Hi all, my first post here (I think) so I would like to start by saying a quick thanks to everyone her for sharing there knowledge. Thanks. I run a closed room CO2, A/C, Dehumidifiers etc. and I have a few technical questions that I can't find answers to. I hope that one of the locals might be able to help. 1 - At the end of the dark cycle, with no air exchange, how low would my O2 levels be. Is it possible that low O2 could be a factor thats limmiting respiration / growth? 2 - The cost of dehumidification is emense and I'm wondering if theres much efficiency gain to be had from scrapping my two high output domestic models and replacing them with a greenouse dedicated model (big $) Anyone Know? 3 - My roots sit at about 82 degrees F and theres not much I can do about it. To deter pythium, I use free draining pots with 100% perlite and pump heaps of air, from my room into the root ball (Co2 and all). At the end of harvest my roots look healthy but I still cant help but wonder if I'm loosing weight because of high root temps. I'm not having problems with this system but I'm always interested in looking for tweaks that might add a bit to the end result. Thanks all, hope someone has some Idea about this stuff.
Unlikely that it is a limiting factor of your growth for 2 main reasons. 1. Very very few home setups are truly airtight sealed environments. There is usually some leak somewhere that allows for air exchange. No matter how small this is prevent the room from being completely filled with Co2. If that were the case your plants would not do very well if there was not abundant O2 during your dark cycle. 2. Your plant in general should be creating more O2 during the light cycle then it uses during the dark cycle, so even if you had a 100% fixed volume of air it would always have enough O2 created during the light cycle to use during the dark cycle. At some point it will reach an equilibrium and plant growth will slow. That is a common problem with a smaller grow space that is sealed and using CO2. One of the least expensive ways to remove RH is to use an exhaust fan system. Besides that you are going to be stuck using the dehumidifer. Most of the commercial setups that I help plan that used sealed Co2 rooms work like this. Co2 is generated to bring it up to 1200ppm. Co2 is monitored and allowed to decrease till about 600ppm. At this point the exhaust fans kick in and air is exchanged. Fans turn off, and co2 generation repeats. This is all automated with controllers so its really efficient. If its working for you so far then thats great. Your plants will definitely do better if you can keep the temps lower, but if you do the bleach or hydrogen peroxide treatment you should be ok. You just have to keep a careful eye on things. Root rot can destroy a crop pretty quickly if left unchecked. Nothing worse then to be at the end of flowering only to have root rot kill your plants. You could build your own water chiller or order one from alibaba. Look up aquarium chillers or water chillers.
Thanks for the response Gelato, I was beginning to wonder if anyone knew what I was talking about. You've given me more confidence about my low O2 concerns, thanks. My room is not perfectly sealed but enough so that the CO2 rises from 400ppm'ish to >2000ppm during the dark cycle. This causes slight tip burn, which I tolerate because I really don't want to vent. I live in a very funky tropical environment where humidity is often >80% and the temps are rediculous. I'm right next to the rainforest and the air is thick with mould spores. This is why I'm so very reluctant to vent as it seems counter productive. Given your advice it sounds like I'm stuck with the expensive dehumidifiers. I really wonder about the root temp thing, do you know what limits growth at higher temps? Common theory states that colder water holds much more O2 but my minds eye says that air that Im pumping through my pots should have a much higher conc'n of O2 than a nutrient solution ever will, especially seeing that the air delivered into the roots is O2 enriched by my plants. Perhaps I'm naive in thinking this. What I do know is that I really dont want to install a chiller because of initial and on going costs. If I was having root pathogen problems Id look at it more seriously but the roots are healthy, white and smell fresh. Are O2 levels and firewalling agains pathogens the only reasons for maintaining lower root temps or is there something else that limits root function when running a bit warm. Thanks heaps mate, It hasn't been easy to find anyone with clue? Most look at me like I'm from Mars
Hmm that is an interesting problem. I've never had to deal with a tropical/rainforrest environment before. That does create some interesting problems. The only thing I can think of other then dehumidifiers is to use desiccants that you can "recharge." Silica Gel desiccant is going to be your preferred method, and you will need a pretty big quantity of it to remove the amount of moisture I imagine. Calcium chloride will be your other option but pose some risk. Ideally you will have 2 sets. 1 set removing humidity while the other set is in the oven being dried out "recharged". Its going to be costly to get a significant amount of the stuff to make any real difference, but it is reusable so the cost can be spread over time. A larger "commercial" dehumidifier will be more energy efficient but it the initial cost and continued energy usage will be costly. AS for the exhaust fan option. You can find some effective air filters that will remove the mold spores and contaminates from the air that will help in that situation. This might be a good lower cost (over time) then constantly running a dehumidifier. The key here is to have staged filtration unless you have access to a large exhaust system. Most staged filtration systems I help plan work in 2 stages. You have a prefilter that traps larger molds and dust particles that have very little air restriction. The 2nd filter/stage is for your fine particles that will trap the remaining smaller mold particles and other smaller contaminants. You could go with using just the fine HEPA filters instead of the 2 stage setup but it doesn't work as efficiently because it will restrict air flow and need to be changed more often. I've never really tried to find the max temp a reservoir could hold and still maintain healthy growth. I might try that one day just to find the answer. As you stated the higher the water temp the less O2 it can hold essentially. So no matter how much aeration you provide to to solution it can only hold so much O2. Have you completed any grows yet? If so and you haven't had any problems then I wouldn't worry about it. If this is your first grow with this setup and things are going well, then I wouldn't worry too much, but make sure to keep an eye out for any slime of discoloration of your nutrients or roots. One option would be to do a sort of flood and drain setup that would allow your roots to "air out" and not constantly be drowning in the water.
