Day/Night Temperature Differetial

[NormanNugget]

Can anyone tell me whether there’s any benefit to having a cooler Night (lights off) temp? Or is this normally just the case due to the lights?

 

On my first grow throughout the Seedling and Veg stages I maintained a 2 degree difference between Day and Night temps. Through flower I increased this to a 4 degree difference.

 

I’m mainly concerned about the dew point when the temps change.

 

With my new space I seem to be able to maintain temps easily, so I expect I can keep them consistent between Day and Night if that’s preferential.

 

Advice and opinions please!

[green_machine]

In my experience mate you want to keep the differential with the day and night temps.  I use a 4 degree differential, duing veg I run it at the 28 lights on and 24 lights off.

 

What might be more important than differentials, especially if you have advanced control of your environemnt is reducing the temperature in flower. I no longer flower my plants at 28oC. I start reducing the temp from the 1st week of flower dropping it down by a degree every week or two (depending on the strain and flowering time) so that by the last couple of weeks of flower my air temps are around 21oC lights on and 18oC light off.

 

I was going to write about this in my diary as I have been doing this for around a year now and getting amazing results. My weed has never looked or tasted better and I have become convinced that flowering at 28oC is destroying terpenes.

[MindSoup]

Not sure this will answer your question directly, but it's relevant information that kind of bleeds over. 

 

 

I think the big thing to avoid is a sudden drop in temperature at lights off which will coincide with a sharp rise in humidity, that's the bud rot zone from what I'm aware of. 

[stu914]

Ideally day/night temps shouldn't be more than 5c difference between the two.

Mine stays 25c on 23c off  (I don't swallow the bullshit about leds needing higher temps)

[GreenVision]

I'm not sure if there is any specific science to having night time temps drop only a degree or two lower than daytime helps the end result in any way, so I tend to just go with my gut to try and avoid rot, shock etc. But TBH mine drop about 4-5C max every night as is and I don't have any complaints.

[NormanNugget]

Thanks guys, good food for thought. 
 

I mostly wanted to know whether this is something I should actually aim for, or just an effect of having the lights off.

 

Seems like there’s not really a benefit to having a colder night then? 
 

@MindSoup I’ve read through your negative diff thread with interest, but it’s not something I’m trying to achieve. 
 

@green_machine interesting stuff about flower temps, I may try that (and save some pennies on heating). Don’t have any evidence to support keeping your 4c differential?

 

I’m leaning towards just maintaining a consistent temp and humidity between day and night, as it might avoid rot problems. But if this proves had to achieve I’ll revert to a 2-4c difference.

[iShouldCoCo]

6 hours ago, green_machine said:

What might be more important than differentials, especially if you have advanced control of your environemnt is reducing the temperature in flower. I no longer flower my plants at 28oC. I start reducing the temp from the 1st week of flower dropping it down by a degree every week or two (depending on the strain and flowering time) so that by the last couple of weeks of flower my air temps are around 21oC lights on and 18oC light off.

My best results have been from these temps too! I did it originally to bring out purples at finish but then saw something by Bruce Bugbee about 0 dif being good

 

General advice is start high and end low
 

 

Edited Thursday at 12:49 by iShouldCoCo

[ukscroger]

Any idea where the papers he talks about are? I was looking for this today and cannot find much. I want to know how valuable it is to heat the tent at night and how the plant responds to overly cold nights with regard to yield and energy costs to heat the space. 

[growinggold]

@ukscroger I find in my shed. 

 

If i flip 12 /12 at night lights on plants don't stretch as much but eventually don't like it being cool in the night lights on n hot in the day light off  n eventually get nutrient issues but it does reduce stretching. 

 

If I do it the other way round n flip to 12/12 lights on at day time n lights off at night. it's more normal n stretch is lot more. 

 

this is all in shed where the walls are 20mm. i have massive temperatures differences as its not central heated house sat 20c. 

 

you can mix both to. you can do 12/12 lights on a night, keep stretch low n then swap to lights more normally in the day. juts make sure you given the extra long darker period. 

[green_machine]

7 hours ago, NormanNugget said:

@green_machine interesting stuff about flower temps, I may try that (and save some pennies on heating). Don’t have any evidence to support keeping your 4c differential?

 

I’m leaning towards just maintaining a consistent temp and humidity between day and night, as it might avoid rot problems. But if this proves had to achieve I’ll revert to a 2-4c difference.

 

 

Hey Mate I have a bit of further information I can add if you want including some evidence. Let me just add as a caveat that my evidence comes from commercial tomato crops grown in greenhouses, not cannabis but the principle is just the same.

 

I couldn't remember the first paper I read this in, it would have been around '96 when I first started growing. I do remember it talked about temperature differentials in tomato crops in greenhouses so I did a quick Google search using these terms and loads of papers came out supporting lower night time temp than day time temps. In short higher night time temps lead to lower fruit production and by reducing night time temps to 21oC fruit numbers increased by as much as 39% while fruit weights increased by as much as 53%. There is a great paper that explains this, ' The effect of night time temperature on greenhouse-grown tomato yields in warm climates' but it is behind a paywall and you will need to pay to access it.

 

I watched the Bugbee video that @iShouldCoCo posted on differentials and he states that both positive differential and 0 dif have similar results so with this in mind why would anyone want to spend all that extra money on energy and heat when they could drop them and get similar results.

 

If you are worried about bud rot I again re-iterate that the solution is not to keep temps the same through the grow but to drop the temps as you proceed into flower. If you are following a VPD chart as you drop your temperature your RH will also be falling. As I said I now finish my plants off at 21oC and my RH at this temp is only 48% but I am still driving my plants as hard as possible and maintaining a roughly 1.5KPa VPD. At 48% RH my chances of developing bud rot are much lower than maintaining the 28oC and 60%RH I used to.

