GenetixBayEST. 2009
Systems · Intermediate · 2 min read

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.

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VegFlowerEbb and flow

Being reviewed. Our editors are double-checking a figure in this article against primary sources. Marked passages may change.

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 nightbeing reviewed [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 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 White crust, burnt tips on lower leaves Lower EC, periodic top-water flush

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.

Questions this article answers

How often should I flood an ebb and flow table?

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.

What media works in ebb and flow?

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.

How long should each flood last?

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.

Why are salts building up on top of my pots?

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.

Sources

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

Related

SupportedBeginner · Draft

Bottom-watering, soak trays and wick (self-watering) systems

Bottom-watering lets pots draw solution up from a tray, either by a timed soak (flood 2–5 cm for 10–30 minutes, then drain) or by a constant shallow level held by a float valve or wick. It is gentle and labour-saving and suits soil, but salts accumulate at the top of the pot, so feed at lower EC than top-feeding and top-water to runoff every week or two.

SupportedIntermediate · Draft

Recirculating drip (top-feed, runoff returns)

A pump pushes nutrient solution through drip emitters into pots of coco, rockwool or clay pebbles and the runoff returns to the reservoir. It saves water and nutrients but the reservoir slowly drifts as runoff brings back salts and root exudates, so monitor runoff and refresh when reservoir chemistry diverges. Typical shots are 2–6 % of pot volume, first shot 1–2 h after lights-on, none in the dark.

SupportedAdvanced · Draft

Growing in rockwool (stonewool cubes and slabs)

Rockwool is an inert, unbuffered mineral fibre that holds a lot of water and air and gives precise control over the root zone. Pre-soak at pH about 5.5, run pH 5.5–6.0 and EC 1.8–3.0 mS/cm in flower, and manage it by volumetric water content and slab EC. It is the native medium of crop steering, and it punishes neglect quickly because nothing buffers mistakes.

SupportedIntermediate · Draft

Growing in coco coir

Coco coir is a hydroponic substrate that looks like soil: inert-ish, airy, and fed every irrigation with a complete mineral solution at pH 5.8–6.2 and EC 1.2–2.4 mS/cm with 10–20 % runoff. Use pre-washed, buffered coco or buffer it yourself with a calcium/magnesium solution, because raw coir holds potassium and sodium and grabs calcium and magnesium.

Grower practiceBeginner · Draft

Choosing your growing system: soil, coco, rockwool or hydro

Choose by how much attention and measurement you can give, not by yield claims. Soil and living soil forgive mistakes for days; coco and drip need daily feeding; DWC, NFT and aeroponics grow fast but can lose roots within hours if a pump or air supply fails. No controlled study shows one system out-yielding the others when light and climate are equal.

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