How Many Times to Flood and Drain?

Flood and drain (or ebb and flow) hydroponics systems are easy to build but how often – how many times – do you flood and drain the system? As with other types of gardening, there are no hard-and-fast rules.

How many times you will need to flood and drain (the number of “cycles”) depends on several factors which will vary from one hydroponic garden to the next. Here we look at what these factors are to help you estimate how often you will need to flood and drain your own hydroponics garden. Once you get growing, observe you plants and tweak your flood and drain cycles to perfection.

Plant size

As you can probably guess, a key factor affecting flood and drain times is the size (“biomass”) of your plants. All plants evaporate (“transpire”) water into the atmosphere through tiny holes in the undersides of leaves, called stomata. In response, the roots draw in more nutrient-carrying water which likewise travels up the plants and eventually transpires out the leaves. This process (“transpiration”) is essentially how plants feed themselves.

Larger plants have more stomata so they lose more water to transpiration. It is up to you to make sure more is available at the roots! In other words, the larger your plants, the more times you need to flood and drain your hydroponic garden. Of course, as your plants grow you will need to flood and drain more often.


When estimating flood and drain times, consider the strength, proximity and duration of light, particularly if your hydroponics system features powerful artificial lighting such as grow lights. Light affects transpiration rates and therefore how often you will need to flood and drain.

Light not only stimulates the stomata in the leaves to open but its warmth speeds up evaporation of water exiting the stomata. The stronger, closer and longer the light source, the more times you will need to flood and drain your hydroponics garden to make up for water being lost through the leaves.


As mentioned, the warmth of light on the leaves speeds up evaporation. However, other factors such as climate and domestic heating systems may create a hot atmosphere for your hydroponics garden – and accelerate water loss through transpiration. You may also need to adjust flood and drain times throughout the year, to suit seasonal variations in temperature and humidity levels.


Plants transpire more easily in dry atmospheres which, like dry sponges, easily soak up water vapour transpiring from the leaves. The drier the atmosphere, the more often you will need to flood and drain your hydroponics garden.

Humid atmospheres, on the other hand, are like a wet sponge – already fairly or heavily saturated with water. This hampers transpiration so you will not need to flood and drain so often as in dry atmospheres.

Grow media

You can use a variety of grow media in your flood and drain system, from expanded clay pebbles to rockwool or coconut coir. However, each medium varies in terms of water absorption and retention rates.

For instance, clay pebbles need both longer and more frequent flood and drain cycles than rockwool because they absorb water more slowly and retain it for less time.

Size of your hydroponics system

The larger and deeper your hydroponics system, the less often you need to flood and drain your garden. Most homemade flood and drain hydroponic systems are typically quite shallow. However, new-style, deeper systems are available which take longer to flood but considerably more time to dry out.

In other words, deeper systems need less frequent but longer flood and drain cycles than conventional flood and drain systems.

Type of plants

Some plants are naturally more thirsty than others, or prefer the “little and often” approach, and so on. Learn about the plants you are growing and increase or reduce how often and for how long you flood and drain to suit their watering preferences.

If you are growing a variety of plants, you will need to tweak how often you flood and drain until you find a compromise that seems to suit them all. Alternatively, have separate hydroponics gardens containing plants with similar needs.

Calculating flood and drain times

Now you know which factors affect flood and drain times, you may still be feeling somewhat clueless about exactly how many times to flood and drain.

Here are some broad guidelines for three popular types of grow media showing flood and drain times for seedlings/starter plants through to large plants. Think about the factors above. You should come up with an estimate for your own hydroponics garden that is a good starting point from which you can adjust flood and drain times up or down.

Note that “day” means approximately 16 hours, when the lights are on. Never flood and drain in the “lights off” period.

Expanded clay pebbles: 4 to 8 times a day (every 2 to 4 hours)

Coconut coir: 3 to 5 times a day (every 3 to 5 hours)

Rockwool: 1 to 5 times a day (once a day to every 3 hours)

Remember that these are guidelines only – feel free to flood and drain outside these ranges. For instance, if you are growing large plants in clay pebbles in a hot, dry, atmosphere with powerful lighting, you may need to flood and drain 9, 10 or even 15 times a day.

Finally, err on the side of caution – never let your hydroponics garden dry out.

Copyright 2007 Caroline Mackenzie

2 Responses to “How Many Times to Flood and Drain?”

  1. I was thinking of building a good sized hydroponic system so I could grow a variety of vegetables. I was wandering how much voltage it would take if my system covered about 300 to 500 sqft. based on your experience. I would like to build a solar energy system to power anything that needs electricity so I was just trying to get some figures.

  2. Hello Sarah,

    I am not sure what you mean by “how much voltage.” Do you mean to ask what your estimated power consumption will be? If so, this would be a question of watts – how many kilowatt hours of electricity your system would use/need.

    This would depend on many variables. For instance, the type of lighting used, the quantity of lighting, “on” hours per day, plus of course any power used to deliver nutrients to your hydroponics garden.

    Based on the information you have given, we can at least consider lighting which you will need if natural lighting is insufficient and is likely to be your prime consideration when estimating power.

    Your hydroponics system would need between 20 and 40 watts of light per sq foot. A 400 watt light will deliver about 25 watts per sq foot covering a total area of 16 sq feet. So, a 400 sq ft system would need about 25 x 400 watt lights. If they run for 12 hours a day, total kilowatt hours per day would be 120 kilowatt hours. Your solar powered system would need to be able to deliver this. I am by no means an expert on solar power but trust your local expert would be able to translate this into something useful.

    Regarding volts, if you do plan to use artificial lighting, a 240 volt circuit can handle twice as many lights as a 120 volt ciruit and would be advised for a large system like yours.

    Hope this helps,

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