A dryback is the drop in substrate water content between the last irrigation of one day and the first of the next. It is the single most useful curve a root zone gives you, and most rooms never see it because nothing is measuring the substrate overnight.
What the curve tells you
Water content falls as the plant drinks and the medium loses moisture. The shape of that fall is a readout of plant behaviour:
- A steep dryback means the plant is transpiring hard: healthy in veg, a generative push in flower.
- A shallow dryback means the plant drank little: cold, dark, oversaturated, or stressed.
- A flat line overnight usually means the medium never drained. The plant spent the night in water it could not use.
Timing the first irrigation
The first shot of the day is a steering lever. Hit it too early and you cut the dryback short, keeping the room vegetative. Hold it and let the medium dry further and you push the plant generative. You cannot make that call on a schedule; you make it on the number.
A rough frame to read against:
| Dryback overnight | What it usually means | Typical move |
|---|---|---|
| 3–8 % | Gentle, vegetative | Fine for early veg |
| 10–15 % | Balanced steering | Standard flower target |
| 20 %+ | Aggressive, generative | Push, but watch for stress |
The exact targets depend on medium, genetics and stage. The point is that you are reading a measured curve, not following a clock.
Steer the plant, not the timer
Once the dryback is on a chart, irrigation stops being a fixed recipe and becomes a response. You dial the size of the dryback to steer the crop toward the plant you want, and you catch the night the curve goes flat before it costs you a week.
The substrate has been telling you this every night. A probe just lets you hear it.