High RH at Flowering - Tinker with Settings, or Cull some Plants?

[spinosa]

Hello Guys,

I am a first time grower (although I have a reasonable amount of horticultural experience), and I'm having problems controlling high relative humidity in my tent now that we are 10 days into flowering. I think that I have all the tools needed to control this properly, but suspect that I just haven't got it all dialed in correctly yet, so I am hoping some wise heads here can help me.

My setup is in my garage, which is not ideal as my intake air will be cold (current ambient temp is about 13 degrees, but this will drop to 5-8c before harvest). During lights off, I have a very effective small oil filled radiator which kicks in at about 20 degrees. There is nothing I think I can do about the intake air temperature except try to manage it. I am using a fan controller to control my intake and exhaust. This is set to kick in at 25 degrees, although it has an idle speed setting to ensure some exhaust / intake occurs below that temperature. I currently have it set to a moderate idle speed, and I have the balance controller slightly biased toward the exhaust, in order to maintain negative pressure. In addition to that I am using a single 600 watt HID for lighting, which currently gives me plenty of lights-on temperature.

I only bought a RH monitor a few days ago, and was shocked to see the RH at between 70-80% at different times through the day. So yesterday I bought a dehumidifier and put that in the tent as well. The probe of the RH monitor is set at the root zone level of the plants, and is adamant that RH is an average of 75%. I don't know where the gauge on the dehumidifier is, but that is reading closer to 55-60%. But the dehumidifier has been running at full blast all night, and is still not winning the battle, so the electricity usage is also getting to be a bit of a concern.

My feeling is that the RH is higher at the root zone because I have a very dense canopy. I have a 1m x 1m tent containing three sativas, which have been kept low and tight using LST. So they are not too high but are nicely filling the width and depth, to the extent that I may need to do some trimming soon to stop any leaves sticking to the sides. We are almost 2 weeks into flowering and some lovely looking flowers are starting to form. But I am concerned about root rot, and possible later bud rot.

The thoughts I have currently are:

1. I only bought the dehumidifier yesterday - perhaps it will take a little while to get things under control (I know this is probably wishful thinking, but hey).

2. Am I overwatering? The plants are taking a total of about 6lt per day between them. They don't look or feel overwatered, but perhaps they could manage on less. I am using coco by the way.

3. I could completely kill the intake fan, so that all intake would be passive. Maybe this would reduce the amount of cold air coming in.

4. I will buy another fan today and aim it directly at the root zone to try to make the RH more equal throughout the tent. (update, I already did this and there is no noticable difference)

5. If there was no other option I could potentially remove the radiator and place it instead in a sort of pre-chamber, so that I wasn't heating the tent directly, but heating the intake air. However this would cost me time, space and money, and so I'd appreciate opinions on whether it would be worthwhile before I go ahead and do it.

6. Lastly, would it be more sensible to simply cull two of my plants? The original intention was always to cull down to a single one, but they've all been really healthy to date and I haven't seen any real need to thin them out. But if that would reduce the transpiration and help keep things more manageable, then it makes sense to me to take one plant all the way to harvest rather than lose all three in the next couple of months to rot. Although there would be tears in my eyes at killing two of my ladies ;-)

Thanks for any suggestions, and sorry for the long post - I wanted to get as much info as possible, but I guess there may still be some questions.

Thanks!
spin

 

[spinosa]

Just a little update to this.

One of the problems is that as the temperature in the tent rises, the RH rises and the dehumidifier kicks in, which raises the temperature further still. So I end up in a bit of a vicious circle.

So my plan for this evening is to reduce the target temperature for the intake/exhaust controller to 21 degrees, in an attempt to keep things a bit cooler. I may also ease the dehumidifier target back to the 50% range initially, since we're only 10 days into flower and I hope I can get away with that in order to just feel my way into it more gently. I'm also going to place the radiator on a low setting, eg around 15 degrees.

Hopefully those actions will start to ease the situation a little, because the past couple of days have been a bit of a nightmare. My lights on temps hit almost 30 degrees overnight because of the lights and dehumidifier combined. I hope that won't be a problem - I do have a Co2 bag in the tent and the plants are supposed to be ok with higher temps, but I don't like the erratic fluctuations that this is causing.

 

[labexperiment]

Place the dehumidifier so the intake for it is pulling the cold outside air into it and dehumidify the air and warming it at the same time. So far it is my only idea on what I have read.

