Irrigation Management

Crop Steering Irrigation Phases: P0, P1, P2 and P3 Explained

Hyper Yield Team·6 min read

Quick Answer: The four crop steering phases are P0, P1, P2 and P3, and they describe the daily irrigation cycle in a soilless commercial grow. P0 is the overnight dry-back — no irrigation, the substrate loses water content while the lights are off. P1 is the morning ramp: a series of shots that refill the substrate to field capacity and trigger runoff. P2 is the maintenance window: smaller shots that hold water content within a target band through the photoperiod. P3 is the final pre-dark phase, where you cut irrigation early and let an intraday dry-back begin. The size, timing and frequency of those shots — not the calendar — are what steer the plant generative or vegetative. Phase structure is the lever; water content, EC and dry-back are the readouts.

Why Phases, Not a Schedule

Most facilities that struggle with crop steering are not running a steering strategy at all. They are running a watering schedule — six shots a day, fixed mL, fixed clock times — and hoping the plant cooperates. It does not. A schedule treats every zone the same regardless of stage, light intensity, substrate condition or how the crop is actually responding.

Phases are the alternative. The P0, P1, P2 and P3 framework breaks the 24-hour cycle into four functional windows, each with a job. Once you think in phases, you stop asking "how much should I water" and start asking "what is each phase supposed to accomplish today, and are the numbers confirming it." That is the difference between irrigating a crop and steering one.

A watering schedule is a guess repeated on a timer. A phase strategy is a decision you can read, confirm and adjust every single day.

This guide walks each phase in order, covers shot sizing, explains how phase structure produces generative versus vegetative steering, and connects the whole thing back to lb/light — because the only reason to steer at all is to put more weight under each light without adding cost.

P0 — The Overnight Dry-Back

P0 is the phase where nothing happens, and that is the point. The lights are off, irrigation is paused, and the substrate slowly loses water content as the plant continues low-level transpiration and the root zone equilibrates. The result is your overnight dry-back — the single most important steering signal in the daily cycle.

P0 sets up the next day. A deep overnight dry-back — substrate water content falling well below field capacity before lights-on — applies generative pressure: it stresses the plant toward flower development, tighter internodes and bulking. A shallow dry-back keeps conditions vegetative: more lush, more stretch, softer structure. You are not steering during P0; you are reading the consequence of yesterday's P1, P2 and P3 decisions.

The mistake here is treating P0 as dead time. It is the most informative window you have. The WC% at lights-on, compared to the WC% at field capacity the evening before, is your overnight dry-back percentage — and that number tells you whether yesterday's irrigation actually steered the crop the way you intended. If the dry-back stalled, the crop drifted vegetative whether you wanted it to or not.

P1 — The Morning Ramp to Field Capacity

P1 begins shortly after lights-on. Its job is to refill the substrate from its dried-back overnight state back up to field capacity — the point where the substrate holds all the water it can against gravity — and to push enough volume through to generate runoff.

P1 is not one big shot. It is a ramp: a series of moderate shots spaced minutes apart, each one raising water content a step at a time until the substrate is saturated and drain begins. Ramping rather than dumping does two things. It rewets the substrate evenly instead of channeling water straight down the path of least resistance, and it gives you a controlled point to read drain EC — the EC of the runoff tells you what salt level the root zone actually sat at overnight.

P1 is also where you reset EC. If you steered generative overnight and let drain EC climb, P1 is your chance to flush the root zone back toward your target feed EC with fresh nutrient solution. The number of P1 shots, how aggressively they ramp, and how much runoff you target are all steering decisions. A fast, high-runoff P1 resets the root zone hard and leans vegetative; a slower, lower-runoff P1 leaves more residual EC and holds generative pressure.

Reading drain to confirm P1

Drain is the confirmation layer for P1. Two numbers matter:

Runoff percentage — what fraction of applied volume came out the bottom. Too little and you are not refreshing the root zone; too much and you are wasting water and nutrient and flattening your EC strategy.
Drain EC — compared to feed EC, the delta tells you whether salts accumulated overnight (drain higher than feed) or whether the root zone is being diluted (drain near or below feed).

If P1 ends and the drain numbers do not match what you intended, the phase did not do its job — and P2 inherits the problem.

