How important is 100% darkness at night really?

Hey all,

I came to a bit of conclusion with how important (or not) 100% darkness is in photo periods and id love to hear what everyone elses experiences are.

Outdoor plants flower regardless of street lights or the changing moon light shining on the plant. I’ve seen lots of debates on why this happens, such as different light spectrums from the moon or street lights being too dim to have an effect (if far enough away i guess)

Indoor growers and articles often say it needs to be 100% darkness, such things like ‘a small led on a power lead or a fan can cause a plant to hermie, reveg etc.’

My experiences seem to tell me: its important the light at night - needs to remain very similar to what the plant is used to from the moment it is born - so if there is weak street lights, moonlight, power leds or small light leaks in your grow space then its okay… as long as its remains similar to what the plant knows as “night time”… No sudden turning the lights on at night or shining a torch on top of leaves kinda thing or outdoors avoid bright streetlights nearby and any faulty/flickering lights and seems you should be okay.

But i love the debate this topic raises, there’s people that agree and some fully reject the idea.

Whats your experiences? :).


I live right next to a major retail store and their lighted sign definitely fucks up my outdoor.


good way to make seeds if you like hermie seeds

I think the need for 100% pitch black uninterrupted darkness is a bit overstated.

Keep it well under moonlight level, and should be no problem.

I take flash photos of my dark cycle plants. I probably have pinhole light leaks. Sometimes I even unzip the tents and water and shit at night if it’s more convenient for me. Never been a problem, I really have never had a problem with herms. A few plants here and there will occasionally throw a late flower nanner or two but that’s just genetics. Chems and sour are prone to it, landrace stuff for sure, etc. I don’t even think I’ve ever had a plant full-on herm on me.


Is the sign always on or
does it only come on when it gets dark enough by a store employee? (varying times maybe)
or its already dark enough for the plant when the sign is switched on? (same as a torch shined into a indoor when the lights are off)?

Im interested in looking further into this debate if you could share more info

It is on all the time. The parking lot lights come on when it gets dark.

The plants don’t herm, they get stuck somewhere in between veg and flower. Like leafy airy buds that never fully form.

Granted I’ve only done this once and plan to use auto flowers next year to hopefully avoid these issues.


Gotta remember the moon lets off light, if its not enough light for growth than ur ok, I had a home mad micro flower tent for clone flowering and random bean trials, made out of cardboard with a taped dlap to access the inside… it depends on the intensity and period of light exposure.


What I have found over the years…one of my plants simply does not like being disturbed. If you do, it will herm on me. And doesn’t need to be light either…anything to piss it off. My other one, doesn’t care.
I think it comes down to genetics on how finicky they will be.


Ime plants indoors are more susceptible to hermie do to several reasons - genetics, intensity of the sun, and root space being a few.


It’s different for everyone. We all assume that light stress will cause hermies much easier then other stress. But its just not so. Ive had a plant that hermed before due to poor temps in flowering but the clones passed the strobe light test and have never hermed since no matter what you do.


Nature can be far from perfectly dark …


Inconsistent light is no problem (such as from a headlamp or camera flash), it’s when you have a consistent light leak that causes a build-up of the photochrome levels in the plant that will lead to herms.

The reason the moon doesn’t affect plants is the lunar cycle. The phytochromes only react to the light around a full moon but the the rest of the cycle allows these to return back to normal before it repeats.

Indoor plants are much more susceptible because we grow in tightly controlled environments where the plant doesn’t have to deal with the stresses of a moon cycle, so any additional light can drastically throw the photoreceptors off.


Thanks, that’s an interesting point of view, it makes me wonder if reg Autos doesn’t hermie at all as they are independent of light cycles … icon_e_confused|nullxnull

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Thanks all, certainly some great comments on both sides of the arguement so far.

So the plants dont ‘count the hours of darkness between the lights being switched on again’ ? I thought a photo flash at night would be enough to wake the plant up during night time…
or is it just about avoiding absorbing too much longterm, from any source, even dim lights during night time?

My noob theory is that certain vigourous traits make them more sensitive to herm. Like the wrong amount of light/PAR at the right time makes it think it missed its pollenation window and want to throw off seeds to preserve genetics. High urge to self preservate lol.

Imho the power strips with a red led on them can cause issues. Maybe it’s the red causes issues.


I agree there, i have a power lead with bright blue leds illuminating each on/off switches and a green power surge indicator, no red leds… And i didnt even realise until week 4 of flower and the plants shown no issues…


My stuff outside is by a streetlight, hps, probably 20 yards away. It shines on my plants all night a mild glow. They are fine.

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There is something to the “indoor” theory. I have two plants, the same clones. One in our greenhouse and is constantly having the outdoor light (400w MH) on and off of it. No issues. The very same plant inside, will Hermie with the slightest change or interruption of it’s darkness hours.


Is the outdoor light close at all? I wonder if its the intensity or the spectrum or ?@MysteryMoog In nature you can have very well lit nites and then there’s lightning storms…makes me wonder

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