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Lighting is crucial when you are growing cannabis. Whether you are growing outdoors or indoors the amount of light your plant receives will affect how it grows. When growing cannabis outdoors the light cycle will happen naturally as the seasons change but for indoor growers there are many different light cycles you can use for growing cannabis.

 

What Are Light Cycles?

The light cycle is the number of light hours and dark hours the plant receives in a 24-hour period. 

In the outdoors a cannabis plant will start to grow in Spring, flower during Summer and be ready to harvest in the Autumn. The amount of light it receives will differ depending on what season it is in.

When growing indoors you will need to replace the sun by using a grow light. You will then need to imitate the length of the days for Springtime, or summertime. This will help the plant know when to 'veg" and when to "flower".

 

Photoperiod Cannabis Plants

Photoperiod cannabis plants are plants that react to light cycles. These plants rely on a change in the light to start flowering. For example, plants that are vegetating under the 18/6 cycle will not flower until the light cycle is changed to a 12/12 cycle.

Cannabis Light Cycle for Outdoor Growing

When plants are grown outdoors, mother nature takes its course. During the year, the seasons change from Spring, into Summer and then Autumn. As these seasons change the days get longer and then shorter. A plant can detect the amount of light it is receiving, and this tells them what season they are in.

 

Spring Equinox

Towards the end of September in the Southern Hemisphere, we start to head into spring. The days will begin to get longer and the nights shorter. 

During this time growers will begin germinating their seeds to begin growing outdoors. As the days get longer, the plant will continue to grow more and more in its vegetative stage.

 

Summer Solstice

In summer, days are at their longest. Up till now the plants have been in the vegetative stage developing strong stems and lots of leafy green foliage. In midsummer the plant will start preparing for flowering.

 

Autumn Equinox

Autumn begins from around 20 March. Flowering naturally occurs when the plant receives less than 12 hours of light a day as the summer days shorten. In nature, the lengthening dark periods which occur in the late summer and early autumn triggers the cannabis flowering cycle. The days are usually 12 hours long, and the night 12 hours long as well. By this time small flowers should be starting to form. 

Autumn time is harvest time! From 15 March to 30 June is when most growers will be harvesting their cannabis.

Sativa dominant strains will normally be in flowering for longer than indica dominant strains. Different strains may also vary in their harvest time.

 

Winter Solstice

By wintertime, your plants should have already been harvested. In winter, the nights are much longer than the days. In nature, the cannabis plant would not be able to survive the cold temperatures in the height of winter.

In cannabis plants a change in leaf colour from green to purple is commonly a sign of cold temperatures.

As the climate starts warming up again and Spring is on its way, the grow season starts again and you will be able to repeat the grow cycle. 

When germinating your seeds for the next outdoor season, it can be helpful to keep them inside until all forms of danger like frost have passed. Any sudden drops in temperature while moving from winter into spring, could kill your newly germinated seedling. Once any risks have passed, your plant will be able to continue its vegetative stage outdoors from late spring into summer.

 

Outdoor Grow Calendar - Southern Hemisphere

 

  

   

   

   

   

   

   

   

   

   

 

   

   

Light Cycles for Indoor Growing

In indoor growing you become the sun and the moon to your plants. By using light cycles, you will be able to imitate the length of the day and night to prompt your plant in certain growth stages. This is not only done with the length of day and night but also by the colour light on the spectrum the plant receives.

Growth	Hours of Light	Hours of Darkness
Seedling	16 - 24	8 - 0
Clone	18 - 24	6 - 0
Vegetative	18	6
Flowering	12	12

 

Vegetative Lighting Cycle

The vegetative stage is where most of your plant’s growth happens. In this stage the roots and foliage will be developing rapidly. During the vegetative stage you want to give your plant a minimum of 18 hours of daylight per day. Growers usually keep their plants in the vegetative period for 4-8 weeks. There are two light cycles that are most used by growers. These are the 18/6 and 24/0 light cycles.

 

24/0 Light Cycle

In the 24/0 light cycle. Plants are given 24 hours of light, meaning the light is left on all the time without the plant ever having any darkness. This light cycle tends to be used on seedlings or cutting in the early stages of growth, but some growers use this light cycle for the entire vegetation stage. Growers who use this method claim that it maximises vegetative growth by forcing the plant to photosynthesize continuously. The 24-hour light cycle also uses the most energy.

 

18/6 Light Cycle

In the 18/6 light cycle. Plants are given 18 hours of light and 6 hours of darkness. This is the most common light cycle for growing cannabis. It can be used for photoperiod cannabis plants, or to grow autoflowering plants for their full grow cycle. 

A plant can be kept in this vegetative stage for years as long as it is kept under the 18-hour light cycle. This is how mother plants are kept for clones. 

A cannabis plant will not start flowering until it is given at least 12 hours of darkness. 

Growers will argue for and against the 24-hour light cycle. Most growers however favour the 18-hour light cycle. Just like humans and animals, plants need to rest as well. If your plant is constantly exposed to light during the vegetative stage, it may become unhealthy and stressed.

 

Flowering Light Cycle

The flowering stage is the final stage of your cannabis plants growth cycle. This stage normally takes between 8 - 11 weeks. In nature flowering naturally occurs when the plant receives less than 12 hours of light a day, so indoor growers will need to shorten their light cycle to trigger the flowering stage in their plants. This is done by using the 12/12 light cycle.

