Do seedlings need dark periods?

It is common for people top do 24 hours of light per day in this stage to promote growth. Most then switch to 18 hours light 6 dark for vegitation. Then 12 12 for flowering. I am no expert. But I suggest to promote a good veg to follow that cycle, also nutes and watering are just a vital part. Keep us posted.

Any one else before i go to bed? I'm tired and need to know if my plants need darkness.

24 hours of light is fine, so isn't 18 hours of light. Personally, I like 18/6. Darkness is not ABSOLUTELY needed, but your power bill will appreciate it.

Hey I also would like to know this, I have 6 day old seedlings. I am still working out the kinks on my grow box but am still wondering when to switch to 18/6.

Keep them on 24 hour once the seedlings pop out of the soil. I dont see any reason to have a dark period, unless you want to save on electricity, but IMO its not that big of a difference.

multiple people have discussed this and plants actually grow bigger and yield more when given a dark period. i went 18/6 from seed, and my plant did great in those 6 hours of darkness.

Originally Posted by hyperan

multiple people have discussed this and plants actually grow bigger and yield more when given a dark period. i went 18/6 from seed, and my plant did great in those 6 hours of darkness.

Nope. Totally wrong. The evidence supports the contrary.

Most green plants are classified as either C3 or C4 which represents how carbon(C) is used during photosynthesis.


C4 plants temporarily store carbon dioxide(CO2) over the dark period to use for photosynthesis during the day. C4 plants slow down photosynthesis once the stored CO2 is used up and they need to gather it from the air. Which is why trees slow down photosynthesis in the afternoon even though the sun is still bright. This does NOT apply to cannabis.


C3 plants(cannabis/veggies) gather CO2 only during the light period when they are photosynthesizing. During the dark period these plants only use oxygen for their metabolic life processes. They don't uptake CO2, nor do they use it. As soon and as long as the light is on, C3 plants gather and use CO2 for photosynthesis.


C3 plants also have the ability to use higher concentrations of CO2 than what is found in the air. If the light is bright enough and the plants have sufficient nutes, their growth rate will accelerate from it(2000ppm vs. 400ppm of CO2), which increases yield. They can do this continuously, without a dark period throughout the vegetative stage.


The dark reaction is a process of photosynthesis that takes place in both darkness and light. It uses ATP and NADPH molecules that hold energy absorbed from light to break apart CO2 into it's base components. Because it's called a dark reaction and can occur in the dark, some people have said darkness is needed for this to occur. This Is Not So.


Again people get a little obsessed with their plants needs & think of them the same as humans. People need rest, so plants must too. This is false as well. Light means growth. Scientifically. Although 18/6 will 'shock' your plants less when you switch to 12/12, it's a personal choice whether you would rather sacrifice a little growth for a quicker adjustment or less photo confusion. If you want to save money or energy that's a personal choice too. Do what you need to do to make your growing situation work.

Don't worry about the dark cycle during veg/seedling stage. As long as they get 12-24 hours of light per day they'll be fine. If you want you can have a dark period every few days. I turn my light off in my veg room when I remember. It's usually on 24/7.