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.
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.
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.
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.
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