# Ebb and flow (flood and drain)

> A pump floods a table or tray of pots to a standpipe height, then the solution drains back to the reservoir. Each flood refreshes water and pulls fresh air into the medium as it drains. Typical schedules: clay pebbles 3–6 floods per light period of 10–15 minutes, coco or rockwool 1–3. It is robust because the media buffer pump failures for days, but salts can build up at the top of the medium.

- Canonical: https://genetixbay.com/knowledge/systems/ebb-and-flow.html
- Category: Systems · Level: Intermediate
- Evidence: Supported · Status: Draft: awaiting expert review (some figures are being checked)
- Stage: Veg, Flower · Systems: Ebb and flow
- Updated: 2026-10-31 · Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay Knowledge Base (genetixbay.com/knowledge)"

## Questions and answers

**How often should I flood an ebb and flow table?** Flood an ebb and flow table 3–6 times per light period for clay pebbles, or 1–3 times for [rockwool](https://genetixbay.com/knowledge/glossary/rockwool.md) or [coco](https://genetixbay.com/knowledge/glossary/coco-coir.md), with none or one short flood at night. These are grower-practice starting points to verify for your own medium. Avoid night floods in rockwool, which keep it saturated, and adjust frequency to how fast the medium dries.

**What media works in ebb and flow?** Ebb and flow works with clay pebbles in net or [mesh pots](https://genetixbay.com/knowledge/glossary/net-pot.md), rockwool blocks, or coco in pots with a [drainage](https://genetixbay.com/knowledge/glossary/runoff.md) layer. Clay pebbles hold the least water and need the most floods, while rockwool and coco hold more and need only 1–3 floods per light period. Because the media hold water, a dead pump buys you days rather than hours.

**How long should each flood last?** Each ebb and flow flood should last about 10–15 minutes in total, covering fill, dwell and drain, with the solution reaching about two-thirds of pot or block height as set by the standpipe. Time the first fill and set a pump max-on timer at about 1.5 times that. As the table drains, fresh air is pulled into the medium.

**Why are salts building up on top of my pots?** Salts build up on top of ebb and flow pots because [subirrigation](https://genetixbay.com/knowledge/glossary/bottom-watering.md) carries water upward and evaporation leaves fertiliser salts in the upper layer, which extension research shows can reach about 10 times the [EC](https://genetixbay.com/knowledge/glossary/ec.md) elsewhere in the pot. Run EC slightly lower than for top-feeding, and hand-water each pot with plain water every week or two to push salts down and take runoff.

**Short answer.** A pump floods a tray to an overflow standpipe; after a set time it stops and the solution drains back by gravity. Draining pulls fresh air into the medium, so every flood is a water-and-oxygen exchange. Jardin's starting schedule: **clay pebbles 3–6 floods per light period, 10–15 min each; rockwool or coco 1–3; none or one short flood at night** [4]. Because the media hold water, a dead pump buys you **days**, not hours [4]. The trade-off: subirrigation concentrates salts at the **top** of the medium [1][2][3].

## Numbers that matter

| Parameter | Target | Notes |
|---|---|---|
| Flood depth | To ~2/3 of pot or block height | Standpipe sets it |
| Flood time | 10–15 min | Fill + dwell + drain |
| Floods per light period | Clay 3–6; rockwool/coco 1–3 | [4] |
| Reservoir volume | ≥ 1.5–2 × the volume to fill the table | Level must stay above the pump during a flood |
| [pH](https://genetixbay.com/knowledge/glossary/ph.md) | 5.6–6.2 (media dependent) | |
| EC | Slightly lower than top-feed | Salts concentrate on top [1][2] |
| Drain confirmation | Reservoir back within ~2 % of pre-flood mass in 20 min | Jardin rule [4] |

## How to run it

1. Table above the reservoir; overflow standpipe plus a fill/drain fitting; a screen on the drain.
2. Media: clay pebbles in net pots or mesh pots, rockwool blocks, or coco in pots with a drainage layer.
3. Learn your fill time on the first flood; set a pump max-on timer at ~1.5 × that.
4. Top-water each pot by hand with plain water every week or two to push surface salts down (and take runoff).
5. Inspect the table for root mats and clean the standpipe every reservoir change.

## Failure modes

| Problem | Sign | Jardin response [4] |
|---|---|---|
| Standpipe blocked / float stuck | Table overflows | Reservoir mass drop exceeds flood mass → pump off, alert; leak pad cuts rail |
| Drain blocked | Table stays full, roots sit in water | No further floods; alert "roots sitting in water" |
| Pump dead | No floods | Alert; media buy days |
| [Salt crust](https://genetixbay.com/knowledge/glossary/salt-build-up.md) | White crust, [burnt tips](https://genetixbay.com/knowledge/glossary/nutrient-burn.md) on lower leaves | Lower EC, periodic top-water [flush](https://genetixbay.com/knowledge/glossary/flushing.md) |

## Jardin notes

Jardin method "Flood & drain": reservoir mass, table high float, fill-line flow meter required; flood scheduler by medium and stage; pH/EC hold between floods. Time-to-harm: days. Automation level 3 [4].

## Common mistakes

- Reservoir too small, pump sucks air mid-flood.
- Running hydro-strength EC and ignoring the salt layer.
- Night floods in rockwool that keep it saturated.

## Evidence

Subirrigation behaviour and top-layer salt accumulation are *Established* extension knowledge [1][2][3]. Cannabis flood schedules are *Grower practice*.

## Sources

1. UMass Extension. Subirrigation for greenhouse crops. https://www.umass.edu/agriculture-food-environment/greenhouse-floriculture/fact-sheets/subirrigation-for-greenhouse-crops
2. NC State University. Subirrigation (top-layer EC up to ~10× elsewhere in the pot). https://hortscans.ces.ncsu.edu/uploads/s/u/subirrig_51e435ff2e468.pdf
3. University of Arkansas. Greenhouse management, Unit 9 §6: sampling subirrigated media (salts in the upper layer). https://greenhouse.hosted.uark.edu/Unit09/Section06.html
4. GenetixBay Jardin engineering notes, 2026 (system profiles and failure margins).