guides go to 4 h), with 4 to 6 points.
- Always set a hard minimum VWC. If the substrate reaches it, P0 ends regardless of the clock.
If VWC does not fall at all during P0, do not treat that as a reason to wait longer. It points to low light, cold roots, very high humidity, a root zone that is still saturated, or a faulty sensor.
P1: ramp back to field capacity
Several small shots work better than one large one. The substrate has time to take up each shot evenly instead of letting water run through channels.
- Shot size: typically 2 to 6% of substrate volume. Vegetative runs use the smaller end, generative the larger.
- Interval: roughly 15 to 30 minutes between shots, longer when steering generatively.
- A typical P1 is 5 to 9 shots. It ends at your target VWC or when runoff starts.
Count all connected substrate when you calculate shot size. A rockwool block sitting on a slab is one volume. See how to calculate shot size.
Runoff early in P1 while VWC barely moves is a warning sign, not a success. It usually means water is channeling through dry or hydrophobic substrate.
P2: maintenance shots
P2 holds the root zone where you want it through the productive part of the day. Each time VWC falls a set amount below target, a shot restores it.
- A neutral starting rule: shot volume about equal to the water lost since the last peak.
- Shots larger than the dry-back produce runoff and hold substrate EC down (vegetative). Shots smaller than the dry-back let EC build up (generative).
- Dry-back between P2 shots of 1 to 3 VWC points is vegetative; 3 to 8 points is generative.
- P2 is optional. If P1 alone gives the next-day VWC you want, skip it.
P3: the overnight dry-back
P3 is where most steering happens, and you do not control it directly. You control it through the time of your last P2 shot.
- Vegetative: last shot 0 to 2 h before lights-off. Overnight dry-back of 10 to 15% of peak VWC.
- Generative: last shot 2 to 4 h before lights-off. Overnight dry-back of 20 to 30% of peak VWC.
- To increase the overnight dry-back, move the last shot earlier. To reduce it, add a later shot.
Look at the whole overnight curve, not just the start and end values. A curve that flattens early, drops in steps, or differs between sensors tells you more than a single dry-back number. How to measure dry-back covers the calculation.
Common faults
| What you see | Likely causes | Check first |
|---|---|---|
| Shot overshoots target | Shot too large, water not yet spread through the substrate, wet sensor spot | Emitter flow and valve time; wait before judging; compare sensors |
| Shot barely raises VWC | Clogged emitter, leak, channeling, dry substrate, faulty sensor | Measure emitter output and runoff; inspect the substrate |
| VWC stays high overnight | Last shot too late, low transpiration, oversized pots, poor drainage | Climate at night; move last shot earlier gradually |
| VWC drops below safe minimum by morning | Last shot too early, rising plant demand, small or root-bound pots, hot room | Confirm with a second sensor or pot weight; move last shot later |
| Substrate EC climbs every day | Too little runoff, strong feed, large dry-backs | Input, drain and substrate EC together; runoff volume |
Phase names in other guides
Commercial programs use the same structure with slightly different labels. Some start counting at P1 and call the post-lights-on wait "additional dry-back" rather than P0. Some fold P1 and P2 together. The logic is the same: wait, ramp up, hold, then let the substrate dry overnight.