# Drain-to-waste drip and crop steering (generative vs vegetative dryback)

> Drain-to-waste drip sends fresh solution every shot and discards runoff, so you control the root zone by irrigation timing, shot size and substrate EC measured with a VWC/EC probe. Vegetative steering uses earlier first shots, larger shots and small overnight drybacks (about 10–15 % VWC); generative steering delays the first shot, uses smaller shots, longer P3 and larger drybacks (about 20–35 %) with higher substrate EC. The ranges are grower practice; one cannabis study found controlled late-flower drought raised cannabinoid concentration.

- Canonical: https://genetixbay.com/knowledge/systems/drain-to-waste-crop-steering.html
- Category: Systems · Level: Advanced
- Evidence: Grower practice · Status: Draft: awaiting expert review (some figures are being checked)
- Stage: Veg, Transition, Flower · Systems: Drip, Coco, Rockwool
- Updated: 2026-10-31 · Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay Knowledge Base (genetixbay.com/knowledge)"

## Questions and answers

**What is crop steering?** Crop steering is controlling cannabis growth through irrigation timing, shot size, dryback and substrate EC, measured with a substrate moisture and EC probe. In drain-to-waste drip every shot is fresh solution, so the [root zone](https://genetixbay.com/knowledge/glossary/root-zone.md) becomes the control surface, and growers push the plant toward leafy bulking or toward flowering. The specific ranges are grower practice adapted from greenhouse tomato work.

**What is generative vs vegetative steering?** Vegetative steering pushes leaf and stem growth: first shot 0.5–1.5 hours after lights-on, larger shots, a small 10–15% VWC overnight dryback and input EC around 1.8–2.4 [mS/cm](https://genetixbay.com/knowledge/glossary/ms-cm.md). Generative steering pushes flowering and tighter internodes: first shot 2–3 hours after lights-on, smaller shots, last shot 4–6 hours before lights-off, a 20–35% dryback and EC 2.4–3.2 mS/cm.

**What are P1, P2 and P3 irrigation phases?** P1, P2 and P3 are the three daily irrigation phases in crop steering. P1 is the ramp-up from lights-on, with small shots every 15–30 minutes until the substrate reaches field capacity and runoff starts. P2 is maintenance, shooting whenever moisture falls 2–4% below the daily maximum. P3 is the dry-down, with no shots from a set time before lights-off so the substrate dries overnight.

**How much dryback should I run in flower?** Dryback in flower depends on the steer. Grower practice uses a vegetative 10–15% VWC overnight dryback for bulking in flower weeks 3–5, and a generative 20–35% dryback at the flip and in late flower. Push dryback a few points at a time, watch for midday wilting, and pull back toward 15% if leaf tips burn or growth stalls.

**Do I need a substrate sensor for crop steering?** A substrate sensor is effectively required for crop steering, because steering means acting on measured volumetric [water content](https://genetixbay.com/knowledge/glossary/moisture-content.md) and substrate EC; steering by feel is a common mistake. Use a calibrated probe in a representative pot, not an edge pot or the biggest plant. Two or three probes with median voting are better, and spend the first 2 days learning field capacity.

**Does drought stress increase THC?** Controlled drought stress late in flower can increase THC concentration, according to one cannabis study. Researchers found controlled late-flower drought raised [THCA](https://genetixbay.com/knowledge/glossary/thca.md) and [CBDA](https://genetixbay.com/knowledge/glossary/cbda.md) concentration by roughly 12–13% and increased inflorescence [dry weight](https://genetixbay.com/knowledge/glossary/dry-weight.md), but warned against generalising to every cultivar. GenetixBay endorses mild late drought: dry back a few points at a time and stop at the first sign of stalling.

**Short answer.** In drain-to-waste (DTW), each shot is fresh solution and runoff is discarded, so reservoir chemistry is simple and the **root zone** becomes the control surface. With a substrate **VWC/EC probe** you steer the plant: **vegetative** = earlier first shot, bigger shots, small overnight dryback (~**10–15 % VWC**), lower substrate EC; **generative** = later first shot, smaller shots, earlier last shot, larger dryback (~**20–35 %**), higher substrate EC [5]. These numbers are *Grower practice* adapted from greenhouse tomato work. The one controlled cannabis study we have found that a **controlled drought late in flower** raised THCA and CBDA concentration by roughly 12–13 % [1].

## The vocabulary

- **VWC:** volumetric water content of the substrate (%).
- **Field capacity:** max VWC after drainage. Learned per pot/slab in the first days.
- **Dryback:** drop in VWC between the last shot of one day and the first of the next (overnight), or across the day.
- **P1 (ramp-up):** from lights-on, small shots every 15–30 min until VWC hits field capacity and runoff starts.
- **P2 (maintenance):** shots whenever VWC falls a set amount (2–4 %) below the daily max.
- **P3 (dry-down):** no shots from a set time before lights-off; the substrate dries overnight.
- **Vegetative steer:** encourages leaf/stem growth and bulking. **Generative steer:** encourages flowering, tighter internodes, and is used in transition and late flower.

