Ebb/Flow Tips

Unlike most active hydro systems where water enters containers from the top, ebb/flow generally employs subirrigation to flood the media from the bottom up. Because growing media is totally inundated it should be fast draining and provide plenty of aeration. To this end an inert hard coarse gravel with a particle diameter of approximately 1/2 inch is suitable. However, because of its weight, gravel is clumsy and laborious to clean between crops. Contemporary media includes manufactured alternatives with brand names such as Hydroton and Geolite, which are not only lightweight but offer better water retention due to their improved surface texture. Commonly referred to as grow rocks, these replacements take the guesswork out of selecting a suitable gravel and are the default media used in this article.


Covering the reservoir and growing bed serve several purposes.

Use opaque inert material for covers. For growing beds, a white color facing the plants helps make better use of light. And though a black color is preferred to face the root side, as long as the material is opaque the purpose can be served with any color. Inert materials should be used wherever nutrient solution comes into contact with it, this is especially important for reservoir covers where condensation, or spray from air stone bubbles, constantly drips from the cover back into the reservoir volume. If the material is not inert, unwanted chemicals can leach from the material and contaminate the nutrient solution.

Do I need to flood during the dark phase?

No. Little or no water is taken up by plants during the dark phase. Set your last irrigation to begin shortly before the lights turn off, and the first to begin shortly after they turn on, then space the remainder of your irrigation cycles to occur equally between these two points.

How deep should I flood?

Ebb/flow systems generally flood through an overflow set approximately one to two inches below the top surface of the grow rock media. This prevents soggy conditions from persisting at the base of the plant's stem, and prevents salts from accumulating on exposed media due to evaporation (see Covers).

For how long should I flood?

The duration only needs to last until the overflow is reached, or if you have multiple growing beds until the overflow on the last bed is reached. Flooding for longer periods decreases the system's aeration potential.

How often should I flood?

Though a common question, it's not simple to answer at face value. The question implies that a given amount of water has been used over a given amount of time and now it needs to be replaced. Problem is, the given amount of water is relative to the volume of water held by your media, and the given amount of time is relative to the rate at which your plants are using that water, both of which require some effort from the grower to discover.

As far as irrigation frequency is concerned, an ideal situation exists when the quantity of media used in each pot, and the size of the plant grown in each pot, compliment one another. Or said another way, when the rate of water use and the available media water reserves are proportional, the same irrigation frequency aptly applies to every potted plant in the garden. If, for example, one potted plant twice the size would be grown with others half its size, a pot big enough to hold twice as much media as the others would be used in order to make twice as much water available to that plant between irrigations. By doing so, the same irrigation frequency would still apply to every potted plant in the garden because plant size and pot size were taken into consideration. Were no consideration given to plant or pot size, the irrigation frequency would need to be tuned to the smallest pot with the largest plant, relatively speaking, in order to avoid having that plant's water reserves compromised between irrigations.

Irrigation cycles are important because they must provide adequate water, nutrients and aeration to roots. Water and air cannot occupy the same space at the same time in the media, and because of that we use the timing of irrigations to strike an optimum balance between the media's air and water content over time. Too many irrigations compromise the optimum aeration ebb/flow is capable of delivering; too few and the water reserves held in the media between irrigations will not be sufficient to maintain an optimum supply of water and nutrients.

The Hidden Reservoir

Ebb/flow users pay close attention to their reservoir water volume (the holding tank), a volume that snaps into action when the water pump is switched on. But many fail to appreciate the hidden reservoir, the water volume held in reserve by the media, a volume that snaps into action after the holding tank pump is switched off and the excess water has drained from the media. If a grower doesn't know how much water the media in a pot can hold, and how fast the water is being used by the plant, he has no basis on which to benchmark his irrigation frequency. Using ebb/flow with grow rocks is often regarded as a method where overwatering is impossible, and it does indeed offer the user great latitude in the irrigation frequencies he uses. Although some growers are satisfied with their results after some trial and error or randomly guessing which frequencies to use, others growing under more challenging circumstances may want to know more about what's going on in their root zone before deciding. This implies finding the media water holding capacity, and the maximum water volume taken up by the plant, both of which can be attained rather easily.

Finding your media's water holding capacity

A simple test can tell you how much water your grow rocks hold. Measure two gallons of water and put it in a bucket. Measure one gallon of bone dry grow rocks and put them in a pot with drainage holes. Place the pot of grow rocks into the bucket of water, cover it, and let it sit overnight. Then remove the pot and allow the excess water to drain back into the bucket. Measure the water remaining in the bucket, the amount of water missing from the original two gallons is what one gallon of your grow rocks hold.

By applying this test data to the actual amount of media in your working pots you will know how much water is available to each plant between irrigations. For example, if one US gallon (3.8 liters) of your grow rock media holds 12 ounces (354ml) of water, but your working pots hold only 1/2 gallon of media, the potted plant would have 6 ounces (177ml) of nutrient solution available to it between irrigations.

Assessing your maximum rate of water uptake

This is done at a point in a crop's flowering cycle when the canopy is full and the plants are using the most water they'll ever use during the crop cycle, usually about 3/4 of the way through flowering. As water is used from the reservoir over time, the volume is periodically replenished by adding enough fresh water to bring the volume back to the full line marked on the reservoir. The volume of these add backs are recorded in a journal or log, from that data can be found how much water the system as a whole uses in one day. Because relative humidity, temperature and air flow affect water uptake, it's recommended to record this data for several consecutive days. Use the highest daily figure if your garden has no climate control or water uptake fluctuates markedly from day to day, you can use the average if conditions are reasonably constant.

