Dry-Back in Crop Steering: How to Measure and Set It
Dry-back is how much water the substrate loses between irrigations. It is the main number growers steer with, and the one most often misread, because guides calculate it in different ways.
Three ways to calculate dry-back
Take a substrate that peaks at 60% VWC in the evening and is at 45% VWC at lights-on.
| Method | Formula | Result for 60% to 45% |
|---|---|---|
| VWC points | peak − low | 15 points |
| Percent of peak | (peak − low) ÷ peak × 100 | 25% |
| Percent of field capacity | (field capacity − low) ÷ field capacity × 100 | Depends on field capacity; 25% if the peak was at field capacity |
A target of "20% dry-back" can mean 20 VWC points or 20% of the peak. On a 60% peak those are 40% and 48% VWC in the morning: a large difference in plant stress. Always check which method a guide uses before copying its numbers.
This guide uses percent of peak VWC unless it says otherwise. It stays meaningful across substrates with different field capacities.
Overnight dry-back
The overnight dry-back runs from the last irrigation of the day to the first irrigation the next morning. It is the main steering signal because it happens every day and it is set by one decision: when you stop watering.
| Direction | Overnight dry-back (percent of peak) | Last shot before lights-off |
|---|---|---|
| Vegetative | 10 to 15% | 0 to 2 h |
| Generative | 20 to 30% | 2 to 4 h |
To make the overnight dry-back larger, move the last shot earlier by 30 to 60 minutes and watch for three nights. To make it smaller, add a shot later in the day. Change one thing at a time.
Daytime dry-back
The daytime dry-back is the fall in VWC between shots during P2 maintenance. Small daytime dry-backs of 1 to 3 VWC points keep the plant vegetative. Larger ones of 3 to 8 points are generative. The size of each shot relative to this dry-back also decides whether substrate EC rises or falls; see P2 maintenance.
How to measure it
- Log VWC continuously. A reading every few minutes from a substrate sensor. Spot checks miss the peak and the low.
- Find the peak after the last shot. Wait until the water has spread through the substrate: 15 to 60 minutes depending on substrate and pot size. The first reading right after a shot is usually too high.
- Find the low just before the first shot. That includes the extra drop during P0 after lights-on.
- Calculate with one method and write down which method you used.
- Compare sensors. If two pots in the same zone differ by more than a few points, investigate before changing the recipe. One sensor is not a zone.
Pot weight is a good cross-check. Weigh a representative pot at the evening peak and at lights-on. The weight lost is water lost, and it does not depend on sensor placement.
Read the curve, not just the number
Two nights can have the same dry-back and very different curves. Look at:
- Slope after lights-off. A steep early drop means the plant was still transpiring hard; a flat line means it stopped quickly.
- Plateaus. A curve that stops falling in the middle of the night can mean water has finished draining and redistributing, or the plant has shut down.
- Steps. A sudden jump up or down usually means an unplanned irrigation, a leak or a sensor fault.
- Night-to-night change. A dry-back that grows steadily through a week is the plant getting bigger. A sudden change is usually the equipment or the room.
Common mistakes
- Copying a dry-back number without knowing its method.
- Measuring the peak straight after a shot, before water has spread.
- Steering from one sensor in a spot that is always wetter or drier than the rest.
- Changing last-shot time, EC and climate in the same week, then not knowing which one worked.
- Pushing dry-back so far that the next shots run straight through instead of soaking in. Watch for early runoff with little VWC rise.