 

12/12 Light Cycle

In the 12/12 light cycle an equal amount of light and dark is provided for the cannabis plant. You won’t see the changes right away. After a few weeks in this light cycle you will start seeing buds developing.  

 

Using Timers

The best way to monitor your light cycle is by using a timer. A timer will allow you to set exactly when your light switches on and off. This means you do not have to physically switch you lights off as the timer will do this automatically for you. Using a timer will ensure consistency and precision when managing the light cycle. You can find many different timers such as a basic manual timer, an automatic timer and a smart timer Analogue timer, a digital programmable timer, and a wireless smart timer

 

Analogue Timers

A manual timer is the cheapest option but does the job just as well as any timer. You simply plug your grow light into the timer plug. You can then use the dial to set when the lights will switch on or off. These timers are programmable in bouts of 15 minutes for 24 hours. 

 

Digital Programmable Timer

Digital timers are more expensive and are a bit more complicated to use. This timer can be set down to the minute, which means you will be able to precisely time your light schedule as well as choose which day of the week you want it to function.

 

Wireless Smart Timer

Wireless timers are the most expensive option but give you the most control over your light cycle. These are a type of digital switch with a wi-fi facility. It connects to your wi-fi router at home and then the time switch can be monitored and programmed anywhere via an app on your smartphone. These are a good option if you are away from your plants for long periods of time and need to keep a watch on your light. 

 

  

Autoflowering Cannabis Plants

Unlike photoperiod plants, autoflowers do not rely on a change in light cycles to start flowering. They will automatically go through their whole life cycle in about 3 months. They do still however use light to photosynthesise and produce buds.

 

Growing Indoors:

The 18-6 light cycle (18 hours of light followed by 6 hours of darkness) is the most common light schedule that growers of autoflowers use. Using this cycle will provide your plants with enough light to encourage good yields, while also allowing your plants a brief but significant dark break to encourage healthy growth.

 

Growing Outdoors:

As long as your plant is getting plenty of light during the day, it will automatically stay in the vegetative stage from late spring until late summer. Keep in mind though that each strain is a bit different.

 

Importance of The Dark Cycle

Cannabis plants are "long night" or "short day" plants. This means that, for their hormones to be triggered into changing from vegetative growth to flowering, they need long periods of uninterrupted darkness.

The science behind this is that plants have light receptors called Phytochrome Red and Phytochrome Far Red. These receptors absorb red light in specific measurements and create a chemical reaction that tells the plant to stay in the vegetative stage or to begin flowering.

 

So how does red or far-red work?

Phytochrome Far-Red can be manipulated through the timing of the light. If there is Phytochrome Far-Red present in your cannabis plant, it will keep it from flowering. When there is light, Phytochrome Far-Red and Phytochrome Red are balanced in number. During darkness, the Phytochrome Far-Red becomes Phytochrome Red over time. This gradual change continues throughout the dark period until there are too few Phytochrome Far-Red to stop the plant from flowering, without it, the plant will enter the flowering stage. 

Growers should not underestimate the importance of uninterrupted period of darkness for transitioning your plants into the flowering stage. The conversion from Phytochrome Far-Red into Phytochrome Red is slow, however the change back from Phytochrome Red into Phytochrome Far-Red is instant once the plant has been exposed to light.

Exposure to light during this time can cause the plant to stop flowering and revert back to a vegetative stage. It can also cause plants to turn into Hermaphrodites and produce seeds.

Due to high daytime temperatures in some areas, some growers choose to run their 'lights on' period at night. These growers should take extra care to avoid light leaking in during the day in your 'lights out' period. 

Here are some ways you can create darkness and avoid any light leakage into your grow area:

 

Grow Tents:

• Invest in a good quality tent that has thick canvas, tight stitching, and quality zips.
• Make sure all vent flaps, drawstring ports and other openings are properly sealed.
• If you have any equipment that has display lights, make sure to cover them up with duct tape.
• Avoid opening the tent up during 'lights out'. Rather wait till the lights are on again to open the tent to check your plants.

 

Grow Room:

• Go into the room yourself, turn off all the lights and close the door (sounds scary I know)
• If there are windows where natural light may get in, you can cover them up with reflective sheeting.  
• Once your eyes have adjusted to the dark, check around the room if you can spot any light leaking in at all.
• Equipment with light-up displays can disturb periods of uninterrupted darkness, so it’s best to cover those displays with duct tape.

 

Growing Outdoors:

• Floodlights and streetlights can disrupt the flowering period of plants growing outdoors. It is important to make sure your plant is not being exposed to any of these lights.  

 

Due to high daytime temperatures in some areas, some growers will choose to run their 'lights on' period at night when its cooler. These growers should take extra care to avoid light leaking in during the day in you 'lights out' period.

 

Some Final Notes on Light Cycles for Growing Cannabis

Its best to keep things simple, choose which light cycle works best for you and decide if you are going to grow photoperiod or autoflowering plants.

Grow lights produce quite a bit of heat, some more than others, so it is advised to run them at night and turn them off during the day. When choosing a light cycle, it’s a good idea to set the 'lights out' period to be over the hottest part of the day. This will make it easier to keep temperature under control.