Which Growing Mediums?

The growing media you use with the IWS Flood and Drain system comes down to personal preference. Excellent results can be achieved with many different growing media’s, but the key features you should look for are:

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

Expanded Clay Pebbles

Clay pebbles are an ideal growing media for flood and drain systems as they don’t require anything to be added or mixed in. Clay pebbles have good water retention and plenty of air space between them, creating a high air filled porosity.

Clay pebbles allow flood and drain systems to be flooded more frequently than other hydroponic systems, with a low risk of over watering. This frequent feeding cycle allows the oxygen in the root zone to be replenished more often, leading to greater nutrient uptake and bigger yields.

Did you know – A 10L pot of dry clay pebbles can absorb up to 5L of nutrient solution! This shows just how porous they actually are.

Coco / Clay Pebble Mix

If using a coco / clay pebble mix, the percentage of coco should be  25-40% of the total volume of the growing media. Adding coco will allow more time between feeds which means fewer feed cycles per day.

Other Mixes

The IWS Flood and Drain system is very adaptable and can be used with many different growing mediums. Just remember to adapt your feeding schedule, depending on the growing medium’s water retention capacity.

Environmental Considerations

As with all hydroponic systems, the nutrient strength and feed cycle should reflect your environment. For example; if you’re using clay pebbles and the environment is hot and the humidity is low, these hot and dry conditions will cause the plant to use more water and less nutrient, causing the nutrient strength to rise in the reservoir. In these circumstances we recommend setting the nutrient strength lower than usual to account for rising strength. In these conditions the pots should be flooded more often, up to once per hour.

The same room during cooler conditions, with a higher humidity, will mean higher nutrient strengths can be used and the number of feed cycles can be reduced to once every 2-3 hours. Always consider the effect that the environment will have on your plants and adjust your feeding strategy accordingly.

Understanding The Flood Cycle

To get the best possible results from the IWS Flood and Drain system you must ensure you get the flood cycle right. The flood cycle is made up of 3 elements;

  • Flood Frequency – This is how often you flood the pot, which depends on the growing media you’re using and how well established your plants are.
  • Flood Height – This is how high the water rises up the pot. In most cases we would recommend flooding to the maximum height.
  • Flood Duration – This is the total time of each feed cycle  and will depend on the number of pot in your system and the growing media used.

These key factors play an important role in getting your feed strategy as accurate as possible.

This guide can be followed for all growing media and should provide accurate flood times and optimum results.

Once you have assembled your IWS Flood and Drain system, it’s time to prepare the growing media.

Please note: No matter what growing media you use in your pots you must use 5-10cm (2-4 inches) of clay pebbles in the bottom of the inner pots. This will help prevent the bottom from holding too much water. Always wash clay pebbles thoroughly to remove dust and small particles, this can be done with tap water.

Insert the root control disk into your inner pot copper side up, add 5-10cm (2-4 inches) of clay pebbles in the the bottom of the pot, then add your chosen growing media on top, only filling the pots three quarters full at this point.

Fill the reservoir with water and add nutrients as directed by the nutrient manufacturer.

Finally adjust the pH in the nutrient tank to 5.5 – 6.5.

Setting The Flood Height

Once the nutrient solution is ready, turn the timer on and start a flood cycle. Make sure the feed-duration dial is turned clockwise, allowing the longest possible flood time. The brain pot will start to fill and so will the pots. As the nutrient solution slowly fills the pots the growing media will take in the nutrient solution preparing the growing media for planting.

The aim is for the maximum flood height to be the same as the amount of growing media in the pot. You should be able to see the water level rising to the surface of the growing media. Lightly push down on the media surface with the palm of your hand, this will make sure the media isn’t floating up. If the water rises over the surface of the growing media add more media until the level of the nutrient solution and the growing media is the same. Once the brain has stopped filling and all the pots have the right amount of growing media in them, leave them soaking in the nutrient solution for at 1 hour.

After the soak period, initiate the drain cycle on the timer by turning the duration dial anti-clockwise, this cancels the feed. When the drain cycle is finished, check the pH and nutrient strength in the reservoir with suitable pH and EC meters. The pH may have changed, if so adjust if necessary.

Please note: If the pH or nutrient strength has changed dramatically, empty the reservoir and change the nutrient solution.

Now you need to time the flood cycle. Start the flood cycle and and time how long it takes for the ALL THE POTS to fill back to the maximum flood height. As soon as the maximum flood height is reached, record the time taken, then start the drain cycle.

With the time recorded you can now set an accurate flood duration on the IWS system. If you are using clay pebbles, add 1-2 minutes onto the time recorded, to help to saturate the clay pebbles and purge the clay pebbles inner core and outer shell of old nutrients. Clay pebbles at the top of the pot will also draw up water and nutrients by capillary action. We do not recommended holding the flood height for longer than 5 minutes as this can cause poor root function and invite disease.

