IWS Flood & Drain - How to Do it Right

IWS Flood & Drain - How to Do it Right

05 Mar 2014

Ready to flood roots with nutrients and oxygen? You've got to try Flood & Drain. But don't just dabble - if you're going to do it, do it right. Here's we show you how.


What's Flood & Drain?

It's a growing method that fills pots from the bottom up.

As the nutrient solution rises, old air and is pushed out of pots.

When your solution's drained away, fresh air is  pulled in, enriching the rootzone with oxygen.

The more you flood, the more oxygen you trap. 

Flood and drain Flood and drain

Which growing media?

There's no right or wrong answer - it's up to you. BUT you do want a media or mix with a:

  • Low / medium water holding capacity (WHC)
  • High air-filled porosity (AFP)   

In short, clay pebbles are your new best friend.

They're lightweight and there's lots of air space between clay pebbles, creating a high AFP.

The porous structure also absorbs and releases nutrient solution over time. This means you can flood often with very little risk of overdoing it. You know what that means - more oxygen.

You can use clay pebbles on their own, or mix them with another media 


Did you Know
A 10L pot of dry clay pebbles can absorb up to 5L of water! That's how porous they are! 

Lots of growers mix coco with clay pebbles to further increase the WHC. This gives them more time between floods and fewer floods each day.

If you do this:

  • we recommend 25% coco to 75% clay
  • 20% - 40% coco with 60 - 80% clay is OK

Mixes with 50% coco or more can stay wet for too long. This kind of defeats the point of flood and drain! 

Clay Pebbles Clay Pebbles

What's the ‘Flood Cycle’

Your flood cycle is made up of 3 things: flood frequency, height & duration.

You need to be very precise and accurate when setting your cycle.    

Flood Frequency

This is how often you flood. 

It really depends on the media being used and how well plants are established.

Flood Height

This is how high the water level rises in each pot.

Generally, you should always go for the maximum height 

Flood Duration

This is how long each flood lasts.

It depends on the number of pots your systems has and your growing media. 

Setting a Flood Height & Duration

Once you've put your system together, it's time to set your flood height and duration.

1. Fill pots

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Insert the root control disk (copper side up) and add a 2 - 4" layer of washed clay pebbles to the bottom of each pot.

Fill pots to 3/4 with your media or mix

2. Fill your tank


Ideally your nutrient strength should be 2 CF (0.2 EC) higher than it was in propagation.

Add a root stimulator for quick establishment, and adjust the pH to 5.5-6.5. 

3. Start a Flood


Set your timer to minutes, and turn the dial all the way to the right for the longest possible flood time. Turn on the timer.

The brain and pots will start to fill.  

4. Check your Water Level


Your aim is for the water level to reach the top of your media. You should be able to see the water level rising to the surface.

To check the media isn't floating up, push down on the media with the palm of your hand.

5. Add media if needed


If the water rises over the surface of the growing media, add more media.

6. Turn off and soak


When the brain stops filling and all the pots have the right amount of growing media and nutrient solution, leave them soaking in the nutrient solution for 30 minutes to 1 hour.  

If you've got a slightly uneven floor, you'll find that the flood height may be slightly different in each pot.

7. Start to drain


After the pre-soak, start the drain cycle on the timer by turning the dial to 'cancel feed' setting.

Once the drained, check the pH and CF in your tank and adjust if you need to.

If the pH or CF has changed dramatically, change the nutrient solution. 

8. Time a Flood


Now you need to get a stopwatch.

Start the flood cycle and the stopwatch (you are timing how long it takes for ALL POTS to fill back to the maximum flood height.)

As soon as the maximum flood height is reached in all the pots, stop the stopwatch and start the drain cycle. 

9. Set your timer


If using clay pebbles
, add 1-2 minutes to the time recorded. 

If using a coco mix, the time recorded is your actual flood time. You don't need to add extra time as it holds more water than pebbles.

You can see it took us just under 7 minutes to flood, so we set our flood time to 8 minutes.  

Don't hold your flood for longer than 10 minutes. Leaving roots completely submerged for too long can cause poor root function and invite disease.  


Your pots should still be around three quarters full at this point.

1. Aim


You want young plants 2-3 cm from the max flood height.

Then, during a flood cycle, the flood height will reach the bottom of the propagation block.

2. Expose Roots


Pull back the wrapper to expose 2-3cm the block to expose roots. Then adjust some of the pebbles, plant the block into the pot and cover back the media. The raised wrapper will now sit at the same height as the flood height. 

3. Add media


Once you are happy with the planting depth of all the plants, fill the rest of the pot with more prepared growing media. 

If you place your young plants too deep into the pots your flood height will rise too high and saturate the propagation block causing poor initial root growth.   

Setting the Flood Frequency

How often you flood the pots is determined by:   

  • The growing media
  • The plant size and water requirements
  • The environmental conditions

The Growing Media

For growing media that does not retain much water, like clay pebbles, you need to flood the pots with a high frequency. When using clay pebble and coco mixes the WHC registers higher so flood frequency will be reduced. 

Clay pebbles

You can generally flood every 1-2 hours when the plant's growing vigorously.

Your exact frequency depends on:

  • the size of your propagation blocks
  • the size of your plants
  • your grow room environment

A large plant in a small block that needed watering once a day in propagation - try flooding every 4 - 8 hours.

A small plant in a large cube that is not using much water - 1 or 2 floods per day will be enough. 

flood and drain with coco flood and drain with coco

When you see a big increase in veg growth, increase the frequency to once every 3-4 hours. As plants get bigger still, flood once every 2 hours.

