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Ask-a-Grower #173-How often do I fill my flood and drain table and for how long?

I just set up an ebb and flow system with a 3×3 tray. How often do I flood the tray and for how long?

That’s a really good question.  “How often” and “How long” to flood will depend on a few different factors such as type of media being used (rockwool, hydroton or other types of rock, coco, etc..), size of the plants as well as the size of container or cube.  These will all have different watering schedules.  For instance, rockwool “holds” water, a lot more water than hydroton rock. So, the hydroton would need more frequent watering than the rockwool.


When I use rockwool I always pre-soak.  I use my base nutrient at 1/2 strength at the early veg dilution rate and I also like to add House & Garden Root Excelurator.  Adjust the pH of your soak solution to 5.5-5.8 and soak the rockwool for about an hour to stabilize the pH of the rockwool.  Some people like to use specific rockwool soak products like Europonic rockwool soak or Supernatural rockwool soak.  After the rockwool has soaked I place the plant directly into the rockwool, then I put all of the plants in the flood tray.  I usually don’t have to water them again for 2-4 days, however it is a good idea to check the plants daily to see if the rockwool has somewhat “dried out”.  I want the rockwool to still have some moisture but not be over saturated.  I check and water manually, based on the saturation of the cubes, for the first week or so then I set the system to water automatically once a day.  By week three I’m usually on a twice a day flood and  I always flood for 15 min at a time with mechanical timers.  Keep in mind the amount of water the rockwool holds.  No matter how much rockwool you are using, occasionally check the rockwool right before the next scheduled flood to make sure it is not still saturated.  If the rockwool is still saturated I skip the scheduled watering.  Also, rockwool has great “wicking” capabilities so when I flood, the water level only has to cover about 30-50% and the rockwool will “suck up” the rest.


Hydroton is expanded clay and during shipping some sediment will occur, always be sure to rinse away this sediment with fresh water, essentially “cleaning” the rock before you use it.  When flooding with hydroton I have to bring the water level up almost as high as the container I am using, most importantly I am looking to make sure that the roots are getting saturated.  The water level has to be higher than rockwool because hydroton doesn’t have the same wicking capabilities.  Also, I always start off with a once a day flood for 15 min at a time.  By the end of week 2 I am able to bump up the watering schedule to twice a day.  As the plants grow and I get close to flower I move to 3 times a day, still for 15 minutes each time.  Remember, this all depends on the size of the containers used for the plants.  I start in 6-8″ baskets and flower in 2-3 gal pots.  If I kept the plants in the 6-8″ pots I would probably end up watering more often.


When using a coco medium I like to water my plants and check them daily to see if they have “dried out”.  I always water when the containers are still moist but not over saturated.  The frequency of watering when using coco has a much larger range.  The frequency will be related to the amount of perlite/rock/vermiculite used with the coco.  Some coco is straight coco and some is pre-mixed for anywhere from 1-7 waterings a day.  Just make sure to keep an eye on the medium daily.

Generally speaking, you can judge when your plants need watering by familiarizing yourself with the difference of the weight of the container when the medium has just been watered, and is totally saturated, versus the weight of the container when the medium is totally dry.  After the initial watering, and when the medium is completely saturated, pick up a container each day to see how the weight changes as opposed to how dry the medium is.  Know the weight of the container when it needs watering and you will have a quick test that will tell you when and if your garden needs watering.  Remember you never want to let your medium dry out completely.

I hope this has been helpful.  If you have any more questions or need further explanation on your current question please email or call any one of our locations.

4 thoughts on “Ask-a-Grower #173-How often do I fill my flood and drain table and for how long?

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