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Night time temp drops


New Member
New room setup and still trying to get down my cooling plan.

My lights will actually be running at night for multiple reasons. Day time temps coming up will be high 80's so from what I have been reading I need to drop the temp down quite a bit. AC issue is being worked in a different thread in D.I.Y.

Question here is: For how many hours will my girls need the -10 (Apprx) degree drop? Power can be crazy expensive down here especially at certain times. I am hoping that running the AC for 7 hours during the hottest part of the day 10am to 5pm will cover it.

Thanks in advance,


Well-Known Member
I hear ya Wiz. Your question was not really answerable without knowing what kind of grow.
Nobody, including yourself, can predict "how many hours will my girls need the -10 (Apprx) degree drop"... and you shouldn't really be thinking of it as "how many hours".
What you should be focusing on in the actual temperature. Above 80 in hydro for any significant time period will cause issues... assuming your res is in the same temp zone as your plants. I
grow in a horribly hot grow room. I use a window AC unit to keep it cool. My AC is set to the temp I want and cycles on when needed.
So... to directly answer your question... however many hours it takes to keep your temps down.

There are MANY other tips and tricks for keeping temps lower. One big one is to use a sealed light fixture of some sort (i.e. cooltube) and vent it outside your grow area.

Did I answer your question... or am I still missing a point?


New Member
No - your right - I should have given a little more info here that was in the link on the OP.

My temporary tent (Veg now) is RH 52% Low and 81% High - Currently Heavy rainy season in the Tropics. I will have a Dehumidifier for new room but won't need it during the 7 dry months. Tent Temp is low 73 high 81. Outside Temps will go up an avg of 8 degrees in next 3 months and that is what I want to prepare for. Currently running 6" 440 CFM exhaust fan very low with var controller for Co2 Exchange and trying to keep quite as the temp setup is kind of "exposed". 2 x 4 inch passive intakes. Res temps seem steady at 74 degrees - sits on "Cool" tile outside of tent.

Lights are assorted LED's for now while setting up Permanent room.

Sooooo I don't think I will have any issue keeping everything a steady under 80 degrees 24/7. My concern is that I keep "reading" that during lights off time or "Sleep time" the girls need a drop in temp approx 10 degrees lower that what the "lights on" temps are. Various reasons have been listed for this need but I will go back and post some threads here if needed.

I would like to know for How long the lights off temp drop period is needed?

I could be getting way too much into this but trying to tighten this setup as best as possible.

Thanks for your help,


New Member
Hey Wizgrow, if you want a fun fix for your girls in terms of lowering the temperatures at night (or in general) try using a PC liquid cooling kit. You can use vinyl tubing to transfer the cooled liquid across every growbucket/through grow mediums/coiled around netpot etc..
At that point you can let it run indefinitely, or apply a timer to it.
you can even use the liquid is your res, so it doubles as a chiller too. ^.^


New Member
LOL Jinn,

Funny thing is I used to be HUGE gamer and tech project manager for a Blue chip Corp before leaving the US and setting down here in Paradise ..... So I just happen to have one of those kits on my old gaming rig i was getting ready to chuck. I'll grab the cooler and HD's first! BUT..... I need to focus on the basics first before i "Play" too much.....already biting off a lot on this "Project" - kind of "Go Big or Go home" scenario ..... but give me some time and a couple grows and I will get there though :)

Just keep in mind that when I say "night".... it means "lights off" for the girls


New Member
I'm tracking night=lights out. But that sounds like providence eh? just happening to have a liquid cooling kit haha.
And i thought about the cooler through the res, its a horrible idea to run the res solution through the cooler due to leeching copper.
However, having a separate small res of distilled water will work. And to cool the grow res, just run the vinyl tubing through the res and have it cool down that way.


New Member
All good stuff but let's not forget the original question :)

My temporary tent is just that..... temporary - while the flower and veg rooms get set up. Girls just in Veg mode at this time.

