Flood and Drain Systems: How Ebb and Flow Hydroponics Works
Flood and drain - also called ebb and flow - is a hydroponic method: a pump floods the roots with nutrient solution, it drains back to the tank, and a timer repeats it through the day. It can be done on a table that floods a whole tray of plants or, as in the systems we sell, pot by pot. This guide covers how it works, the IWS flood and drain systems, what to grow in, tank care, and how it compares with DWC, NFT and Autopot.
How flood and drain works
- Flood. Nutrient solution rises through each pot from below, soaking the medium and pushing stale air out.
- Drain. It runs back to the tank and draws fresh air down into the medium.
- Repeat. The roots never sit wet for long or dry out.
The IWS systems we sell
The IWS is modular: every plant has its own pot (an inner pot of medium inside an outer pot), piped to a control bucket IWS call the brain, all feeding from one tank. It was launched some 20 years ago and very little about it has changed since; for a pot-based active hydroponic system we still think it is the best on the market.
| System | Sizes | Good to know |
|---|---|---|
| IWS Flood & Drain (Std. Remote) | 6, 12, 24, 36 or 48 pots | Choice of three inner pots; 16 mm pipe; flexible or rigid tank |
| IWS PRO Flood & Drain Systems | The same, plus 18 pots | Larger-bore 25 mm pipe; the 48-pot comes with a 400 or 750 litre FlexiTank |
| IWS Trident Flood & Drain - 16L and IWS Trident Flood & Drain - 25L | As the PRO | 16 or 25 litre pots |
Which one? Trident for larger plants, PRO for larger-bore pipe or an 18-pot size. The standard system has three inner pots to choose from:
- Punched: solid wall, moulded mesh base. Suits most media. We fill the bottom third with pure clay pebbles and the rest with a coco and clay mix: in our experience, the best approach.
- Aqua: a net pot, for maximum drainage and air. For pure clay pebbles, which we usually only recommend on the 6-pot and 12-pot systems.
- Culture: net base, wall net at the bottom and solid above. The second option for mixed media.
What to grow in
- Clay pebbles. Canna Aqua Clay Pebbles are expanded clay, which their page calls perfect for flood and drain. Rinse new pebbles before they go in, so the dust does not reach the pump - see clay pebbles and rockwool.
- 60/40. Gold Label Hydro/Coco 60/40 is 60% clay pebbles and 40% coir. The coco holds some moisture and protects the roots, so you can flood less often, which helps on larger systems, where a flood takes longer.
- Feed. Canna Aqua is designed for recirculating systems such as flood and drain, and contains buffering agents to hold the pH once it is set (doses in our Canna Aqua feed chart). Intense 60/40 is a two-part feed developed for the 60/40 mix.
How often to flood
There is no fixed number of floods: it depends on the medium, the plants and the room. Flood often enough that the roots never go short of water, with gaps long enough for the pots to drain and take in air. As a rule of thumb, pure pebbles in full growth want several floods a day and a 60/40 mix fewer. Keep each flood short: just long enough to fill the pots. Plants drink most with the lights on, so growers commonly put most floods in the light period.
Start low and add floods as the plants grow. Leaves drooping before the next flood can mean too few; a medium that stays wet on top and grows algae can mean too many.
Keeping the tank right
- pH and EC: take targets from your feed chart. House & Garden's chart gives pH 5.7-5.9 for recirculating hydro; the Plant Magic Hydro A & B page gives 5.5-6.2. The tank drifts as the plants drink, so check both every day or two (see our pH and EC guide), and empty and remix it regularly - weekly is the usual rule - rather than only topping up.
- Temperature: the Hailea Water Chillers page says to keep the solution at 18-21°C. Warm water holds less oxygen and suits pythium (root rot), so if the tank runs warmer in summer, a chiller is the fix. In a cold room, use one of the Eheim Tank Heaters.
Flood and drain vs DWC, NFT and Autopot: which should you choose?
| System | How the roots are fed | Grown in |
|---|---|---|
| Flood and drain | Pots flooded from below on a timer, then drained | Clay pebbles or a 60/40 mix |
| NFT | A constant flow over bare roots on a tray | A rockwool cube on a spreader mat - no other medium |
| DWC (deep water culture) | Roots hang in a bucket of aerated solution | A net pot of clay pebbles |
| Autopot | A valve fills each pot's tray to 20 mm, then waits until it is empty | Soil on a bed of clay pebbles |
- Flood and drain: for 6 to 48 plants in pots on one tank.
- NFT: for tight spaces and first systems. See the GT205i and the Multiduct NFT 2-Channel.
- DWC: for one to four large plants. We do not stock a boxed DWC kit; our hydroponics page lists the parts to build one.
- Fewer than six pots: a dripper system such as the four-pot Origin System Small (60cm X 60cm). We recommend clay pebbles in it.
- Autopot: for soil growers who want nothing to plug in. See our Autopot guide.
Common problems
- Root rot. Pythium can be an issue in any hydroponic system, and we recommend treating against it in summer. Keep the tank cool and dark, and clean between crops. Silver Bullet Roots is a root-zone cleaner: its page gives 0.5-0.75 ml per litre weekly for maintenance, 1 ml per litre for an outbreak, and 2 ml per litre to flush pipework between crops. Its page describes it as silver ions and hydrogen peroxide, and the Cannazym page says enzyme products should not be used with liquid oxygen (hydrogen peroxide) - so do not use it in the same tank as Cannazym, which the Canna Aqua chart includes from day 6 onwards. Follow the label.
- Slow or uneven floods. Stand every pot level and at the same height, and check pot filters and fittings for roots and bits of medium between crops.
- Dry pots. Pure pebbles hold little water, so a dead pump or stuck timer shows quickly. Watch a flood now and then.
Quick answers
Can I use coco in a flood and drain system?
Yes, mixed with clay pebbles: that is what the 60/40 mix is for. The PRO and Trident pages list coco as a medium too, but coco on its own holds far more water than pebbles, so flood it far less often.
Flood and drain or DWC?
Flood and drain for more plants. DWC is simpler for one to four large plants, but the roots hang in the solution all the time, so its temperature and the air pump matter more.
Flood and drain or NFT?
NFT for small, fast crops in a tight space, with no pots to fill. Flood and drain for bigger plants, each in its own pot of medium.
More Growers' Guides
Not sure what you need? Call the shop on 0117 971 3000 or pop in to the Bristol store and we'll help you pick.