# Watering to runoff

> Watering to runoff means applying enough water or solution that some drains from the bottom of the container, typically 10-20% of the volume applied.

- Canonical: https://genetixbay.com/knowledge/glossary/watering-to-runoff.html
- Also written: water to runoff, watering till runoff, drain-through, 10-20% runoff, leachate
- Glossary category: Cultivation and training · Evidence: Established
- 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"

Watering to runoff is irrigating until a portion of the applied water [drains](https://genetixbay.com/knowledge/glossary/runoff.md) out of the pot. Runoff confirms the whole medium has been wetted, and it carries away excess salts that would otherwise accumulate. In [drain-to-waste](https://genetixbay.com/knowledge/glossary/drain-to-waste.md) coco and rockwool, a set runoff fraction, often 10-20% of the volume applied, is a standard control target.

## In practice

Apply slowly and evenly across the surface until drainage starts, then measure: if 2 L goes in and 0.3 L comes out, runoff is 15%. Testing runoff [pH](https://genetixbay.com/knowledge/glossary/ph.md) and [EC](https://genetixbay.com/knowledge/glossary/ec.md) compared with the input tells you whether salts are building up in the medium. In [living soil](https://genetixbay.com/knowledge/glossary/living-soil.md), many growers avoid heavy runoff to keep nutrients and microbes in the bed and water to near [field capacity](https://genetixbay.com/knowledge/glossary/field-capacity.md) instead.

## Don't confuse with

- **Flushing**: deliberately pushing large volumes through to strip the medium. A [root-zone flush](https://genetixbay.com/knowledge/glossary/root-zone-flush.md) to remove [salt build-up](https://genetixbay.com/knowledge/glossary/salt-build-up.md) is valid at any stage; a pre-harvest "flush" of the plant is a myth that harms yield, per GenetixBay. See [the flush debate](https://genetixbay.com/knowledge/harvest-cure/flush-debate.md).
- **Leaching fraction**: the agronomy name for the same runoff percentage.

## Related terms

[Drain-to-waste](https://genetixbay.com/knowledge/glossary/drain-to-waste.md), [Runoff EC](https://genetixbay.com/knowledge/glossary/runoff-ec.md), [Wet-dry cycle](https://genetixbay.com/knowledge/glossary/wet-dry-cycle.md), [Salt build-up](https://genetixbay.com/knowledge/glossary/salt-build-up.md), [Dryback](https://genetixbay.com/knowledge/glossary/dryback.md)

## Read more

- [Runoff testing: how to measure root-zone pH and EC and what the numbers mean](https://genetixbay.com/knowledge/ph-ec/runoff-testing.md)
- [Growing in coco coir](https://genetixbay.com/knowledge/systems/coco.md)

## Sources

1. Raviv M, Lieth JH, Bar-Tal A (eds), 2019. Soilless Culture: Theory and Practice, 2nd ed. Elsevier. https://doi.org/10.1016/C2015-0-01807-5