Permanent - Build in process now:
Veg room: 8x7' with about a 7' high ceiling. Also has a small bathroom off to the side where i took out the toilet for "MOM" space - 4 buckets. In the shower sits the 66 gallon Res. Under the sink the controller. 6 inch 440CFM Fan exhaust with scrubber and 6 inch passive intake. Veg room will have 16 5-gallon OPS buckets to support the 2 flower tents in a large spare bedroom. New to this so it could change to 8 of them supporting the flower room - EEEKk I am really digressing here..... of course you guys know better than I the different options available here including "Autos" etc. I just need to play with this for a bit and see what works best for "me".

Flower room: AC and Dehumidifier as needed and plan in process still. Room size 14x11' (Big Vaulted ceilings) with 2 Tents: A 4x8 and a 5x10. Air flow, AC, passive, active etc has been discussed at one point above :)

Still the original question:
I would like to know for How long the lights off temp drop period is needed for?



New Member
Sorry man, I tend to get caught up on the diy aspects I'm a tinkerer by nature haha. To answer your REAL question, the 10degree drop doesnt even really need to be a 10 degree drop.

That temperature difference you're after deals with internode spacing,
but with zero difference between night and day temps will cause leaf chlorosis, and possible death.
so as long as you maintain a lower temperature during lights off, even if its a few degrees you'll be keeping your girls happy and healthy.

all that being said, keep the temps during 'lights out' lower than 'lights on', for the entire period. (keep temps above 60f to prevent any chance of stunting your growth or shutting down photosynthesis[55f] )


New Member
Well Buddy Jinn,

Check this Sh!t out: I knew one of the side effects was node length but I had it Backwards......

Taken from High Times' 2006 Seed Guide.

What's The Dif?
Manipulating Plant Performance with Professional Temperature Control.
By Proffessor Potter.

Growers know that ambient temperatures play an important role in plant development.You don't have to have a Ph.D. in plant physiology to understand that temperature determines the rate of bio-chemical reactions,the relative amount of products created by enzymes within plant cells,and those products' transport and allocation in plant tissues.Temperature also influences the production,interaction and activity of plant hormones.Mayb a Ph.D. would help,but even a novice grower can use advanced temperature control to help grow a better crop.Here is the gist of it.

Plants are generally and obviously seen to be best adapted to temperature regimes in which daytime growing temperatures are,on average,higher than nightime growing temperatures.However,for reasons not fully understood-or at least not fully explained here-the difference in day/night temperature influences stem and internodal elongation.Higher relative night temperatures result in plants with shorter internodes (the distance between branches or nodes).By creating night temperature equal to or higher than day temperatures in the growing area,we can grow shorter,more compact plants without sacrificing flower number or size.This method of temperature control is called DIF.

DIF is not an acronym;it's simply short for "difference."DIF is the difference in average day temperature (ADT);in practice,the "average" is usually dropped and the equation is given as DT/NT.If your growing area's temperature is 76F during the day and 70F at night,then the DIF is +6.The term DIF was originally coined to refer to conditions in which the average day temperature is lower than the average night temperature,producing a negative DIF.For example,if DT= 76F and NT= 85F then the DIF= -9F.

Dr. Royal Heins,a distinguished proffessor of horticulture at Michigan State University,coined teh term DIF.He recognized the phenomenon of plant response to higher night than day temperatures;it was an accidental discovery that arose from and experiment being conducted by one of his students.DIF is not a natural phenomenon-in nature,night temperature seldom exceeds day temperature-and was unknown to horticulture until the 1980's.By manipulating it,we can confuse the natural hormone balance in the plant to influence it's growth.

A partial explanation for the DIF phenomenon is that the syntesis or action of a gibberelic acid,which contributes to cell-wall elongation,is inhibited.Similarly,auxin,anot her plant hormone that influences cell elongation,may also be inhibited.Given that plant hormones seldom work alone,it may be a combination of these or other factors.No one really knows for sure,but we are working on it.Whatever the reason,the effect of the DIF means that the plant will grow more compactly with warmer night temperatures than with cooler-than-day night temperatures.