P2 — The Maintenance Window

P2 is the longest phase. It runs from the end of the morning ramp through most of the photoperiod, and its job is to hold substrate water content within a target band — not to refill to field capacity every time, just to keep the crop in the zone you want.

P2 shots are smaller and less frequent than P1 shots. They are top-ups: each one nudges water content back up a few points before the plant's transpiration pulls it back down. The size of the band you let water content swing within is itself a steering decision. A tight P2 band — small, frequent shots, minimal swing — keeps conditions stable and leans vegetative. A wider P2 band — larger shots, more time between them, a bigger intraday dry-back between shots — applies generative pressure through the photoperiod, not just overnight.

P2 is where most of the day's total irrigation volume gets delivered, and it is where multi-zone facilities lose the most consistency. A zone in flower-stretch and a zone in week-six bulk need different P2 bands, different shot sizes and different intervals. Run them on the same P2 settings and one of them is being mis-steered. This is exactly the per-zone problem that makes manual steering at 109-zone scale so hard to hold together by hand.

P3 — The Final Shot and Intraday Dry-Back

P3 is the pre-dark phase. Before lights-off, you stop irrigation early — earlier than the plant's transpiration would otherwise call for — and let an intraday dry-back begin while the lights are still on. The substrate starts losing water content before P0 even starts.

P3 controls how the crop enters the dark period. Cutting irrigation early means the plant goes into lights-off already dried back somewhat, which deepens the total overnight dry-back and strengthens generative steering. Carrying irrigation late into the photoperiod does the opposite — the plant enters the dark period closer to field capacity, the overnight dry-back is shallower, and the crop leans vegetative.

The timing of that final shot is one of the cleanest steering levers you have. Move the last P3 shot 30 to 60 minutes earlier and you have meaningfully changed the next P0 without touching shot size or frequency. P3 is small in volume but large in consequence.

How Phases Produce Generative vs Vegetative Steering

Here is the whole framework in one view. Every phase has a generative setting and a vegetative setting, and you steer by choosing where on that range each phase sits:

P0 — deeper dry-back is generative; shallow dry-back is vegetative.
P1 — slower ramp, lower runoff, higher residual EC is generative; fast ramp, high runoff, hard EC reset is vegetative.
P2 — wider water-content band, larger less-frequent shots is generative; tight band, small frequent shots is vegetative.
P3 — earlier final shot, longer intraday dry-back is generative; later final shot is vegetative.

You do not flip one switch. You set all four phases in the same direction for a clear steering signal, or you split them deliberately during a transition. The veg-to-flower handoff, for example, is a gradual walk of all four phases from vegetative toward generative over the stretch period — not an overnight change.

Substrate matters here too. Rockwool holds and releases water fast, so its dry-backs are sharper and its phases are more responsive — small changes show up quickly. Coco buffers more: it holds water tighter, dries back more slowly, and forgives a mistake longer but also responds to a correction more slowly. The phase framework is identical across both; the shot sizes, intervals and dry-back targets are not.

Shot Sizing: The Number That Makes Phases Real

Phases are a strategy. Shot size is how you execute it. A shot is expressed as a percentage of substrate volume — a 6% shot into a 6-liter block delivers roughly 360 mL. Get shot size wrong and the phase structure is just a label.

General commercial starting points, to be tuned to your substrate, genetics and stage:

Phase	Objective	Typical timing	Shot-size guidance
P0	Overnight dry-back	Lights-off to lights-on	No irrigation — read the resulting dry-back %
P1	Ramp to field capacity, trigger runoff, reset EC	Starts ~30–90 min after lights-on	Series of ~2–6% shots, ramped, until runoff begins (target ~10–20% runoff)
P2	Maintain WC% within target band	Bulk of the photoperiod	Smaller ~1–3% top-up shots; interval set by how fast WC% falls
P3	Final shot, begin intraday dry-back before dark	Last ~1–3 hrs of photoperiod	Stop early; one small final shot or none — earlier = more generative

These are starting points, not prescriptions. The right shot size for your facility depends on substrate volume per plant, plant count per zone, genetics, environment and stage — which is exactly what a calculator is for.