 

 

 

 

Edited Thursday at 20:06 by green_machine
spelling

[Gsyrup]

7 hours ago, NormanNugget said:

it’s not something I’m trying to achieve

 

It doesn't work anyway

 

11 hours ago, stu914 said:

Mine stays 25c on 23c off  (I don't swallow the bullshit about leds needing higher temps)

 

Come on mate dont be like that - its not bullshit,  just because youre getting a  result with your fixture better than you did under HID lighting doesn't mean you're getting the best result from your fixture. If you're happy with it like, then thats cool, but you're not getting the best out of your system, plants or light.

 

14 minutes ago, green_machine said:

my evidence comes from commercial tomato crops grown in greenhouses, not cannabis but the principle is just the same.

 

Studies into the effect on tomato fruit are totally useless when it comes to cannabis cultivation because a cannabis plant doesn't grow fruit at all. You'd be better off looking at Spinach, or Quinoa, perhaps.

 

22 minutes ago, green_machine said:

As I said I now finish my plants off at 21oC and my RH at this temp is only 48% but I am still driving my plants as hard as possible

 

you've just reduced the rate of transpiration (cool temps) but increased the amount lost through transpiration. I appreciate my position is probably against the grain, though - check out a book called Growing by Plant Empowerment.

[green_machine]

39 minutes ago, Gsyrup said:

 

Studies into the effect on tomato fruit are totally useless when it comes to cannabis cultivation because a cannabis plant doesn't grow fruit at all. You'd be better off looking at Spinach, or Quinoa, perhaps.

 

 

 

I am not sure that spinach or quinoa are closer to cannabis, I believe the most similar relative would be hops and hops are certainly a fruiting plant. I agree that tomatoes may not be the perfect example (that is why I added the caveat) but I do think there is some truth to having a positive dif when growing any plant be it fruiting or not.

 

I am aware of the work of the team at Wageningen University and I generally believe in the principles behind plant empowerment especially when it comes to trying to maximise the openings of the stomata and micropores in the plant, I do think there are some limitations to bringing their work into the home environment, but I am not sure if this is the place to discuss that.

 

I wonder if maybe I have a misunderstanding of VPD. I thought that stomatal regulation is directly responsible for controlling leaf-level transpiration in response to VPD. When stomata are open leaf level transpiration rises in a linear fashion in relations to VPD up to round 2KPa after which transpiration starts to drop. By setting my conditions (Temp and RH) to maintain around 1.5KPa ( this varies from lows of around 1.4 - to highs of around 1.6 during lights on) I am ensuring my stomata are always open and this is driving my plant growth during lights on (provided other factors water, nutrients etc... are available). In terms of VPD during lights off, for brevity, I don't think I was clear enough in my last post, but my lights off VPD has always been lower than my lights on closer to the 1.2KPa level so I don't think I should be a stress on the plant.

 

Am I misunderstanding this @Gsyrup  ?

[Gsyrup]

12 minutes ago, green_machine said:

Am I misunderstanding this @Gsyrup  ?

 

I think you're missing the key component. Light is the driving force of transpiration, VPD is just a tool to calculate amount of water lost through transpiration. As per what we're discussing, temperature effects rate of transpiration via metabolism, with photosynthesis (and rate of transpiration) increasing between 20 - 30c, assuming we're at a static PPFD. The cooler it is, the slower they are to photosynthesize, and if its too hot photosynthesis stops, to stop transpiration.

 

We're keeping the stomata fully open, and LST in the most optimum range (28c), so as to not impede gas exchange... to use the light!

 

 

e2a - to bring this around to the actual topic of the thread, any differential up to -5c is fine but the closer together the better, and only because it takes less time for your plants to start metabolising.

 

 

 

 

Edited Thursday at 22:08 by Gsyrup

[green_machine]

Shane from Migro recently internviewed Mitch Westmoreland, a graduate research assistant at Bruce Bugbee's labs. I would highly recommend everyone watch it as it discusses crop steering of cannabis and how various factors can control this. 

 

One of the things they discuss is temperature. The most recent research from their lab (still unpublished) suggests that higher temps are benficial during veg to help the formation of the canopy, however, once the canopy is fully formed and absorbing all the light possible, the photosynthetic rate was pretty similar regardless of the temperature ( there are of course limits to this and there does seem to be strain dependence as well).

 

Their most recent advice is to start the plants at high temp and RH to get the canopy filled as quicly as possible, this does not have to be done through veg growth it can be accomplished by just using more plants. Once the canopy is full drop temps down to 25 for the start of flowering and then towards the end of flowering drop the temps down further to 21 to maximise potency and terpenes.

 

One thing that was very interesting to learn was that cannabis buds have fewer stomata than the leaves so they have a much harder time regulating the temperature. This means that buds are several degrees higer than the durrounding air. If you are flowering your plants at 28 then at late flower your bud temp is probably hitting 30+oC and these temps are too high for terpene preservation.

[Gsyrup]

18 minutes ago, green_machine said:

If you are flowering your plants at 28 then at late flower your bud temp is probably hitting 30+oC and these temps are too high for terpene preservation.

 

If the heat source is top down in a room with inadaquate air movement and wind direction, maybe.  Whole temp is ideally the same top to bottom, sinks and sources, that is quite hard to achieve in confined spaces though, I'm not going to pretend I've got it right 100% of the time. As for secondary metabolites, (its in the name really) are you preserving because of reducing metabolism vs am I producing more for longer due to increasing metabolism? As I see it, preserving secondary metabolites is for when metabolism has stopped and ideally thats when they're hung up. Im not saying I am right or wrong, I just know if I tried to do what you're doing in my situation it'd all go tits up.