By applying the daily maximum water uptake data collected during the 12 hour photoperiod to the number of potted plants, the rate of water uptake (per plant per hour of photoperiod) is discovered. When we know the hourly rate at which plants use water, and how much water they have in reserve, we can fine tune irrigation cycles to trigger at a time when a certain percentage of their water reserves have been used up. This insures that a plant's water and nutrient supply will be neither too plentiful nor inadequate, and in the process of doing that we have optimized aeration potential as well.

If you've collected the data mentioned above, you can use this form to calculate your irrigation frequency.

Benchmark Your Irrigation Frequency Interval
(all fields are required)
Enter Your
Flood & Drain Times
Flood Time minutes
(The time the pump needs to run to flood the system to the overflow level)

Drain Time minutes
(The time it takes the system to completely drain after the pump shuts off)

Because plants are no longer using limited media water reserves between the time the pump starts and the time water finishes draining from the media, the actual flood and drain times are not considered as time spent between irrigations in the results below.

Enter Information
About Your Media
1 US Gallon of your media holds milliliters of water.
(See Finding your media's water holding capacity)

One working pot contains milliliters of media.
Here's some handy conversions for finding milliliters:
1 US Gallon = 3785 milliliters
1 quart = 946
1 cup = 237
1 fluid ounce of water = 29.6
Enter Information
About Your System
Your system's maximum daily water uptake is milliliters.
(using a 12 hour photoperiod, see Assessing rate of maximum plant water uptake)

Total number of potted plants used in the system

Sometimes an estimated maximum water uptake figure needs to be used until a crop has reached a point where the actual figure can be known. In this case, temporarily use one liter (1000 milliliters) per each square foot of canopy space, or 10.76 liters per square meter.

Fine Tune Your
Water Reserve Trigger
Select the % of used media water reserves to trigger irrigation cycles

The lower the trigger setting, the less water is used from the media between irrigations, and the more frequent the irrigations will be. A lower setting could be used with stronger fertilizer solutions to prevent the solution from becoming too concentrated between irrigations. Too many irrigation cycles, however, can reduce the aeration potential offered by your ebb/flow system. The extreme case, constant irrigation, loses its ebb/flow functionality altogether.

The higher the trigger setting, the more water is used from the media between irrigations, and the less frequent the irrigations will be. A higher setting could be used with weaker fertilizer solutions, or to reduce the number of irrigation cycles controlled by a timer. Too few irrigation cycles, however, can temporarily cause solution remaining in the media to become overly concentrated with fertilizer salts. The extreme case, dry media, can cause crop failure, or salts to bond with media in a less soluble form.

A general rule of thumb that's worked well for me over the years while using solution concentrations of around 2.0 ECms (1400ppmTDS@.7), is to irrigate when 25% to 40% of the nutrient solution held by the media has been used.

Your irrigation frequency interval is every minutes.

As flood & drain times are already accounted for in the calculations, the minutes shown here would be used from one on-timer event to the next on-timer event. For example, the second irrigation would be scheduled to begin that many minutes after the first irrigation was scheduled to begin.

Other Information:
Water reserves held in one working pot of media: milliliters
Irrigation cycle trigger (spent media water reserves): milliliters
Maximum uptake per plant per photoperiod hour: milliliters
Computed number of irrigation cycles during a 12 hour photoperiod: (round up or down to suit your situation)

About the Example Data

The grower in the example has a system taking a total of 29 minutes to flood and drain. One US gallon (3785 milliliters) of his grow rock media holds 12 ounces of water (355 ml). Because each working pot holds 1.5 US gallons (5680 milliliters) of media, each plant has 18 ounces (533 ml) of water stored in the media between the time the system drains and the next irrigation cycle begins. After logging and reviewing his crop's water usage, he found the most water used in one day was 1.35 US gallons (5110 ml) during late flowering. Considering he used 4 potted plants, and that the flowering photoperiod was 12 hours, each of the 4 plants used about 3-1/2 ounces (106 ml) of water per hour. Opting for a 40% trigger, he chose to irrigate after each plant used about 7 (213 ml) of the 18 ounces of water available to it between irrigations. This resulted in an irrigation frequency of 150 minutes (2-1/2 hours).

His first irrigation was set to begin 1 hour into the 12 hour photoperiod, the last to begin 1 hour before the end of the photoperiod, and the three remaining irrigations to occur between the first and last at 2-1/2 hour intervals.

Do irrigation timer settings need to be changed for the vegetative phase?

No, new timer events can be temporarily added to cover the longer (but less demanding) vegetative photoperiod. Using the example in the above form, although he only needs 5 on/off events for flowering, he uses a timer with 7 on/off events. When switching from a 12 to an 18 hour photoperiod, he first adjusts his light timer to add 3 hours to the beginning and another 3 hours to the end of his existing 12 hour photoperiod. With his irrigation timer, he adds two new on/off events to cover these additional temporary spans of time. When he switches back to flowering, he only needs to delete the two new irrigation timer events and change his light timer back to its original 12 hour setting.

Because the irrigation frequency benchmark is based on peak water demand, once optimized, the irrigation interval settings never have to be changed for less demanding crop phases. However, if the grower decided to harvest larger plants or change his pot size, he may want to reevaluate. And because a fully vegetated canopy will use less than half as much water between irrigations as it will at peak during flowering, it's safe to extend these temporary vegetative irrigation cycles somewhat beyond the benchmarked interval should no more timer events be available. The table below gives the number of timer cycles for common vegetative photoperiods when the above irrigation frequency interval is being used. This information may be helpful to a grower planning within the constraints of a timer he already owns, or in choosing a new timer that has enough on/off cycles to meet his needs.

Computed number of on/off timer cycles for common vegetative photoperiods
(round up or down to suit your situation)

16 hour

18 hour

20 hour

24 hour


Back to Cannastats

Copyright © 1997-2004 pH