Thanks very much for your time and detailed response. Yes, the climate here has created challenges that most are unfamilar with, nothing that heaps of tech / knowledge cant fix. I'm old hat, done heaps of grows in temperate zones, this will be #3 in the current setup. There is much work for the academic crowd to in the field of canna. Many unknown's. Thanks again, I really value your knowledge.
How big is your grow space, I didn't see you mention this? 82 degree's is a bit high for hydroponic res. but I thought you mentioned that you're growing in soiless medium? Speaking for rDWC high temps can promote pathogens and low temps hold more oxygen -- however slightly higher temps promote root activity and cooler temps slow root activity. Sort of a balance with rDWC. I purchased the Saturn 6 controllers to use for each flower tent with CO2 tanks as well. My understanding is that you need to dump the tents at lights off because it'll harm your leaves if CO2 concentrations are too high during lights out. Regarding humidity and CO2... you want high humidity when using CO2. You shouldn't need to use dehumidifiers constantly. I purchased two Haier dehumidifier 30 pint units newly refurbished from eBay for $100 including shipping and then received $25 gift cards for each unit as rebates from my local energy company. Units are very efficient, nice and thin (perfect for taking up minimal space), and only 25 lb's each.
Set your CO2 enrichment to stop about an hour or so before the lights go out. This will give them time to use up the extra co2 and not waste it.
That's what I do, reduces C02 to about 500ppm when the lights go off. Co2 peaks at about 2000ppm at the end of the dark cycle. The leaves show minor tip burn, not a big deal IMO Um, yeah OK. Are you working from experience or readings from the net. BTW, my dehumimidifiers look just like yours only mine are a little bigger. Wait until you get your power bill, you'll realize that those units are working pretty hard. I've deliberately avoided posting tolals, that's Why all of my numbers have been presented on a per watt of light basis
If you're using CO2 then your temps should be between 85-90 degrees in your grow space. Based upon simple VPD tables that puts your RH % between 80-90% for max. photosynthesis activity. If you're dehumidifiers are running around the clock in your grow space... then what are you setting your dehumidifiers at? I posted pictures of 30 Pint units which are rated for upwards of 1500 sq foot. My tents are 25 sq ft each...? Am I missing something?
Thanks for the VPD chart. Correctly using VPD isn't as simple as printing off a chart, you need inputs specific to your garden, leaf temps etc. Running at high humidity is great during the first 4 or so weeks in bloom, beyond that you'd be mad to operate at >55%. Even most VPD guru's reccomend higher VPD during bloom. The trouble is that once your space is filled with living vegetable matter the amount of water that's pumped into the air is huge. You can measure it pretty accurately, pretty well all of the water thats consumed by your plants is delivered into the air around them (except for a little thats assimilated). Also during the last three or so weeks if you don't drop the humidity you're pretty well guaranteed bud rot. Dehumidifiers become very inefficient when trying to reduce RH below 50%. Hence domestic model ratings eg. 50L/day @ 33 degrees celcius and 90%RH. At 50% humidity the dehumidifier in the example will only remove, maybe 20L per day. Have you finished a crop using that VPD to the end? I know my bud would rot for sure. EDIT - I highly reccomend scrapping that vpd chart and checking this link for a good VPD chart maker / calculator. This will make charts specific to grow and bloom. The bloom chart looks very different to the one above. The Ultimate Vapor Pressure Deficit (VPD) Guide - Pulse
Yes, I have had many non-CO2 grows. And I hear your concerns, but below 50% humidity is a little excessive (even if it is at the end of flower). I'm not sure where you keep your temps during 50% RH, but I'm assuming you end up with a crazy high kPa VPD value, much higher than desired. And of course -- if I ran my dehumidifier set at 50 RH% it would probably just run all day and night. That's crazy low, especially for CO2. It would run all night in day just in my basement without plants at 50 RH% Take a look -- this guides you for VPD from veg, early-flower, and late-flower. And sure you want a slightly higher VPD towards the end, but you may be going too high:
Thanks, that guide suggests that my humididy should be approx 45-55% during late flower. Pretty well the same as the pulse calculator.