Day/night temp variation requirement is a MYTH

ItsGrowTime said:

I just grew my biggest crop (2 strains), by far, with average lights-on temperature LOWER than average lights-out temperature. For the entire crop cycle.

A new dehumidifier ran during lights out. Most fans and all AC shuts off for lights out. The dehumidifier actually raised the room temps while lights were out, up to 85f! When the lights and cooling kick on, the temps never break 80.

The established rule is day temps 10 degrees higher than night temps. Of course don't go over 90 if you can help it or under 65. I don't dispute the max/min too much. But I call BULLSHIT on the rest. The plants don't care what the temps are as long as they are within normal range.

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Jnugg said:

Here ya go!



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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