Use the interactive Irrigation Shot-Size Calculator below to run your own numbers — enter your substrate volume per plant, target shot size as a percentage, and plants per zone, and it returns mL per shot per plant and liters per shot per zone, with quick P1/P2 guidance.

The Hyper Yield Angle

Here is the connection that matters: the phase framework in this guide is not a separate thing from what Hyper Yield produces — the morning directives are the per-zone P1 and P2 parameter set.

Every night, the pipeline pulls live Aroya data for all 109 zones — substrate water content, EC, VPD, CALC%, temperature, RH and drain metrics. It reads each zone's overnight dry-back against its stage-aware target, checks drain EC against feed EC, and grounds every decision in the facility's own SOP. Then it generates the directive: P1 and P2 shot parameters, EC target, pH target, shot count, runoff percentage and interval — per zone, every morning, before the team walks the rooms.

The grow team's job becomes review and execute — Accept, Modify or Dismiss — instead of originating 109 phase decisions by hand at 4 AM. Every override is logged, so the head grower can see exactly where and why the team deviated from the directed phase strategy. When a zone is missing data, the directive says so and recommends a conservative adjustment rather than steering blind. The phase framework is the strategy; the nightly directive is that strategy made specific, consistent and repeatable across every zone.

That consistency is what shows up in lb/light. Steering that is correct on Tuesday and rushed on Friday produces variance, and variance is the largest controllable drag on yield efficiency in a commercial grow. Holding the phase structure steady across every zone, every day, is how you push lb/light up without adding a single watt or a single labor hour.

Frequently Asked Questions

What are the four crop steering phases? P0, P1, P2 and P3. P0 is the overnight dry-back with no irrigation. P1 is the morning ramp that refills the substrate to field capacity and triggers runoff. P2 is the maintenance window that holds water content within a target band through the photoperiod. P3 is the final pre-dark phase where you cut irrigation early to start an intraday dry-back.

What is the difference between P1 and P2? P1 refills the substrate all the way to field capacity and generates runoff, using a ramped series of moderate shots — it also resets root-zone EC. P2 only maintains: it uses smaller, less frequent top-up shots to hold water content within a band, and it delivers most of the day's total irrigation volume.

How do irrigation phases steer generative vs vegetative growth? Each phase has a generative and a vegetative setting. Deeper dry-backs (P0), slower lower-runoff P1s, wider P2 bands and earlier P3 final shots all apply generative pressure. Shallow dry-backs, hard P1 EC resets, tight P2 bands and late final shots keep the crop vegetative. You set all four phases in the same direction for a clear steering signal.

How big should an irrigation shot be? Shot size is expressed as a percentage of substrate volume. P1 shots typically run around 2–6% of substrate volume, ramped until runoff begins; P2 top-up shots are smaller, around 1–3%. A 6% shot into a 6-liter block is roughly 360 mL. Tune these to your substrate, genetics and stage.

Do rockwool and coco use different phases? The four-phase framework is identical for both. The shot sizes, intervals and dry-back targets are not. Rockwool releases water faster, so its dry-backs are sharper and phases respond quickly. Coco buffers more — it dries back slower and responds to corrections more slowly — so it tolerates wider intervals.

How do I confirm a phase actually worked? Read the substrate and drain data. P0 is confirmed by the overnight dry-back percentage. P1 is confirmed by runoff percentage and drain EC versus feed EC. P2 is confirmed by water content staying inside its target band. P3 is confirmed by the intraday dry-back showing up before lights-off.

Summary

The four crop steering phases — P0, P1, P2 and P3 — turn a fixed watering schedule into a daily steering strategy you can read, confirm and adjust. P0 is the overnight dry-back you read; P1 is the ramp that refills to field capacity and resets EC; P2 is the maintenance band that carries most of the day's volume; P3 is the early cut-off that begins an intraday dry-back. Shot size, expressed as a percentage of substrate volume, is how you execute the strategy, and water content, EC and dry-back are how you confirm it. Set all four phases in the same direction to steer generative or vegetative. At commercial scale, the hard part is holding that phase structure consistent across every zone every day — which is exactly what a nightly per-zone directive does, and exactly why consistent phase execution shows up as higher lb/light.

Related reading:

See what Hyper Yield does for lb/light at your facility. Book a demo →