DIF is useful in negating the effect of plant crowding,which,due to the plants response to the far-red light (heat) emitted by it's neighbors,stimulates stem elongation.The influence of radiant far-red light from non-biological sources also cuase stem elongation and "stretching" under lower light conditions.Under high light and otherwise optimal conditions,DIF is used for reducing plant height and promoting denser flower formations.

Some of the first research with the DIF was done with Easter lillies and poinsettias,bot important horticulture crops.In commercial horticulture,it is important to track height and time blooms in order to meet specific target dates for marketing.
The DIF,and variations of it,are now used by professional plant growers around the globe for crops as diverse as corn,sage,tomatoes,impatients and,occasionally,cannabis.

How much should the DIF be?It depends on the crop.Ester lillies show the greatest effect at a DIF of -15 degrees celsius,poinsettias at -12 degrees celsius,and fuchsia at -20 degrees celcius.For some crops,there are tadeoffs;the greatest effect isn't necessarily the most economic or physiologically desirable.Additonally,the DIF change from positive to zero has a more marked effect than from zero to negative.Commercially,the typical maximum DIF is usually no more than -6 degrees celsius and is typically -2 or-3 degrees celsius,since night heating and cooling raises costs.Further,there is a tradeoff between hight control and flowering time,leaf area and fruit development.Reducing the day temperature to accomadate DIF reduces the growth rate in heat-loving plants,and a high negative DIF has been demonstrated to reduce relative flower number and size in several species.

For cannabis,the best DIF for day/night temperatures is probably zero if you grow in a medium-light garden and maintain day temperatures of 80F - that is,day and night time temperatures should be the same.In a low-light garden,a negative DIF may actually have a detrimental effect,as seen in chrysanthemums.A possitive DIF has traditionally been recommended for indoor cannabis horticulture,and it may be desirable for a low-light garden.But if you have free reign to control temperature and you grow in a medium- or high - light garden,you might try a DIF of -2 degrees celsius or -3 degrees celsius.But given the cost and difficulty of heating and cooling,a high-light growing area-especially one pushing the plants with supplementary Co2 and higher day temperatures-might require and alternative.

There is an alternative to using the DIF as a function of DT and NT.This technique doesn't have an official acronym,but in commercial growing it's referred to as "cold-air dump" *CAD).This is when the temperature in the growroom is lowered after sunrise (lights on) to below the NT for one or two hours.The CAD relies on the plant's response to the change between night and day.Stem elongation has been found to be at it's highest rate at the end of the dark period and the beginning of the light period.Cooling during only the first two hours of the day dramatically stem elongation in poinsettias.This technique is usually the most economocal and is used more frequently in commercial flower production.

In a cannabis growing operation that I am familiar with,a more refined version of CAD is being used with excellent results.This "invisible greenhouse" hydroponics operation uses 85 percent of available sunlight,with a supplemental 400-watt high-pressure sodium light and Co2 fetilization up to 1200 ppm.With the high-light and Co2 fertilization,dat temperatures are run at 85F-90F and the growth rate is outstanding.Because of the high DT,raising the NT to create a negative DIF would be uneconomical and would probably stress the plants.In this operation,the day temperature is extended for several hours after dusk,then gradually diminished to a night temperature of 27 degrees celsius.One hour befor "dawn" (lights on),it is raised to 30 degrees celsius and then lowered to 25 degrees celsius to stimulate a CAD,before returning to the day temperature of 30 degrees celsius.The effect is most notable with sativa cultivars,but indica cultivars also do exceptionally well in this system.Internode length is shortened,and the flowering tops are dense and compact but very large.This is only one of the horticultural techniques used in the operation;however,the effect is noticeable and seperable from other influences in this high-yeild system.

Manipulating the day and night temperatures is a proven method in the world of horticulture,has helped reduce the use of chemical growth regulators,and is used to increase productivity and assist with timing crops to the day and hour.The cold-air dump similarly is coming into it's own as a cropping method.So DT/NT or CAD,what's the DIF?For cannabis,either can be used to improve plant performance,depending on what growing parameters are used.It's just a matter of how you do that funky stuff you do.
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