 

[jinn]

Route the cold air from your intake, via aluminum ducting, to your HID. It'll cool your light and warm the air in the tent. I don't know about the radiator, as I've never really had to worry about cold temp growing before.
Humidity and condensation in a building - How to control it

 

[spinosa]

Thanks for the replies guys - those are useful ideas, although after my actions today I don’t know if I’ve misdiagnosed the issue.

I noticed that when I opened my garage door, the RH inside the tent dropped to the sort of levels I was looking for. Obviously that’s not a practical solution long term, so I added some ducting to the intake so that the air is being drawn from the outside. This may work temporarily although at some point the ambient temps will become too cool and I will have to look at the suggestions you guys have made (both very practical, thanks!).

Right now, I’m having a really tough time just getting any sort of consistency. During lights off (daytime) the ambient temperature is just about where I want it, but every now and then the RH raises enough that the dehumidifier kicks in, and it heats the air inside the tent and the RH seems to take hours to come back down again - it’s like it’s creating its own problem. Aside from not getting the RH dialled in, it means I’m getting temperature fluctuations of +/- 5 degrees which is worrying me more than RH at the moment.

So this evening I am going to remove the dehumidifier and just keep an eye on temps and RH to see if I can get a stable base to start from. Which brings me to a couple more questions:

1. I am now wondering if watering or overwatering might be adding to the problem, and so I am considering dropping my rhizopots into an Autopot system. I don’t know if this is advisable, but it seems a logical way to ensure only the needed amount of water is going to the plant. Any thoughts would be much appreciated.

2. It might be that with better temperature management, I can keep the RH at bout 50%, instead of the 35-40% that I was aiming for. If that’s as good as I can manage, could I expect a reasonable harvest still? I guess I should use some root & bud rot solution as a preventative measure in this case.

3. I’m still not averse to the idea of culling or a bit of defoliation if people think that will help. The canopy is thick enough that the lower leaves are getting practically no light, so I guess removing those can only help matters a little.

Thanks again for any replies and your patience - I realise I’m kind of feelingly way with this and just giving you the info as I identify it.

Thanks,
spin

 

[jinn]

I imagined you would have already thought of defoliating a bit >.< But yes that will actually help a bit not only with the RH but the growth of your plants too! Just don't go over board because the can go into shock.
I agree with taking out the dehumidifier, as its one more variable in the mix for the moment.
Since you're in the garage, opening the door to vent the air 2x a day is the easiest way to control the RH surrounding your tent.
cement/concrete love wreaking havoc on RH in cold weather.

As for the watering I didn't want to say anything because i don't know how thirsty your phenos are, or any of the other 100 variables that affect watering schedules lol

 

[spinosa]

Thanks jinn - yes, I had thought of defoliating but hadn't actually got around to doing anything yet. I'll make a couple of simple changes this evening at lights on and monitor to see if I can get things a little better stabilised and then take it from there.

Maybe I should just bin the hygrometer - everything was going just fine until I started using that, lol.

 

[jinn]

its good to have just to keep yourself out of the danger zone for mold. You really really don't want to be 3 days from harvest and lose the entire crop the mold =/

But keep us updated, I love hearing about as many different grows and set ups as possible!

 

[MVG 43]

If you have a passive intake the humidifier will always be running because its constantly having humid air around it. Same thing as placing it outside. The only way you want to run it that way is you have a sealed room with an A/C. Above 400 watts and you really need A/C. 50% RH is fine for flowering, 35% is too low imo

That's the bad thing about running your lights on at night is the RH always will be 20-30 higher then. A/C would take care of both problems, cool your room and lower the humidity. If you use cold air from outside, yes it cools, but the high RH comes along with it. Are you running a cool-tube hood?

Lets say you do adjust it right, then the next week it can be completely different based on the weather outside. You can order a 5,000 btu a/c for 140 bucks on amazon, that would be perfect. Just cool the garage as a whole, then the tent passive within itself, then get a real co2 setup and your ready. That would cost 900 bucks. If you don't want that expense, then keep it in mind for you next grow.

 

[spinosa]

Just to update you after the changes I made yesterday. First of all, an embarrassing admission which I would normally keep quiet, but I can see how other people could make the same mistake, so hopefully I can save someone else the headbanging that I went through.

My Hygrometer is an Accuread thing, which has a probe attached to the main unit in order to measure inside and outside temps, and RH. In order to correctly measure temps, you place the probe inside the tent with the LCD display unit outside your grow room, and then the "outside" temp where your probe is, is actually the "inside" temp of your grow room. Which is nice and easy to understand. What was less clear is that the Hygrometer is contained in the LCD display unit, not the probe, which explains why I was getting such a difference between the dehumidifier and the Hygrometer reading - my hygrometer was happily measuring the RH outside the tent.

