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]being reviewed:
| 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 (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 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
- Install a calibrated probe in a representative pot (not the edge, not the biggest plant). Two or three probes with median voting are better.
- Spend 2 days learning field capacity: shoot until runoff, record the plateau VWC.
- 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.
- Track substrate EC daily. If it runs above input + 1.5 mS/cm, add a flush shot or more runoff.
- 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, 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 to clear salt build-up, at any stage (see the flush debate). 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 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].
