NanoGadget
Well-Known Member
Temps. Under LED maintaining leaf surface temps is ultimately a bigger factor than trying to milk an extra 8 or 10% rH. Under those lights shoot for ambient temps netween 80 and 85 and you should be fine.
Is VPD a catch 22??
- Thread starter Bullmark
- Start date
Cookie Rider
Well-Known Member
Try 82 degrees at 60 in veg.
That’s when I saw them take off.
That’s when I saw them take off.
Markshomegrown
Well-Known Member
I use large pots(29ltr) with raised drainage holes(3") on large trays, water till harvest, fill the trays up with 1-2" of water, I could fill most of my flower room floor up with water, the humidity does rise really quick, low airflow on a ground level, so the trays take a day or to too dry out.
Man I’ve pulled out all the stops trying to increase the RH…..in my small space I have 2 humidifiers running full blast, catch trays, underneath the shelf that the plants sit on, filled with water, 3 wet towels hanging and a bucket of water in the corner……all in a 2x6 closet.
I think the low rh has been a contributor to my plants taking so long to finish up. I searched my town up and down for a speed control for my exhaust and ended up ordering one today from Amazon.
I tried something that may be counterproductive but I wanted to see…..I turned my exhaust off for around 20 min. It immediately raised the rh to 70%. I turned it back on and it took a good hour for the rh level to slowly return to the low 40s.
i guess my thinking was it would give the plants a little bit of time at optimal levels. My luck the rising and falling will have a larger negative effect……that speed control can’t get here fast enough.
I think the low rh has been a contributor to my plants taking so long to finish up. I searched my town up and down for a speed control for my exhaust and ended up ordering one today from Amazon.
I tried something that may be counterproductive but I wanted to see…..I turned my exhaust off for around 20 min. It immediately raised the rh to 70%. I turned it back on and it took a good hour for the rh level to slowly return to the low 40s.
i guess my thinking was it would give the plants a little bit of time at optimal levels. My luck the rising and falling will have a larger negative effect……that speed control can’t get here fast enough.
LeastExpectedGrower
Well-Known Member
The variax or whatever you have coming for your fan control will take care of this. Our ambient RH right now on the floor where my tent resides is currently 33% (lower than ideal, but I've been holding off using the ultrasonic humidifier until I can get an evap unit down there)...but my grow tent is firmly in the 50% range. That's because my fans are generally only pulling lightly when conditions are in their sweet spot and only ramp up when it gets too hot, or the humidity gets too high. the big pots of water/soil & the plants themselves can more than fill a small area with humidity as long as you're not sucking all of that out of your tent/closet/cabinet/room too fast. I have a small humidifier in my tent that I fill with distilled water once every week or two to catch moments where I've opened the tent and humidity has dropped.
What temps should I run for the 6 hour dark period ?? If it matters I’m in the last 2-3 weeks of flower…
thefullspectrum
Well-Known Member
Rather than an aftermarket speed controller for an AC motor 'not' designed to be variable. Simply use an anolouge or digital timer to turn the fan on for eg: 15 mins every hour etc. Try and find that balance. Most of us would kill for low humidity.
Alternatively hook up the fan to a temp/humidity controller. One of these may cost more than a cheap and nasty AC voltage reducer but I can guarantee your exhaust fan will last a lot longer running on and off full power than slowing it down.
Alternatively hook up the fan to a temp/humidity controller. One of these may cost more than a cheap and nasty AC voltage reducer but I can guarantee your exhaust fan will last a lot longer running on and off full power than slowing it down.
NanoGadget
Well-Known Member
I like to keep my lights off temps within about 10 degrees of lights on Temps.
I’ve considered that route and have been manually running back and forth doing that very thing. My fan, on full power, really sucks all humidity out quickly. I was concerned that my overall air exchange would be compromised by having the exhaust fan turned off for such a large percentage of the time.
I’ve read in multiple places where the last 2-3 weeks of flowering it’s best to lower the temps and RH……I can’t recall exactly why. Do you run low mid 80s all the way to harvest??
twentyeight.threefive
Well-Known Member
Do you have a link to these multiple places where you heard it?
I’m a Luddite so providing a link is probably not within my skill set, but one particular group Californialightworks.com has a nice tutorial on growing under the lights they sell. I know they suggest lowering temps by maybe 4-5 degrees for the last couple weeks. I’ll look back and see if I can name a few of the other grow sites that advocate lowering the temps and RH during the final weeks.
As most know the internet can be a great source of info, but it can also contain a lot of total BS.
I’ve grown plenty outdoors but my indoor experience is very limited. When I started my first autos if someone told me I’d have plants barely 2ft tall with the buds that mine have currently, I’m not sure I would’ve believed them. I’m tickled to death to get what I’ve gotten so far, but I know if I’d vegged in 82f instead of 72f things would have gone better.
The quality of the buds is great, my only concern was their super slow rate of finishing. It seems like every week I look at them and say 2-3 more weeks. I know they will finish on their own time, I just wanna do my part by controlling things like temps and RH to their liking.
Now I’m rambling but will say in the course of navigating my trip down this rabbit hole I’ve concluded that there’s a lot of opinion that temps and rh should be lowered in those last 2-3 weeks. Just b/c someone is doing something and producing good results doesn’t mean it can’t be altered to improve those results.
Thanks for your input. Sometimes it takes a village..
madvillian420
Well-Known Member
Mold/rot is also strain dependent, surprised nobody has said this yet.
Theres a guy on IG called Mitten Master, the dude runs a warehouse grow the size of my house and produces some seriously impressive nug. I admire his results and practices, one of which is to dim the lights to 70% in the last weeks of bloom.
Theres a guy on IG called Mitten Master, the dude runs a warehouse grow the size of my house and produces some seriously impressive nug. I admire his results and practices, one of which is to dim the lights to 70% in the last weeks of bloom.
Mont@n@
Well-Known Member
82f imho is the best for led lights.
Here are the numbers I go by based on this VPD chart with great results. I use a growtronix system to monitor VPD (kPa)
25 - cxb3590 5x5 flower room
80.6f - 82.4f (mini split a/c, CO2 700ppm)
My safe zone RH% with air circulation
RH% VPD (kPa)
85% - 0.54 Fungal Pathogens
80% - 0.71 MAX (big chance of fungal)
75% - 0.89 x Safe Zone Max
70% - 1.07
65% - 1.25
60% - 1.43
55% - 1.61 x Safe Zone Min
50% - 1.78 Min
45% - 1.96 High Stress Zone
VPD Chart
An Introduction to Vapor Pressure Deficit | Ceres Cannabis
How well do you understand temperature and relative humidity in your greenhouse environment? Learn more about Vapor Pressure Deficit and how it can affect your marijuana or CBD grow operation.
cerescann.com
Happy Growing
bodderz
Active Member
I run a propogator grow so nice and small to control. The outlet/inlet fans cycle for a minute every 15 mins with a small powerful fan just swirling the air inside it around. The point being I take my RH up to 75% RH at a temp of 29.5c or 85F. The RH gets knocked down to about 55% after a 1 min fresh air intake but it's not long before it's heading up to 70+ again and i've never had any rot or mould. It is all about the air movement with high RH it seems.