So, now at least I know that I am able to measure the right thing, and we might hope to make some progress...

That said, there is not as much of a difference as I was hoping - through lights on with no dehumidifier we managed to get down to 60% RH, and at lights off today, we are back up to 70%. But as MVG43 points out, taking in cold humid air is not going to help that, so I will undo my changes of yesterday now so that I am slightly recirculating the air from the tent. With that, and with a bit of fiddling with the dehumidifier, I hope to be able to get lights on RH down to about 50%, and from there I can see what I can do about lights off.

I am still banging myself on the forehead about the meter. I may stop some time later this week ;-)

 

[spinosa]

If you have a passive intake the humidifier will always be running because its constantly having humid air around it. Same thing as placing it outside. The only way you want to run it that way is you have a sealed room with an A/C. Above 400 watts and you really need A/C. 50% RH is fine for flowering, 35% is too low imo

That's the bad thing about running your lights on at night is the RH always will be 20-30 higher then. A/C would take care of both problems, cool your room and lower the humidity. If you use cold air from outside, yes it cools, but the high RH comes along with it. Are you running a cool-tube hood?

Lets say you do adjust it right, then the next week it can be completely different based on the weather outside. You can order a 5,000 btu a/c for 140 bucks on amazon, that would be perfect. Just cool the garage as a whole, then the tent passive within itself, then get a real co2 setup and your ready. That would cost 900 bucks. If you don't want that expense, then keep it in mind for you next grow.

Thanks MVG 43, that's a really helpful post - especially the comment about 50% being acceptable, which makes life a lot easier for me. I don't really want to go to the expense of AC right now, and I also want to minimise the noise of my grow, but I expect my next grow to be in a different location where I can grow inside the house, and that will make such things possible.

Whilst this has all been a bit of a pain and I've started to get a little paranoid about whether my girls are going to make it to harvest, my learning curve has just gone parabolic. It was all quite simple up to now, so I am happy to be getting some very good experience for my next attempts, even if this one doesn't quite pan out.

 

[Fuzzy Duck]

Mmm real life weather and RH, well RH peaks during the night & lowers in the day... damp/wet weather will cause RH rise & all of this effects the indoor RH value to some degree !

Direct intake from out side into the grow area, well lets just say ya pumping in what ever RH/temp of outside air.... not always a good thing ?


I may go as far as...

1. Passive intake vents on tent opened.

2. Place dehumidifier out side of grow tent set at RH 50% 24/7, a side effect of dehumidifiers is heat generation & in small space like a tent its going to warm it up pretty well... but in garage a larger space ambient heat produced would be spread out to a larger volume of air lowering effective temp.


3. So with this a dehumidifier out side in garage ultimately correcting RH to said level & the exhaust fan is pulling in fresh air through the passive vents hopefully lowering RH should improve grow room RH in theory.



When i think about it a dehumidifier in tent with active intake pumping X cubic m2 of air per hour to into tent & said extraction fan removing what ever volume of air per hour is a dehumidifier really going to be effective in such a small area with a fast flow of air ?

Dehumidifiers are normal rated for X size of room volume etc what it don't tell ya is what ever X cubic meters of air per hour it can deal with ? so what that may suggests is it mainly works best with a reasonable slow air flow & not of being stuck in between active intake & extraction fan...


I hope ya catching meh drift er

 

[spinosa]

Thanks again for all the replies - just to update on the current status.

I pulled the girls out and removed some lower leaves, mainly those that were starting to yellow, but I also took off a couple of more healthy looking branches which were in danger of sticking to the side of the tent. The base of the plants is now much more open, and I feel happier that I probably have better airflow at that level as a result.

Having now worked out how to use the hygrometer properly, the dehumidifier is back in the tent and working on a timer so that it only cuts in during lights out. That is quite comfortably maintaining lights out RH at about 45-50%, and I am very happy with that.

I have had to set the timer to prevent the dehumidifier coming on during lights on, as the heat from it takes the temperature up too high. I noticed a couple of leaves suffering from heat stress after my one night of high temps, and so I have come to the conclusion that I would prefer to have higher lights on RH and no heat stress, and hope that the lights out lower RH is enough to keep rot at bay. I will probably invest in some preventative treatments as well.

Lights on RH fluctuates between 60-70%, which I find odd, since I would expect the increase in temperature to lower it. So maybe the hygrometer is not so accurate anyway. But in all honesty I think this is the best I can do - to try to condition the intake air in the current location would be too energy inefficient and also too noisy, so I am going to have to take my chances and hope that I get away with it this time. It’s all good experience anyway.