# Dryback

> Dryback is the drop in a substrate's water content between irrigations, usually expressed as a percentage of its content at full saturation.

- Canonical: https://genetixbay.com/knowledge/glossary/dryback.html
- Also written: dry-back, dry back, drybacks, substrate dryback, overnight dryback
- Glossary category: Systems and media · Evidence: Grower practice
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

Dryback is how much a growing medium dries between one irrigation and the next. It is measured as the fall in volumetric [water content](https://genetixbay.com/knowledge/glossary/moisture-content.md) (from a substrate sensor) or in pot weight (from a scale), and expressed as a percentage. It is the core control variable in crop steering.

## In practice

A [rockwool](https://genetixbay.com/knowledge/glossary/rockwool.md) block or [coco](https://genetixbay.com/knowledge/glossary/coco-coir.md) pot is brought to near [container capacity](https://genetixbay.com/knowledge/glossary/field-capacity.md) in the morning, then allowed to dry during the day and overnight. If a sensor reads 65% water content after the last irrigation and 52% before the first one next morning, the overnight dryback is 13 percentage points, or 20% of the starting value. Smaller drybacks favour vegetative growth; larger drybacks favour generative growth and can concentrate root-zone [EC](https://genetixbay.com/knowledge/glossary/ec.md). Too large a dryback stresses the plant and can damage roots on rewetting.

Always state which way you're calculating it: percentage points of water content, or percentage of the starting value.

## Units and formula

Dryback (%) = (WC_max − WC_min) / WC_max × 100, or simply WC_max − WC_min in points.

## Don't confuse with

- **Runoff**: water draining out; dryback is water lost by uptake and evaporation.

## Related terms

[Crop steering](https://genetixbay.com/knowledge/glossary/crop-steering.md), [Field capacity](https://genetixbay.com/knowledge/glossary/field-capacity.md), [Wet-dry cycle](https://genetixbay.com/knowledge/glossary/wet-dry-cycle.md), [Rockwool](https://genetixbay.com/knowledge/glossary/rockwool.md), [Coco coir](https://genetixbay.com/knowledge/glossary/coco-coir.md)

## Read more

- [Drain-to-waste drip and crop steering (generative vs vegetative dryback)](https://genetixbay.com/knowledge/systems/drain-to-waste-crop-steering.md)
- [Growing in rockwool (stonewool cubes and slabs)](https://genetixbay.com/knowledge/systems/rockwool.md)

## Sources

1. Stemeroff J, 2018. Irrigation management strategies for medical cannabis in controlled environments. MSc thesis, University of Guelph. http://hdl.handle.net/10214/12125