Flowering Light Cycle Interrupted- Here’s How to Deal with It


Flowering grows plants is the trickiest aspect of the whole cultivation. And in case you’ve messed up lighting during the process, it’s something you need to worry about.

Now, as you’ve landed on this page looking for solutions of flowering light cycle interrupted, calm down and keep reading. We’ve got some quick solutions that will be in rescue.

If you’re in a hurry, let’s give you a quick solution-

If the interruption is sudden and not more than 4 hours, bring the system back to regular schedule ASAP. If it’s 4+ hours of extra lights, that might recall the vegetative stage. Spot the hermies and remove them quickly. On the other hand, if it’s 4+ hours of extra darkness, don’t worry and bring it back to regular schedule.

Done with that? Well, don’t be yet. Because we’ve written down a 2000 words post to explain the whole reasons, risks, and remedies of such interruptions.

Let’s start with learning the ideal flowering light cycles-

The ‘Actual’ Flowering Light Cycle

Before we opt into problems and solutions of light cycle interruption during flowering, let’s learn what the ‘actual’ flowering light cycle is. That might help out to measure how many interruptions you’ve done to the process8.

Grow lights need a precise variation of grow light wattage and the light hours. If we divide flowering into a number of sub-stages, here they go-

Transition> First Buds> Growth of Buds> Buds Ripen

Now here is the best light cycle for flowering at different stages-

Stage 1- Transition Period: 12-10/12-14 Hours of L/D

This is the time when plants transits from vegetative to flowering Stage and the light cycle should be 12-14 hours each day.  

On a broad sense, it’s the first 1-2 weeks of flowering stage while plants enters into flowing from vegetative. The plants are still growing and so, the light cycle won’t differ much from what it was during the vegetative cycle.

Stage 2- Spotting the First Buds: 10-8/14-16 Hours of L/D

This is what we call ‘First Buds’ and this is the time while you’ll get to see the first flower growing up(only for female plants). The light/dark cycle should be around 10-8 hours and 14-16 hours respectively.

It takes the whole 3rd and 4th week to complete the process and you’ll see real buds instead of hairy pistils. The growth will slow down, and therefore, you’ve slowed down the amount of light and increase the amount of darkness.  

Stage 3- Growth of Buds: 6/18 Hours of L/D

Now, this is the stage which we call the real ‘flowering’. The buds will be found growing up and maturing over time, and the ideal light/dark cycle would be 6 hours of light and 18 hours of darkness.

Why? Well, Flowers grow up because of the build-up of Florigen(A flower hormone). And upon many scientific experiments, it’s proved to build up when it’s dark.

Stage 4- Buds Ripen: 6-0/18-24 Hours of L/D

At this last stage, almost everything about flowering is done. So, no need to change the light/darkness cycle so much. You can keep the lights for 6 hours and the darkness for 18 hours.

Based on situations, some growers decide to provide an uninterrupted 24-48 hours of darkness to induce the riping process.

Why Do Light Cycles Get Interrupted?

First thing first, light cycles don’t get interrupted themselves. There are some intentional or mistakenly done changes behind that.

Who do we say it’s ‘Intentional’? Well, it’s a basic indoor growing 101 that some grower trigger to induce quick flowering or stay in vegetative stage by manipulating the light cycles. And done properly, that seems to be no problem at all.

What we’re concerned about is the unintentional and accidental cases of light cycle interruptions, particularly in the flowering stage.

Here are some of the reasons we’ve listed behind extra light during flowering

Case 1: Long Interruptions(4 hours or more)

  • You forgot to put on/off the lights.
  • The dial-up timer didn’t function properly.
  • There is an unexpected leak through the vent or around a door frame.

Case 2: Quick Interruptions(Few minutes to 4 hours)

  • Light burned out and you didn’t notice.
  • Electricity interruptions due to hazards like winter storms or so.
  • Sudden interruptions in the light cycle for a while.

