How To Change Light Cycles In Your Home Garden

Light cycles for indoor gardens are always a concern the first time during an initial grow. But once you get the basics down, it’s easy.
How To Change Light Cycles In Your Home Garden

Wondering how to change light cycles in your home garden? High Times Cultivation Editor Nico Escondido answers all of your grow questions in his weekly Nico’s Nuggets column.

The Question: How Do You Change Light Cycles From The Veg Phase To Flowering?

Dear Nico,

Thank you and High Times so much for staying dedicated to the true cause of the movement. Teaching the world to grow is the best way to mitigate the unrealistic laws of marijuana prohibition.

That said, we are about to start our first indoor garden! (We usually grow outdoor, but winter is coming…)  We want to be sure we have everything covered before we start. We have both MH and HPS bulbs to use with a switchable ballast and want to ask about how exactly (and when) to change light cycles from the vegetative phase to the flower stage?

We want to induce flowering at the right time for optimal harvest and peak potency. Thanks to you and High Times for years of great work!

— Scott & Kelly M.

The Answer: Learning The Basics Of Light Cycles

How To Change Light Cycles In Your Home Garden
These ladies were just switched from 18-6 to 12-12 in order to induce flowering.

Greetings Scott & Kelly,

Thanks for the kind words and for writing in with a question. Light cycles for indoor gardens are always a concern the first time during an initial grow. But once you get the basics down, it will be an afterthought moving forward.

To start, the first consideration will be just how big you want the final size of your plants to be. Working backward, the final size of your plants at harvest will largely be determined by how long you keep your plants in their vegetative phase.

How long you decide to “veg” your plants will also determine when you change the light cycle to induce flowering.

Most indoor home growers have limited space, so a two- to four-week veg is generally the standard.

However, if plants are put into flower production too early, they will not have enough energy reserves (stored in the root structure, which will be too small to store large amounts of energy) to produce big yields. That is why growers who utilize a short two-week veg period are usually growing a large volume of smaller plants in a sea-of-green (SOG) format. (The large volume of plants makes up for the smaller yields per plant.)

Most indoor growers do not have space to grow dozens upon dozens of plants, so many opt for fewer plants with a longer veg time and, thus, a larger final plant size for larger yields at harvest.

It is important to remember that once the light cycle is switched for flowering, plants still have another seven to 10 weeks of growth and will double in size—if not a bit more.

So if, for example, you want to have a final plant size of 3’ at harvest, then put your plants into the flower phase once they reach 15” – 18”.

This is an important aspect to consider, especially if you are using a grow tent or a space with limited height.

Also, remember that indoor lamps hang directly above garden canopies and further cut down on over-head space for plants to grow. (Note: Grow lamps should hang a minimum of 18” above plants and will need to be adjusted periodically as plant height increases.)

Once you decide on your vegetative timetable, it is time to think about how you will switch the light cycle.

How To Change Light Cycles In Your Home Garden
This plant has been in flower for two weeks and displays young, white flowers.

Most growers use an 18-hours on/6-hours off light cycle for the veg phase. Some prefer to use 20 on/4 off, which is also acceptable. However, stay away from 24 hours straight light during veg, as this does not allow any downtime for the plant, not to mention the roots grow and develop the most during the dark cycle when the rest of the plant “sleeps.”

At the end of your vegetative phase, growers then change the light cycle to an even 12-hours on/12-hours off.

Cannabis plants rely on the photoperiod, or amount of daylight received, for their internal biorhythm to determine when to begin flowering. Receiving 12 hours (or less) of light will trigger the flowering phase.

To make the change from 18/6 or 20/4 to the required 12/12 light cycle, growers adjust the timers that their lights are on.

To do this with minimal shock to your plants, simply extend the time of the lights-off cycle to 12 hours at the start of the dark cycle, making sure the lights-on cycle is also set to 12 hours. Check the growroom at the start and end of each 12-hour cycle for the first two days to make sure the timers are set correctly. After that, it is just a matter of time until harvest.

  1. Hello, I am a 64 year old novice grower. Currently I have 2 strains growing, an autoflower (critical purple, and White widow which is not an autoflower. After how many weeks should I adjust the light cycle on the White Widow? They are currently one month old and look healthy and lush.
    The Critical Purple has already begun to flowers.

    1. Dennis – This decision is really yours to make based on various factors, including the amount of space that you have in your grow area. To me that is most likely to be the determining factor for you. Your auto will flip soon on its own and you could decide to flip your lights to 12/12 at that point, or you could keep it at 18/6 till you harvest your auto and then change to 12/12. If you have a limited amount of headspace in your grow area, you will want to consider the fact that plants will almost double in height once they flower. Make sure that you do not paint yourself into a corner by letting the widow get too tall in veg. I hope this helps. There is also a really nice guide to changing to flower over at

  2. Hello., my name is Dave. I start my flowering stage normally, but after 1st week for some reason I change my light sickle, for example it works before 4pm-4am, now I change to 10pm-10am. My question is does it harm my flowers or not. Thank you.

  3. In addition to being satisfying, easy to do, and delicious, it is also critical to growing sustainable vegetables, fruits, and herbs. It can be an essential part of a self-sufficient lifestyle to be able to raise some of the food you consume. It is imperative to plant intensively and according to the season in order to grow a successful sustainable garden.

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