## Numbers that matter

From Jardin's steering table [5]:

| Lever | Vegetative steer | Generative steer |
|---|---|---|
| First shot after lights-on | 0.5–1.5 h | 2–3 h |
| Last shot before lights-off | 2–3 h before | 4–6 h before |
| Overnight dryback | 10–15 % VWC | 20–35 % VWC |
| Shot size (% pot/slab volume) | Larger, fewer (4–6 %) | Smaller, more in P1 (2–3 %), longer P3 |
| [Input EC](https://genetixbay.com/knowledge/glossary/input-ec.md) (coco/rockwool) | 1.8–2.4 mS/cm | 2.4–3.2 mS/cm (slab EC drifts up on purpose) |
| Day/night temperature difference | Small (1–2 °C / 2–4 °F) | Larger (4–6 °C / 7–11 °F) |

A common schedule: vegetative steer in veg and in flower weeks ~3–5 for bulking; generative at the flip (to control stretch) and in late flower [5].

**Mild late drought (house-endorsed).** Letting plants go a bit dry in late flower is *Supported* by one cannabis study, in which controlled drought raised inflorescence dry weight and [cannabinoid](https://genetixbay.com/knowledge/glossary/cannabinoid.md) content [1], and it's house-endorsed: the founder uses it, and some cultivars thrive when allowed to go a bit dry. Every cultivar responds differently. Push dryback a few points at a time, watch for midday wilting, and stop at the first sign of stalling.

## How to run it

1. Install a calibrated probe in a **representative** pot (not the edge, not the biggest plant). Two or three probes with median voting are better.
2. Spend 2 days learning field capacity: shoot until runoff, record the plateau VWC.
3. Set P1 to reach field capacity in 3–6 shots; P2 to hold within 2–4 % of max; P3 to hit your dryback target by lights-on.
4. Track substrate EC daily. If it runs above input + 1.5 mS/cm, add a flush shot or more runoff.
5. Change one lever at a time and keep notes. Plants answer over 3–7 days.

## Failure modes

| Problem | Sign | Jardin response [5] |
|---|---|---|
| Probe fault or out of the pot | Implausible flat or jumpy readings; VWC doesn't rise after a shot | Fall back to schedule-only recipe; alert |
| Waste tank full | Float trips | Pump lockout before next shot |
| Substrate EC runaway | EC > input + 1.5 | Automatic flush shot with plain water |
| Over-steering generative | Leaf [tip burn](https://genetixbay.com/knowledge/glossary/nutrient-burn.md), stalled growth, wilting by mid-day | Bring dryback back toward 15 %, lower EC |

## Jardin notes

Jardin method "Drip, drain-to-waste + crop steering": required substrate VWC/EC/temperature probe, runoff mass or flow, waste tank float, reservoir mass. Fresh reservoir every 3–7 days (no uptake model needed). Time-to-harm 12–36 h. Level 2 at launch; Level 3 steering is unlocked only after 10 logged steered grows on that substrate and genetic [5].

## Common mistakes

- **Flushing before harvest.** Pre-harvest flushing is a *Myth* that costs you yield and resin: it starves the plant while it packs on its final weight, trichomes and resin. Feed through to harvest at stage strength. The only valid flush is a [root-zone flush](https://genetixbay.com/knowledge/glossary/root-zone-flush.md) to clear [salt build-up](https://genetixbay.com/knowledge/glossary/salt-build-up.md), at any stage (see [the flush debate](https://genetixbay.com/knowledge/harvest-cure/flush-debate.md)). A flush shot when substrate EC runs away is root-zone management, not a pre-harvest flush; in coco, a light solution protects Ca and Mg better than plain water.
- Steering without a probe, by feel.
- Copying a commercial dryback onto a small pot that dries 3× faster.
- Running generative all of flower and losing bulk.
- Mistaking a dry corner pot for the room average.

## Evidence

The mechanism (water and [salt stress](https://genetixbay.com/knowledge/glossary/sodium.md) shift assimilate partitioning toward generative growth) is *Supported* in greenhouse horticulture [3]. Controlled drought late in flower raising cannabinoid concentration is *Supported* by one cannabis study, which its authors say should not be generalised to all cultivars [1], and mild late drought is house-endorsed with the same caveat: every cultivar responds differently; reduced irrigation frequency saved water with little impact on yield or THC in another [2]. The specific P1/P2/P3 ranges are *Grower practice* [4][5].

## Sources

1. Caplan D, Dixon M, Zheng Y, 2019. Increasing inflorescence dry weight and cannabinoid content in medical cannabis using controlled drought stress. HortScience 54(5):964–969. https://doi.org/10.21273/HORTSCI13510-18
2. Stemeroff J, 2018. Irrigation management strategies for medical cannabis in controlled environments. MSc thesis, University of Guelph. http://hdl.handle.net/10214/12125
3. Raviv M, Lieth JH, Bar-Tal A (eds), 2019. Soilless Culture: Theory and Practice, 2nd ed. Elsevier (irrigation in soilless production). https://doi.org/10.1016/C2015-0-01807-5
4. Greenhouse Grower. The details behind crop steering in cannabis production. https://www.greenhousegrower.com/production/the-details-behind-crop-steering-in-cannabis-production/
5. GenetixBay Jardin engineering notes, 2026 (system profiles and failure margins).