green75- 10* degrees sounds about right, i would suggest trying to lower it some tho 6-8* degree would be most effective as 10* could cause a shock, or even any fungus could grow. how is your humidity holding with that swing i noticed when i drop 10* at night it causes the humidity to rise in the 60% range, with a 6-8* i noticed only a 4-8% increase from the 48-52% with my lights on.
per site
http://www.growweedeasy.com/temperature 10* is best for day/night swing
also The temp swings are good for the plant IMHO it makes them push more sugars/tricombs/oils
also found this online that is by a dutch seed company.
The effect of temperature fluctuations can be disastrous to plants for a number of reasons. When the temperatures are very low for example, the plant grows at a slower rate because it is not able to photosynthesise as fast as it would in normal temperatures. Photosynthesis refers to the process by which plants use light to convert the nutrients that they absorb from the soil and water into soluble foods used up by the other plant processes that lead to its growth. If the temperature fluctuates once in a while, it will not produce a visible difference in the overall growth of the plant because it will adjust. However, if very low temperatures persist or go uncorrected as far as indoor setups are concerned, the cannabis will completely stop growing.
Growth is stopped because the plants tissues are damaged. Once this happens, it is very hard to restore them and the parts that are affected may not be recovered again depending on the extent of damage. Where the temperature drop is a result of no lighting, the cannabis does not photosynthesise firstly because they don’t do so in the dark but also because they are trying to fix the damaged tissues.
When the temperatures become very high on the other hand, the effect is almost the same as when it is too low. The growth rate of the plant slows down at first and if the temperatures are not corrected in time the plants stops growing completely. Older marijuana plants are usually the worst affected because they are vulnerable due to their reduced vigour as opposed to younger and stronger first time plants. This includes the category of regenerated or re-vegetated plants that have been harvested severally.
In severe heat, plants will lose excessive amounts of the water they normally transpire causing dehydration and can lead to their death. Plants that are grown outdoors have a slight advantage in cases of high temperatures because outside, the plants are blown by natural winds which assist them to cool down. The cool air removes the excess humidity that is caused when the transpired moisture heats up on the leaf surface. In indoor setups, growers would be advised to set up oscillating fans in their grow rooms as well as vents to let out hot air and waste gases to keep the plants cool.
Temperatures should never be allowed to go to the extremes which are beyond 40 degrees Celsius or below 20 degrees Celsius if the plants are to have good chances of surviving. Measures should be put in place to either automatically control the temperature in indoor setups or to shade plants from direct sunlight or extreme cold by moving plants into a greenhouse.