If you are using a mix of coco and clay pebbles then the time recorded is your actual flood time. Do not add any extra time as your growing media has a fast capillary action and holds more nutrient solution than clay pebbles alone.


The pots should still be around three quarters full with the growing media at the same depth as the maximum flood height.

When planting your young plants you should plant them 1-2 cm into the growing media at the maximum flood height. This will allow the growing media the plant was propagated in to soak up the nutrient solution..

Please note: If you plant your young plants too deep in the pot your propagation block will become saturated causing poor initial root growth. Once you are happy with the planting depth, fill the rest of the pot with more prepared growing media.

Setting The Flood Frequency

How often you flood the pots will be determined by:

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

If you are using a growing media such as clay pebbles you can flood your pot more often, if using  coco and clay pebble mixes the flood frequency should be reduced.

Flood Frequency with Clay Pebbles

We highly recommend clay pebbles as the best growing media for the IWS Flood and Drain, as we have found that this media gives the best results. We would also recommend using Grodan 3” or 4” blocks for propagating your plants to grow in clay pebbles, thought not essential.

When using clay pebbles you must use the IWS ‘Aqua Pots’, these large net pots allow better flooding and enhanced root development. To prevent roots blocking the drain pipe, a copper root-control disk is used in the bottom, this will inhibit the roots from growing into the outer pot.

During the flood cycle the pots will fill and drain quickly. For this reason the flood frequency can reach a maximum of 1 flood every 2-3 hours when the plant is in growing vigorously.

The initial irrigation frequency depends on a number of factors; the size of your propagation block, the size of your plant and your grow room environment. The propagation environment will be quite different to the growing environment, most people find when moving plants into their growing environment the plants water demand will increase due to higher temperatures and lower humidity. It is important to make sure the block and clay pebbles are not drying out, or staying too wet, between feed cycles.

Once there is noticeable increase in vegetative growth you can increase the feed frequency to once every 3-4 hours. As plants get bigger and their demand for water increases you will need to adjust the frequency to once every 3 hours,

Please note: If feeding every 3 hours you must allow the drain cycle to finish before counting down to your next feed cycle (3 hour later).

A lot of growers consider 3 hours to be the maximum flood frequency but larger plants in a hot and dry environment may benefit from a flood frequency of once an hour.

During each flood, water and nutrients are delivered to your plant and stale oxygen is expelled. And with each drain cycle fresh oxygen is pulled into the air spaces in the root zone. So  more frequent floods means more fresh oxygen around the roots.

Please note: If plants are not well established within the clay pebbles they will not benefit from frequent floods.

Flood Frequency with Coco / Mixes

When using coco and clay pebble mixes, you must use the IWS ‘Culture Pots’. These pots have a net base which has been designed for finer growing media. The bottom portion of the net pot should be filled with clay pebbles. The rest of the pot should be filled with your mix of coco and clay pebbles.

It is important when using coco in flood and drain systems is to not over-water. For the first 1-2 weeks after planting, while your plants are becoming established, your plants will need feeding a maximum of once a day. This flood should be in the middle of your light period. 

When the plants are established and vigorous growth has started, the plants demand for water will increase. It is at this time you should check the moisture level of your mix before the feed cycle or toward the end of your light cycle, if your growing medium is drying you should increase your feed frequency to 2-4 times a day. The first and second feed cycles should be equally spaced e.g. if your using an 18 hour light cycle your feed cycle should be at 6 and 12 hours.

2-4 feed cycles are, in most cases as much as they need, but plants that have a higher water demand may need irrigating 3-6 times a day during peak growth. Remember when irrigating your coco and coco and clay pebble mixes,  ‘transpiration before irrigation’, this means that must be allowed to use remaining water in the growing media before giving them anymore. This is why we recommend having your first feed cycle at least 1-2 hours after the lights turn on.

Hint: During warm dark periods you may consider having 1 night time irrigation, this is only really necessary if using clay pebbles.

Maintenance – Avoiding root blockages

Because the IWS Flood and Drain system fills and empties through the same tube, roots growing out of the pot can cause blockages in the pipe work. This can be avoided by always using the copper root control disks provided and routinely turning the inner pot every 2-3 days in the same direction. This will ensure the roots stay away from the tube and will help to produce more even growth.

If you suspect your pots or pipe work may be blocked, you can check by quickly flooding the pot with 5-8 litre of nutrient solution in from the top of the pot. If the solution drains away freely it’s okay, if it takes a long time to drain you most likely have a blockage. If you have a blockage you should remove the inner pot and check the inlet/outlet tube for roots or debris.

Most hydroponic system problems are due to not keeping a clean system. You must make sure that float switches in the brain pot do not become dirty or clogged with any growing media or debris. Every time you refill the reservoir, a quick rinse with fresh water over the switches will help prevent problems. If you do notice sediment or debris in the reservoir or brain pot, remove or clean it immediately before a problem occurs.

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