# Runoff

> Runoff is the water or nutrient solution that drains out of the bottom of a container after irrigation; growers test it to see what is happening in the root zone.

- Canonical: https://genetixbay.com/knowledge/glossary/runoff.html
- Also written: run-off, drain, drainage, runoff water, drain-to-waste runoff
- Glossary category: pH, EC and water · 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"

Runoff (drainage, [leachate](https://genetixbay.com/knowledge/glossary/watering-to-runoff.md)) is the solution that leaves a container after watering. Its [pH](https://genetixbay.com/knowledge/glossary/ph.md) and [EC](https://genetixbay.com/knowledge/glossary/ec.md) reflect the conditions in the medium more than the input solution does, because it has passed through the [root zone](https://genetixbay.com/knowledge/glossary/root-zone.md) and picked up or lost ions. Runoff fraction is the share of applied water that drains, for example 1 L out of 5 L applied is 20% runoff.

## In practice

In coco and rockwool [drain-to-waste](https://genetixbay.com/knowledge/glossary/drain-to-waste.md), growers usually aim for about 10–20% runoff per day to stop salts accumulating. In soil and [living soil](https://genetixbay.com/knowledge/glossary/living-soil.md), many growers water to little or no runoff and test it only occasionally.

How to test: water as normal, collect the first runoff in a clean container, and measure EC and pH promptly. For more consistent numbers, use the PourThru method developed for containers by NC State: wait about an hour after irrigation, then pour just enough distilled water on top to collect roughly 50 mL of leachate (Cavins and colleagues). Compare trends, not single readings.

## Don't confuse with

- **Root-zone flush**: deliberately running large volumes through to strip salts; runoff is the normal drainage from routine watering.
- **Input solution**: what goes into the pot.

## Myth check

GenetixBay house position: a **[root-zone flush](https://genetixbay.com/knowledge/glossary/root-zone-flush.md)** to wash accumulated salts out of the medium is the one valid [flush](https://genetixbay.com/knowledge/glossary/flushing.md). It treats the medium, not the plant, and can be done at any stage. A **pre-harvest flush** is a *Myth* and actively harmful: it starves the plant while it is packing on its final weight and resin. See [the flush debate](https://genetixbay.com/knowledge/harvest-cure/flush-debate.md).

## Related terms

[Runoff EC](https://genetixbay.com/knowledge/glossary/runoff-ec.md), [Input EC](https://genetixbay.com/knowledge/glossary/input-ec.md), [Salt build-up](https://genetixbay.com/knowledge/glossary/salt-build-up.md), [pH](https://genetixbay.com/knowledge/glossary/ph.md), [Pour-through](https://genetixbay.com/knowledge/glossary/pour-through.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)
- [Drain-to-waste drip and crop steering (generative vs vegetative dryback)](https://genetixbay.com/knowledge/systems/drain-to-waste-crop-steering.md)

## Sources

1. Cavins TJ, Whipker BE, Fonteno WC, Harden B, McCall I, Gibson JL. Monitoring and managing pH and EC using the PourThru extraction method. NC State University Horticulture Information Leaflet 590. Cited via MSU Extension: https://www.canr.msu.edu/uploads/resources/pdfs/measuring_ph_and_ec_of_crops_grown_in_large_containers.pdf