As you can see in the list, we’ve sorted them out into two kinds of incidents. For obvious reason, the impacts and process of recovery are different in both of the cases. We will take you through that shortly.

Light Cycle Is interrupted During Flowering: Issues and Remedies

A light cycle interruption during flowering stage brings on more impacts on the growth and yield that what it does during the vegetative stage. The reason behind is, plants respond more to light changes during flowering.

Here are the key impacts and their immediate solutions when light/darkness cycles are interrupted/disrupted-

Plants Going Back to Vegetative Stage(Hermie)

If the light cycle stays for longer than what it should, we call it a disruption of the dark cycles. And if that continues for quite a few hours, you will see your plants going back to the vegetative stage from flowering. Because too much light during flowering induces the plants to re-vegetate.

However, if your flowers are nearly matured, this won’t happen anyway.

Reason: You may have left the lights on accidentally for quite some time. As result, plants couldn’t get enough darkness to induce the production of Florigen(a flowering hormone). And that eventually reverses the process of flowering and recalls vegetative stage back.

Buds Maturing Faster

Sometimes, the darkness lasts for longer than the schedule. In case the lights are off for a day or two, you will see the flowers grow faster. Although it seems to be good for a grower, nothing should be pushed out of the natural process.

Reason: Darkness is the obvious reason behind the flower’s growth. Because darkness induces the production of Florigene and eventually helps the flower to mature.

Scrawny Buds and Hermaphrodites(Gender Changes)

Sometimes, things get so out of our control that both of the dark and light cycle get messed up and causes a hammering effect on the plants. If lights are turned on and off immediately, this will result into scrawny buds and probably become hermaphrodites.

Reason: The obvious effect of sudden light/dark cycle interruption is plants developing both male and female flowers. And it’s not only about changes in light/darkness cycle. Over-heat and thermal stress is another side effect of delayed light cycles that induce hermaphrodites.

Pro Tips to Deal With Flowering Light Cycle Interruption

Now that, you have a messed up lighting situation in your hands, you have to deal with it anyways. We’ve listed down bot of some quick and some permanent solutions that might rescue you and your plants.

Let’s start with the 1st scenarios when you have an elongated light cycle during the flowering period-

Scenario 1: Extra Hours of Light in Flowering Cycle

While it’s flowering, plants need to be put in sleep at a correct time. And turning off the light does the job. In case you’ve got some extra hours of light, act according to the list below-

Correct The Regular Light Cycle Immediately

As soon as you noticed the light hours, set back it to the normal schedule. As an example- If the light’s supposed to be turned on for 14 hours till 12 am, and it gets till an accidental 4 am, start the dark hours immediately.

Meanwhile, take care of the unexpected rise in grow temperature and other impacts of these extra hours of light. 

Kill and Dry Anything That Looks Hermie

If you’re in a situation of more than 6 hours of extra lights, there might be hermies growing up in some of the plants. Go on a ball pick mission and carefully remove anything that looks like balls(re-vegetating).

Scenario 2: Extra Hours of Darkness in Flowering Cycle

The second case of light interruption is- extra dark hours. As we’ve said before, these will boost up the process of flowering and you might notice premature flowers. Here is what to do-

Turn on The Lights Again

The first thing you need to do is to turn on the lights as soon as you notice the extra dark hours. You can start over the regular light/dark cycle schedule again and continue till the flowers are fully matured.

Scenario 3: Too Much Interruption During Both Cycles

The last case we have in hand is a messed up light/dark cycles. What I mean is long hours of irregular interruption in both of the cycles. These will end up in scrawny buds and hermaphrodites.

In case you’re in such a mess, do the followings-

Remove Any Plant That Exhibits Hermaphroditism

First thing first, you need to spot any signs that exhibit hermaphrodism in the plants. These are eventually caused male flowers in other female plants and destroy the whole yield.

How To Avoid Flowering Light Cycle Interruptions

As you’re done with the current mess of light cycle interruption, you never ever won’t like it to happen again. In fact, no one would.