A lot of growers leave this as their maximum flood frequency but some large plants in a hot and dry environment with a high water demand may benefit from a flood frequency of once an hour.  

Remember that during each flood, oxygen is trapped around roots - more floods = more oxygen


The trick is not to over water.

During the first 1 - 2 weeks, flood no more than once a day, ideally in the middle of your light period.

Some growers actually hand water during this time, while plants are still establishing.

After the first 1 - 2 weeks, when serious vegging has started, plants need more water. Check the moisture level of your media before irrigating, towards the end of your light cycle. If it's drying, increase your frequency to twice a day. Both floods should be equally spaced.

Plants that have a really high water demand may need 3 floods a day. Your first irrigation should be at least 1-2 hours after the lights turn on.

flood and drain with coco flood and drain with coco


Your nutrient strength flood frequency needs to reflect your environment.

When it's hot and dry, plants use more water and less nutrient - your nutrient strength should be lower and you need to flood once an hour. 

In the same room that's cooler with a higher humidity, plants need less water and more nutrient. You want a higher nutrient strength and flood frequencies can be reduced to once every 2-3 hours.

flood and drain with coco flood and drain with coco

IWS Flood and Drain System Maintenance

Now your system's up and running, you need to keep it well maintained. Here are our top tips.

Avoid Root Blockages

Roots sometimes grow into pipes and block them.

To avoid this, always use the copper root control discs provided.

If you can, routinely turn the inner pot around 45° in the same direction every 2-3 days. This keeps roots away from the tube. Growth will be more even, too.

Checking for Root Blockages

Quickly pour 5-8 Litres of nutrient solution into the top of pots.

If the solution drains away freely, you’ve got nothing to worry about.

If the solution sits there and takes a long time to drain you most likely have a blockage.  

Note - this technique only works well with clay pebbles. If your pot is blocked you should remove the inner pot and check the inlet/outlet tube for roots or debris.  

Keep your system clean

Make sure the float switches in the brain pot don't become dirty or clogged up with any growing media.

Each time you refill the reservoir, a quick rinse with fresh water over the switches will help prevent problems.

Over time some sediment and/or debris may collect in the reservoir or brain pot, remove or clean it immediately.

Common problems and solutions

Problem: My system fills but doesn’t drain

Solution: This is usually because of a problem with the pump. If it's vibrating but not pumping, it will either be air locked or blocked.

Twist it so the inlet faces up – if you see bubbles or the pump starts to work, job's a good'un.

If this happens a lot, check the fill tube. Make sure water isn't being directed onto or near the pump. Turn the tube outlet around to pour solution down the side of the brain pot instead of allowing it to splash the surface of the water.  

If the pump is vibrating in the brain pot and doesn't release any air bubbles when turned, it may be blocked. Turn the power off, remove the front of the case from the pump and inspect the impeller and inlet. Remove any debris and replace.  

If the pump in the brain pot doesn't vibrate or work properly, check the bottom float switch. As long as it's clean and hasn’t been forced open, call for a replacement!

Problem: My system won’t stop filling/my pots are over flowing

Solution: The pump in the reservoir is still filling the brain pot when the power switches off.

Check the anti-siphon valve in the reservoir is not underwater or blocked. It should be near the top of the tank, where it can take air into the pipe when the pump is off to stop the water from flowing.

There will be a small hole drilled in the pipework coming up from the pump at the top of the reservoir. Check that this is not blocked.  

If the problem continues, you may be looking at a faulty top float switch in the brain pot. Check the float switch is clean and free from debris. If in good condition, call us for a new part as you might have a faulty timer unit or float switch. 

Problem: Some of my pots are over-flooding?  

Solution: When you have an uneven floor and some or all of the pots are lower than the brain pot, you will get some over-filling. Put simply, the pots must be sat on the same level as the brain pot – this is essential to how the system works.   

Problem: Why is the EC/CF rising rapidly in the reservoir?  

Solution: Nutrient strength normally rises slowly as plants use more water than nutrients. If your temperature is high (above 28ºC) and humidity is low (below 50%), the nutrient strength will be quicker to rise. Add more water to the tank to reduce the nutrient strength. Consider reducing the nutrient strength by 0.2-0.4 EC units (2-4CF).  

Rapidly rising EC/CF may also be a sign that your flood frequency is too low and the growing media gets too much time to dry out between flood cycles. Consider increasing the flood frequency.  

Problem: Why is the pH rising rapidly in the reservoir?  

Solution: pH tends to rise slowly during normal nutrient uptake. If your pH climbs rapidly and you have to add lots of pH down' it's normally due to the clay pebbles. Failing to wash and pre-treat your media can lead to rises during early veg. However, the pH will stabilize over time.

Try not to add lots of pH down – opt to use a mixture of nitric acid and phosphoric acid. This will help the balance of minerals in the nutrient solution.   

Problem: Why is the pH dropping rapidly in the reservoir?  

Solution: pH drop can sometimes be a normal as plants move from veg growth to flowering. Adjust back up with potassium hydroxide (ph Up) or potassium silicate (silicon).  

If the pH drops rapidly you may be over-watering. A high flood frequency and wet media reduces oxygen levels, leading to a fall in pH. Over-watering also increases plant susceptibility to root disease and can ultimately cause root death. Check that your growing media does not remain wet for too long between irrigations. If this is an issue, consider reducing the flood frequency.  

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