What Overnight Dryback Reveals About Your Irrigation
While you sleep, the substrate is telling you a story about plant health. The overnight drop in substrate moisture β called dryback β is one of the most direct signals of whether your irrigation strategy is working.
What dryback is
Dryback is the percentage the substrate moisture drops from its peak after the day's last irrigation to its low point before the next day's first. If substrate water content (WC) peaks at 60% in the evening and falls to 48% by morning, that's a 12% overnight dryback.
Why the night matters: with no irrigation interfering, you see the plant's real water use plus pure evaporation β the cleanest window for measuring how much the plant drinks and how well the roots are working.
What dryback tells you
- Too little dryback (e.g. <5%) β the substrate stays saturated all night, roots lack oxygen, and you risk root disease and sluggish uptake. Usually means watering too much or too late.
- Too much dryback (e.g. >20% unintentionally) β the plant stresses from water shortage in the early morning. May mean under-watering, shallow roots, or a substrate that holds little water.
- Consistent dryback in the target band β a sign of healthy roots and a well-tuned irrigation schedule. A common reference is ~10β15%, adjustable by strategy (deliberately pushing dryback a little deeper in flower is a generative-steering technique).
Numbers are starting points β they depend on substrate, pot size, strain, and stage.
How to steer with dryback
Watching dryback over several days lets you adjust with data, not guesswork: dryback getting progressively shallower β reduce late-day irrigation volume or frequency; an abnormally deep dryback one day β check for clogged emitters or a pump issue; erratic, unstable dryback β often a sign of uneven water distribution across the table.
Paired with VPD, the picture is complete: VPD is the air's pull on water, dryback is the response at the roots and substrate. Reading them together is the heart of crop steering.
Why it has to be continuous data
Dryback can't be measured by walking over once a day β it's the whole shape of the overnight curve. It needs a sensor logging substrate moisture frequently around the clock. And here's the built-in bonus: the same sensors produce your GACP environmental records automatically. You grow better and get inspection-ready documentation from a single effort.
In short: VPD is what the air does to the plant; dryback is what the plant does to the water. Control both, and you're truly steering the crop.
Continue: What is VPD, and target values by week Β· GACP software vs spreadsheets