Take these actions and seal the chances of happening this over again-

Spot and Seal any Kind of Leaks

In case of indoor cultivating, there is a good chance to have unintentional leaks through which light can enter into the room and interrupt the dark hours. To check for these leaks, close yourself inside the room and keep the lights turned off.

Take a few minutes to adjust yourself to the darkness. Now look at your palm and check whether you can see it or not. If you can find the palm in your vision, then there is some kind of leak in the whole space. Find and seal them with a sealant.

Use A Dual Grow Light Timer

Century 24 Hour Plug-in Mechanical Timer Grounded

As you know, growers need to set different schedules of light and darkness throughout the whole cultivation. To prevent light/darkness interruptions, the most familiar kit is the dual grow light timer. In these days, these timers will let you adjust the lighting time with an interval of 15 minutes. That helps the growers to deal with the lights more precisely.

Our recommendations:

Use An Environmental Controller

Want to avoid the mess of multiple timers and controllers around the whole grow room? You may try a good quality environment controller where you can deal with all of light, humidity and temperature control. 

Our recommendation:

FAQ(Frequently Asked Questions)

How long should you leave grow lights on?

Depends on the stage plants are in, and the type of plants. For example, at the flowering stage, cannabis plants need you to leave the grow lights on for 10-12 hours a day.

Can plants have 24 hours light?

Yes, you can. Specially during the vegetative stage. Because more presence of lights induced better vegetating. But during the flowering stage, that might be a problem as it re-vegetates.

Do plants grow faster in light or dark?

Growth takes place during the vegetative stage of their life cycle. And light induces the growth meanwhile. So yeah, plants grow faster in the light.

How much light will stop flowering?

Maybe 24 hours or more of light hours will stop the flowering of plants and bring it back to re-vegetate. Although, it depends on which level of the flowering phase the plants are in.

  1. It’s really a cool and helpful piece of information. I am happy that you justshared this useful info with us. Please stay us informed likethis. Thank you for sharing.

  2. Heya Saleh, thank you very much for the article here. May I ask where you came across this info? Got any source or papers I can read? I like the technical details ^_^

    • Hey man! Thanks for the appreciation. Means a lot.
      Well, I had to go through hectic hours of research and analysis+ a few practical lessons of mine. Can’t recall any source right now. But in case you have any more queries, I’m here to answer.

      • Lights are on from 6pm to 6 am…. for one day it turned on at 9am accidentally and manually turned off at 545 pm. Turned back on at 6 pm and set it back to 6pm to 6 am…. what could happen?

        5 weeks into flowering buds are nicely formed

  3. Hi. This year I decided to try an experiment. I have 2 potted plants that were grown outdoors and both are healthy and strong.

    To trigger flowering 2 weeks ago I started being them inside and putting them in a 100% dark storage cupboard. I was giving them 10 hrs outside (7.30am-5.30pm) every day, then of course they are receiving 14hrs of darkness.

    I was pretty surprised how quickly they began flowering. Pistols appeared after 3 nights in the cupboard. Now 2 weeks on they are flowering nicely. Today when I put them outside I notice one had hermi pollen sacks on 4 small locations. I picked those off and inspected the plant very closely for more. I think I got them all. I’m hoping it’s the severe heat that has encouraged the pollen.

    On Sunday I have to travel interstate to a dying relative. I will return late Wednesday night. This is going to give me 4 nights where I am unable to move in or out. I don’t have anyone to continue my program of moving in or out.

    I have 2 options – 4 nights of total darkness, or leave them outside whilst I’m gone (currently still receiving approximately 13 hrs of sunlight outside).

    I’ve come to terms with the fact I’ve probably lost a lot of yield already, but can anyone help me ??

    Total dark or back outside????

  4. My plants have been in flowering stage for over 3 weeks and I went up this morning and one light has been on fo 4 days well they where in the dark so the dark stage has been Interrupted for 4 days what should I do please help there are 2 to 3 in h buds on my plants

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