Flood and drain – also called ebb and flow – periodically floods the root zone with nutrient, then lets it drain back to the tank. The drain cycle is the clever part: as the water falls it pulls fresh air down into the medium behind it, so roots get a full recharge of oxygen several times a day.
How it works
A pump on a timer fills the tray or pots from below.
Water rises to a set level, held by an overflow standpipe.
The pump stops and everything drains back to the tank under gravity.
Air is drawn into the medium as the water level drops.
Typically three to six cycles a day in flower, fewer in veg. The medium never sits saturated, which is what separates this from simply leaving pots in water.
Why growers choose it
Excellent oxygenation without air stones or aeration in the root zone itself.
Very even feeding – every pot gets the same solution at the same time, unlike drippers which can block individually.
Few moving parts. One pump, one timer, one standpipe. Little to go wrong.
Scales cleanly from a single tray to a full room.
What to run it in
Clay pebbles are the classic choice – they drain instantly and hold no water, which is exactly what the cycle needs. Rockwool slabs work well too. Avoid anything that holds water heavily; peat-based soil in a flood tray stays waterlogged and defeats the point.
Things that catch people out
Flood height. Fill to roughly two thirds of pot height, not to the top. Flooding the surface invites algae and gnats.
Drain speed. If it drains slowly, roots sit wet. Check the return line is not restricted.
Reservoir temperature. Everything drains back to one tank, so one problem becomes every plant's problem – see water chillers.
Timer failure. A stuck-on pump floods the room; a stuck-off pump dries the pebbles within hours. Use a decent irrigation timer.
We are a hydroponics shop in Croydon and we build these to order. Tell us your tray size and pot count and we will spec the pump, tank and timer together. Discreet UK and European delivery, and excellent customer reviews on Trustpilot.
