time of day

[ericst11]

does any one know about light regiments like 18 hours of light or 24hours of light and do they really need so much darkness.

[Ross]

When growing under lights, it's not a good idea to leave lights on for 24 hours a day. Plants need darkness to complete their life cycles (like food making). I run mine for 12 hours a day during winter and increase up to 14 hours during summer. Hope this helps.

[ericst11]

I use 12on and 12 off in the winter to but usally put my plants out in the summer but this year there staying in because the pest problem that I brought in from last summer . So is 18 on 6off alright instead of 24 .

[Ocelaris]

There are vigorous debates about whether leaving the lights on or off is required.

The facts surrounding the confusion lies in the "light cycle" vs. "dark cycle" of photosynthesis.

The light cycle is the normal photosynthesis producing Oxygen from CO2.

The dark cycle is when the plant takes in Oxygen and outputs CO2, it's a little misleading, in that it doesn't necessarily require darkness, just that it happens without the presence of light.

I'm not enough of an expert to say that plants NEED a dark period, but the typical argument of plants needing a "rest" is too much of a personification of human requirements to plants to be a good arguement IMO.

If you really want to push it, 18 on, 6 off is probably the max I would do, as I'm not confident they don't need dark, and but regression to a somewhat natural cycle sounds most preferable to me. Though many people do grow plants with the lights on non-stop, I am not sure if orchids would be affected, but certainly some plants do not mind light all the time.

[Royal]

Originally Posted by Ocelaris

The dark cycle is when the plant takes in Oxygen and outputs CO2, it's a little misleading, in that it doesn't necessarily require darkness, just that it happens without the presence of light.

Isn't dark the absence of light?

[Ross]

It's pretty much accepted in the plant world that darkness (absence of light ) completes the exchange plants go through. I don't accept the idea that darkness is unproven. It has been proven over and over as a part of the plant processes. I don't know the science, but I know that a definite light cycle and dark cycle are required. It has been discussed here on this board and should be searchable.

Edit to add cite: Photoperiodism

[Ocelaris]

Photosynthesis

With the high energy products provided by the light cycle, plants then use reactions that do not require light to actually produce carbohydrates. The initial steps in the dark cycle are collectively called the Calvin cycle, named after American chemist Melvin Calvin who along with his coworkers determined the nature of these reactions during the late 1940s and early 1950s.

The Calvin cycle essentially has two stages. In the first part of the cycle, several enzymes act in concert to produce a molecule called glyceraldehyde-3-phosphate (GAP). (See Figure 2). Note in the illustration that this molecule has three carbon atoms. Each of these carbon atoms comes originally from carbon dioxide molecules—so photosynthesis completes the amazing task of manufacturing carbohydrates out of air (the source of the carbon dioxide). This stage of the Calvin cycle is sometimes called carbon fixing. In order to carry out this synthesis of GAP, the Calvin cycle consumes some of the NADPH and ATP that was produced during the light cycle.

The carbon dioxide needed for this step enters through pores in the photosynthetic leaf (called stromata). Plants close these pores during hot, dry times of the day (to prevent water loss) so the details of carbon fixing vary for plants from different climates. In hot climates, where stomata are closed for a higher percentage of time, the trapping of carbon dioxide has to be more efficient than in cooler climates. This biochemical difference in photosynthesis helps explain why plants from one climate do not grow as well in warmer (or cooler) places.

[Ocelaris]

Originally Posted by RoyalOrchids

Isn't dark the absence of light?

Exactly! That was my point, that by calling the calvin cycle "dark" it was a misnomer, called wrong, incorrect, dubiously labeled, misleading etc...

people hear "dark" and they think obviously, dark no light... but that is wrong to think that. It's bypassing the truth which is more complicated than "light makes plants grow, dark lets them sleep" 3rd grade mentality...

But it's like atheism, it's not a rejection of god, it's a lack of requirement for god... the calvin cycle is not a rejection of light, it's a lack of requirement for light...