Any DWC/UC/RDWC users running cobs?

I'm curious to here from those who are using these types of systems and what nutes they are feeding, ec and other stuff. Right now I'm kind of running an experimental tent to really dial some stuff in, and I'm finding that I'm feeding FAR higher than I ever have before. On this grow I rebuilt my RDWC to have larger drains and feeds allowing me to recycle the whole thing about once per minute. I also started using temp/humidity controllers to main a proper VPD, as before my temps would be around 83-84 and 28-34% RH in my veg tent.

I'm noticing I have much healthier roots now and explosive growth, but combined with my newest COB running at 37w/sqft at 18" above the canopy, these plants are feeding like no other. I generally have fed 0.9-1.0 EC (.5 scale) through veg and 1.1-.12 through flower, but on this grow I'm already at 1.4-1.5 in veg with no signs of burn, and the plants still look hungry (new growth a bit pale, looks like magnesium or sulfur def). If the EC continues to drop daily I foresee going as high as 1.6-1.7 before finding the sweet spot, which just seems absurdly high to me for RDWC.

Just curious to hear what others are feeding and running at with this style of grow.

Yes, I think my EC is a bit higher too then it was when I was using HPS. Although it differs per strain of course. Perhaps because the plants evaporate a bit less? Or do you give them a lot more light than before?

The plants usually need around 1.4 to 1.5 EC in veg and then much lower (somewhere between 1.0 and 1.2) after the switch to 12/12. When they get only 12 hours of light instead of 18 they quite quickly consume a lot less nutrients.

I switched to a perlite hydro instead of DWC for the last round and came to pretty much the same EC values to keep things stable.

wietefras said:

Yes, I think my EC is a bit higher too then it was when I was using HPS. Although it differs per strain of course. Perhaps because the plants evaporate a bit less? Or do you give them a lot more light than before?

The plants usually need around 1.4 to 1.5 EC in veg and then much lower (somewhere between 1.0 and 1.2) after the switch to 12/12. When they get only 12 hours of light instead of 18 they quite quickly consume a lot less nutrients.

I switched to a perlite hydro instead of DWC for the last round and came to pretty much the same EC values to keep things stable.

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I'm using a bit more light than before, I was running around 30w/sqft during veg and then raising it to 37 for flower. This time I'm starting at 37 and leaving it for the most part, minus adjusting the height when needed. I'm keeping temps around 75-77 and RH at 60-65%, but I don't think they are evaporating too much less, since they seem to be drinking plenty of water. I will probably end up holding these at 1.5 EC for a few days and seeing what happens. in the past I've always dropped the EC down at the start of flower but kept my veg nutes (foliage pro) for the first 2-3 weeks, then when switching to my bloom nutes I'd raise the EC back up a bit.

It's just odd, I've always used lower EC and had read that RDWC/UC systems needed much less, and this has generally been the case with T5's and past cobs. I guess the extra 7-8w/sqft plus proper VPD is making them really consume the nutes.

37W/SF of cobs is a good amount of light. if you find they are not uptaking nutes to your liking,try raising temp or drop humidity a bit to increase transpiration (or add a bit more cal mag to your formula)

CobKits said:

37W/SF of cobs is a good amount of light. if you find they are not uptaking nutes to your liking,try raising temp or drop humidity a bit to increase transpiration (or add a bit more cal mag to your formula)

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They are definitely uptaking nutes, just a lot more than I am used to seeing in the past. I was doing the calmag thing for a while but now I've switched to epsom salts at 1tsp per 5 gal since its far cheaper, and I've found that I need the magnesium more than calcium since the foliage pro already has a good amount of calcium in it.

Sureshot2 said:

They are definitely uptaking nutes, just a lot more than I am used to seeing in the past. I was doing the calmag thing for a while but now I've switched to epsom salts at 1tsp per 5 gal since its far cheaper, and I've found that I need the magnesium more than calcium since the foliage pro already has a good amount of calcium in it.

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Run epsom salt at 1g/gallon.

I was edging up over EC 2.0 and my plants were still hungry, with temps in the low 80s and 60% RH or close.

If they're hungry, feed them!

This is the biggest plant I've ever grown and she was starving to death the whole way;

ttystikk said:

Run epsom salt at 1g/gallon.

I was edging up over EC 2.0 and my plants were still hungry, with temps in the low 80s and 60% RH or close.

If they're hungry, feed them!

This is the biggest plant I've ever grown and she was starving to death the whole way;
View attachment 4007870 View attachment 4007871

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Yeah that's what I'm running it at. I just said 1tsp per 5g since its an easy common measurement, but it comes out to 1g per gallon.

How many watts/sqft were you running? It sounds like my conditions are similar to yours, so as you say, I will just keep feeding them and see what the plants want. This is definitely one of those cases where the "rule of thumb" is simply that, and not to be taken as the gospel.

Sureshot2 said:

Yeah that's what I'm running it at. I just said 1tsp per 5g since its an easy common measurement, but it comes out to 1g per gallon.

How many watts/sqft were you running? It sounds like my conditions are similar to yours, so as you say, I will just keep feeding them and see what the plants want. This is definitely one of those cases where the "rule of thumb" is simply that, and not to be taken as the gospel.

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Quite so.

I think our numbers are pretty close;
900W/24 sq ft=37.5W/sq ft. I ran CXB3590 chips at about 56% efficiency, I used 80 degree lenses for less spillage.