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10 Golden Rules for Flood & Drain

10 Golden Rules for Flood & Drain

Alex Grady

Want to get the best out of your flood and drain system?

Follow these 10 golden rules and you’ll get bigger plants, better yields and healthier roots! 

1. Use the Right Growing Media

The exact media you use comes down to personal preference.

Just make sure your media or media mix has:

  • Low - Medium Water Holding Capacity (WHC)
  • High Air Filled Porosity (AFP)

For frequent flooding…

…we recommend using clay pebbles. A 10L pot of dry clay pebbles can absorb up to 5L of water! They’re incredibly porous!

Their porous structure absorbs your nutrient solution, then releases it over time. This means you can flood your system frequently without over-watering it.

Remember, flood and drain systems are designed for frequent flooding. The fresh supply of nutrients and oxygen leads to super fast growth and bigger yields.

To flood less often...

…mix coco with your clay pebbles - lots of growers do.

Coco holds a lot more water, so you don’t have to flood your system as often.

This means you have more time to respond to issues (such as pump failures) between each flood. It’s also a better option if you're not able to reach your system every day.

If you do this, 20 - 40% of your mix should be coco, 60 - 80% should be clay pebbles.

Use any more coco than this, and your media can stay wet for too long – this defeats the point of flood and drain.

Mix your own, or you can buy it pre-mixed.

2. Keep 5 - 10cm of Clay Pebbles in the Bottom of Each Pot

Even if you’re mixing clay pebbles with coco.

This is to prevent too much water being held at the bottom of each pot.

Flood and Drain

3. Get your Flood Cycle Right

There are three things to think about: Flood frequency; flood height; flood duration.

Flood Frequency: how often you flood your pots

This is determined by:

  • The growing media
  • Your plant sizes
  • The climate
  • Water demand

After propagation, most people find that their plants are suddenly in a lower humidity and a higher heat – their water requirements will increase.

If your growing media doesn’t hold much water (like clay pebbles), flood more often. If mixing coco with clay pebbles, you won’t need to flood as often.

Flood and Drain

Flood Height: how high the water level rises in each pot

The maximum flood height should be no higher than that of your growing media.

  • Once your system’s set up, add your root control disc and washed clay pebbles
  • Fill ¾ of each pot with growing media
  • Fill your water tank with your nutrient solution (2 CF higher than during propagation)
  • Set your timer to minutes and turn it to the longest possible flood time
  • Your brain will then start filling your pots
  • If water rises past the height of your growing media, add more media
  • To check the growing media isn’t floating up, press it gently with your hand
  • Pre-soak for 30mins – 1hr, then drain

Flood Duration: how long each flood lasts

You should never hold your flood cycle for longer than 10 minutes.

  • After draining, it’s time to set your flood duration
  • If the pH or nutrient strength is off, adjust your nutrient solution
  • Start the flood cycle again, to see how long it takes for the max flood height to be reached
  • Add 1 – 2 minutes to this time if using clay pebbles (unless you mix it with coco)

If you have an uneven floor (like we do in our Sheffield Greenhouse), the flood height will be different in each pot. This method lets you work out exactly how much media is needed in each pot.

4. Place Young Plants just 2- 3cm into the Growing Media

Don't place young plants too deeply into the pots, your flood height will rise too far up the transplant block.

By placing them just 2 - 3cm deep, your nutrient solution will reach the bottom of the transplant blocks when it’s at the maximum flood height.

Once you’ve positioned your plants, expose their roots by moving the plastic wrapped around the transplant blocks up by 2 – 3cm, so it’s in line with the top of your media.

Flood and Drain

5. Transpiration Before Irrigation

Or, in English: Wait for your plants to start using the remaining water in the growing media before flooding them again.

Your first irrigation should be at least 30 minutes after you turn the lights turn on.

Normally, it’s enough to flood during ‘lights on’ – you shouldn’t need to flood during ‘lights off’.

You might need one irrigation when it’s warm and dark, but that’s it.

Flood and Drain

6. Ensure your Nutrient Strength Matches your Climate

During summer months, your room will probably run at 28oC, with an RH of 45%. In this hotter, drier climate, plants use more water and less nutrient.

You should lower your nutrient strength, and flood your pots every hour.

A Bluelab CF Truncheon is perfect for checking your nutrient strength.


7. Prevent Roots from Blocking Pipes

Flood and drain systems fill and empty through the same tube. As a result, roots can sometimes block the feed pipe. To keep roots away from the tube and get even growth, rotate the inner pot by 45° (a quarter turn), in the same direction, every 2-3 days.

Make sure you use the copper root control discs provided with your IWS Flood System, too! They prevent roots from ‘chasing’ the water through the drain pipe.

To check for blockages, pour 5-8L of nutrient solution in from the top of the pot.

  • If the solution drains away easily, you probably haven’t got a blockage.
  • If it sits there and takes a long time to drain, sorry, you’ve probably got a blockage.
  • If there's a blockage, remove the inner pot and check the inlet/outlet tube for roots or debris.

Note: this test only works well with clay pebbles.


8. Use the Right Pots

If using clay pebbles in a smaller system, you’ll want to use the IWS ‘Aqua Pots’. They’re large net pots that are great for flooding, and allow for enhanced root development. They're the ones supplied with all IWS Flood Systems (up to 24 pots).

If you have a larger IWS Flood System (36 pots or more), they come with 'Punch Pots' - you need these for bigger set ups.

When mixing coco with your clay pebble, use the IWS ‘Culture Pots’. Their net base has been designed specifically for finer growing media. 'Culture Pots' are special order items. If you need them, start a live chat or visit your local store.

For the very best results, place a RhizoPot in each inner pot to prevent roots growing into your feed pipe.


9. Keep your System Clean

To prevent issues from occurring:

  • Ensure your float switches in the brain pot never become dirty or clogged
  • Rinse your reservoir between each refill
  • Remove any sediment or debris that collects in your reservoir


10. Upgrade to an IWS Pro System if you need to

Go too big in a standard IWS Flood System, and it’ll take too long to flood all pots. By the time the final pot has filled, the first pot that filled may be over-flooded.

You can end up over-saturating root zones, and getting uneven growth.

If you’re growing more than 12 – 18 plants, consider upgrading to an IWS Pro System. You’ll get a larger pump, and 25mm pipes, so all pots are faster to fill.

This will reduce the risk of root blockages, too!

To order this system, visit your local store or start a live chat. Don't forget to check back next week - our blog's updated weekly!


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