Rockwool block big enough for plants in ebb and flow SOG?


I am now starting flowering but I am unsure if the the rockwool blocks are big enough for the plants. Pls have a look at the pictures. The block is 100mm x 100mm x 65 mm

  • If it is too small, what can I do? I still have more of these blocks, can I stack them?
    I have plenty of slabs, maybe I could put the blocks on them. But that would mean I could not move the plants anymore because they would be connected to the slabs?


I recommend “potting” them in pots surrounded by rockwool mini-cubes. It’s a little more forgiving since they drain faster and hold air between the granules. The bigger blocks and slabs require more finesse to get watering right.

You can get away with flowering at that size in that block, you’ll just have to flood several times per day.

You can stack them, or use slabs. If you use the slabs, you won’t be able to move them as the roots will grow throughout. If you are doing ebb and flow, make sure to hand water from the top once a week to flush them, using normal strength solution.


you can grow a very large plant in a little cube with F+D stacking the cubes wont work very well i would leave em and increase the frequency when the plants ask i find rockwoll holds moisture longer than you think…


you can also put a cocomat down in the tray for the roots to go exploring helps with top heavy plants from falling over as well good luck. i like to top drip rockwool works awesome.


As soon as I started to read your post I thought you need to see Argos thread, and then when I scrolled down there he is lol.

Not sure why Argo didn’t give you a link to his grow thread to check it out. I would highly recommend it.
Not sure why Meeshs last comment came up with the link. Just click on the green text.


I couldn’t tell you why either. lol maybe I hit a button I didn’t mean to. Weird!


All depends on how big you want to grow your plants, with those size cubes 14-30+g plants i believe is doable but that is also dictated by watering frequencies and other variables, but if wanting bigger just place on top of a bigger block or the slabs like you suggest which you can cut smaller if need be for movement purposes.


What is F+D? Why stacking wont work well?

I am just worrying if the containment is too small maybe something like root bound can happen? When I started my seedlings almost drowned because I think I waited too long with transplanting and they became rootbound and the water didnt drain properly anymore.

@vernal what would happen if I don’t flush them from the top? Could I just flood the table so high that the nutrient solution would flood over the top of the cubes?

Yes you can flood the cubes but they hold a ton of water when you do it that way F+D = Flood and Drain I dont think stacking will work as far as wicking water up from a lower flood level, but if your roots are in the bottom cubes and they get wet you will be fine a 3-4 " cube will support a medium sized plant thru maturity. As long as it’s not a long flowering plant +90
take a look here a wealth of info on growing in rockwool


I grow big long flowering plants and use a 6x6x6 hugo or unislab so its way overkill but bigger roots bigger buds. but once you get to a 6x6x6 cube it wont wick all the up like a 3" or 4" cube so they suggest top drip from the top and it works great I still am on the wet side at 2x a day 30 sec drip so that tells you how much water it holds it’s way more than you think two unislabs will almost drink a 5 gal bucket with 3 leaving it dry.


You’ll get excess salt accumulation in the tops of the cubes. You don’t want to flood too high, you don’t want to drown roots during flood cycles. The hand watering weekly prevents salt buildup and ensures consistent nutrient levels in the cubes. Running “drip clean” or potassium oxide in your reservoir can help with this.


my flooding takes only 1.30 minute because I have a strong pump. Also the nutrient solution gets pulled out quickly, it is some sort of venturi valve with syphon.

If I flood to high, I get more salt buildup or because of the danger of drowning?

I have flora kleen. But I am not sure how to use it. I dont want to change the nutrient solution. Do I need to change the nutes after using flora kleen?

Actually handwatering is not an option as it is too many plants. An alternative?

Can you explain me how to use potassium oxide? I quickly searched for it but couldn’t find anything.
Actually I am looking for a solution to flush the cubes without changing the nutes and without any fresh water.

You just use regular nutrient solution to flush the cubes. Just normal reservoir water hand-watered in once a week. Flood too low and you’ll get excess salt buildup and improper water exchange. Too high and you’ll drown the roots. 1:30 flood cycles you can flood pretty high since the roots aren’t submerged for very long. I recommend flooding 1/2 way up the media.

If you can’t handwater, just use lower strength feedings.

Potassium oxide, aka “drip clean” helps keep salts in solution and prevents buildup. Flora Kleen may just be another potassium oxide “flushing” product, I am unsure, I’ve never used it.

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in 1:30 minutes I am already half up the cubes. So I can flood without problems to the upper part of the cubes. The solution gets pulled out quickly also. Maybe thats good already? What strength of nutrients would you recommend? Thank you for the advice.

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With short flood cycles you can get away with higher flood height. Be careful the cubes aren’t washed around by water. You may wish to pot the cubes in hydroton or clay balls for weight. It also holds more water and air that way.

Depends on your local tap and what you’re using. I recommend Dyna-Gro, 1.5mL/L in veg, 2mL/L in flower. It’s got everything you need and nothing you don’t, in easy 1-part liquid format, inexpensively. If you have soft water, use CalMag with it, or a base nutrient with higher Ca/Mg. I don’t test ppm/EC, even with active hydroponics. Use a big reservoir, change 1/2 the volume of the reservoir every 7 days, and you won’t run into problems.

I learned hydroponics from the aquarium plant hobby. We called it “estimative index dosing”. The concept is that with large, frequent water changes and re-dosing what you estimate to be the appropriate amounts of fertilizer, you never have to worry about any nutrient being lacking or building up.

You mentioned in another thread that your room temperatures were high, like 30C or so, are you able to keep your reservoir cooler than that? Reservoir needs to stay <22C.


my res is around 15 degee celsius. Its outside the rooms. Also I have now external air (no additional CO2 anymore) so my temps in the room are now around 15 to 20 degree celsius.

I am using RO water and mix the calMag myself. I have no water source. So I need to carry the water. I want to see if I can get away with no res change or maximum one res change the whole grow. The res I have is only for flowering (estimated 60days). I have another res for mothers and clones.

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You carry R/O by hand? Alright, pivot then. Use the biggest reservoir you possibly can, keep it cool (as you have, 15C is perfect), use a low nutrient concentration, try adding full strength recommended nutrient every week with top-off water. If you can change 1/2 the volume of the reservoir at least once during flower you’ll be okay. Since you aren’t changing the reservoir frequently, avoid additives, especially organic ones that will promote bacterial and fungal growth. You want a squeaky clean reservoir.

Further, as you aren’t changing nutrient solution frequently, it is imperative you keep it sterile. Add bleach and/or peroxide frequently since they off-gas quickly. You want to avoid any biofilm/buildup. The reason for using low nutrient concentrations is they won’t precipitate and will remain in solution at low concentrations.


I do have a small pump that I use for flooming but it seems after some time the pump gets cover by some sort of fungus. I think I read that something similar happens with air stones?

My res temps go up now, because of summer. Are there certain temperatures when I will run into problems?

Every now and then I add H2O2. What amount you recommend?

How much is a low nutrient concentration? What problems will I have when it precipates? How can I see if it precipates?

Try to stay under 22C for reservoir temperatures.

Use food grade H2O2 if you can. Just follow the directions on the bottle. I liked to alternate using bleach and peroxide, since they dissipate so quickly, and have an antagonistic effect on one another. Bleach lasts longer in solution than peroxide does, but still evaporates after a few days.

Nutrient concentration, well, I don’t use EC or ppm anymore, but what’s your base water like? I use Dyna Gro at 30mL per 20L. It’s probably around 1000ppm, maybe a tad higher since I have such hard water.


What happens around 22 Celsius?

I use self mad calg mag with RO.

How can I prevent the fertilizer to dissipate on the ground?