How It Works Start My Diary Login Sign Up

Light Deprivation For Cannabis Grow

Created by
JoshuaHolt JoshuaHolt
Added 5 January 2021

Growing Cannabis Using Light Deprivation

Depriving your cannabis plants of light might sound counter-productive but it can actually help you achieve more bud in a shorter period of time. Adjusting your outdoor light schedules is easy and just requires consistency on your part. Let's take a look at light deprivation and how to manage it correctly with outdoor cannabis plants.

What Is The Light Deprivation Technique?

Cannabis In Greenhouse

Light deprivation (also known as 'light dep') is a technique used by growers to control the amount of light their outdoor plants receive. Similarly to how we have control over an indoor light schedule, the plant's cycle can be sped up outside by forcing it to flower.

Light deprivation helps farmers keep their businesses running with more harvests each year. The technique is often used in climates where autumn weather brings a lot of rain and there is higher risk of mold, which can prevent the grower from finishing the cycle properly as winter approaches.

Light deprivation involves cutting the amount of daylight hours to 12/12 using light proof coverings for a few weeks so the plant 'thinks' the days have shortened and begins to flower. After consistently controlling this for at least 3 weeks, a cannabis plant will recognise that it is well into blooming stage and can be left to finish its cycle without coverings, as the days will also naturally be shorter by then.

How Many Harvests Can I Achieve With Light Deprivation?

Greenhouse Cannabis Plants Nearly Ready For Harvest

Depending on the region you live in you can achieve at least 2-3 outdoor harvests each year using light deprivation techniques. With the right management and space, it is even possible to set up a nearly perpetual harvest throughout the season.

Best Time To Start Using Light Deprivation

Light Deprivation Cycle Example

The timing is very important when it comes to light deprivation, both in terms of when you choose to start during the year, as well as your daily routine of putting the covers on and off. Both need to be precise in order to make the most of the light and get plants flowering without problems.

Most farmers who use light deprivation germinate their seeds in early spring so the seedling can be flowered almost straight after they are put outside. Even though the days are longer at this stage, light deprivation allows the farmer to get their plants flowering so there is a harvest ready for mid summer, or July/August in the Northern Hemisphere.

CycleGerminateLight DepHarvest
1st CropEarly SpringLate SpringMid Summer
2nd CropLate SpringAutumn

The second batch is usually started as the first plants start their blooming phase. The plants either flower naturally or the farmer choses to speed up the process by going for a second round of light deprivation (usually if the autumn brings risks of losing crops). Know how the climate and light schedules change in your region so you can plan accordingly. This way you'll get at least 2 harvests.

Before You Get Started

Autumn Light On Flowering Cannabis

Bear in mind that the change in spectra of light throughout the year plays a fundamental role in how a outdoor cannabis plant grows and develops its cannabinoids and terpenes. That means that forcing a cannabis plant to flower early may mean it does not have the full UV spectrum it otherwise would if it was left to grow naturally into autumn.

Make a plan for when you want to start so that you can harvest once in summer and once in the autumn before it gets too cold or wet. Decide which time of day works best for taking the cover on and off your plants so that they have no more than 12 hours of light each day. Stick to the schedule or the process could be interrupted, potentially causing problems during flowering.

Covering Your Cannabis Plants

Cannabis Plants Need 12 Hours Of Darkness To Flower

How you cover your cannabis plants depends on how they are being grown. Outdoor plants can either be in pots or grown directly in the ground, and each comes with its own set of challenges. Greenhouses are commonly used for light deprivation techniques as they provide a structure in which to easily support and remove light proof covers.

Remember that the plants go through a 'stretch' phase during the first weeks of flowering, so the covering needs to be at least double the size of the plant. You may want to go bigger if you know the strain you're growing has a tendency to explode when the light schedule is flipped.

Potted Plants

Outdoor Plants In Pots Are Easy To Move

Potted cannabis plants are generally easier to work with than those planted in the ground because they can be moved around, which gives a grower more options for providing cover. For instance, if you have one or two plants, you could move them to a blacked-out indoor space. This could become quite tedious though, especially if you have a lot of pots to work with.

Being able to move the plants means they don't have to be under the sun while they are covered, which could cause some heat or moisture related problems.

Greenhouses

Advanced Automated Light Deprivation Greenhouse

Greenhouses also give us plenty of freedom to cover our cannabis plants without much difficulty. Black-out tarps can be fitted to allow a grow to simply pull the cover over in the evening and take it off in the morning.

With a proper greenhouse there is the possibility to automate the whole system so you don't have to worry about manually doing the cover yourself everyday. However, these systems are expensive and may not be worth it for just a few plants, but that is up to you.

Planting In The Ground

Open Hoop House Cannabis Grow

Plants in the ground tend to require a support or structure to be built around them. Hoop house style greenhouses are commonly used as they are simple to build and the cover or tarp can be draped over it as and when is necessary (based on the schedule you have set). Ideally, you want to have your plants in a greenhouse or build a structure if you plan to plant in the ground.

Alternatively, you can use plastic bins or build a light proof frame that you can place over the plants easily. Just be careful about leaving it under the sun for too long as the conditions inside could get stressful for the plant.

Tips For Light Deprivation Technique

Light Deprivation Tips

Light deprivation does require a lot of work especially if you have to lift a gigantic tarp everyday. Consider seeing if someone can help you out or if you can't be there make sure someone else can.

  • Experiment and see what works for your plants, as well as your schedules.
  • Do not miss a day or get lazy or the process could backfire and you could end up with hermies (hermaphrodites).
  • Make sure no light enters the space for 12 hours during the dark period.
  • Without ventilation, leaving the cover on overnight may trap moisture and heat so it is recommended to cover 12 hours before sunrise and remove the cover after the sun has gone down.
  • Choose resistant strains that handle stress, mold and pests. 
  • If you can afford it, install a system that allows you to easily place and remove the covering. Advanced systems can even be automatic.

Autoflowers can also help you achieve more harvests per year. Thanks to BongCity710 for the video.

If you found this article useful or have any tips regarding light deprivation techniques, please feel free to share your thoughts in the comments section below!

External References

Vegetative growth of Cannabis sativa L. cultivars in Jamaica using 18/6 photoperiod. International Journal of Plant Science and Horticulture. - Emanuel, Machel & HENRY, Valrick & Robinson, Dwight. (2020).

Hermaphroditism in Marijuana (Cannabis sativa L.) Inflorescences - Impact on Floral Morphology, Seed Formation, Progeny Sex Ratios, and Genetic Variation. - Punja, Z. K., & Holmes, J. E. (2020).

Controlling greenhouse light to a consistent daily integral. - Albright, Louis & Both, A.J. & Chiu, A.. (2000).

Photoperiodism in relation to hormones as factors in floral initiation and development. - Hamner, K.C.; Bonner, J. (1938).

 
This article was updated December 2020.





Comments

Shotokan
Shotokan

Hi. I´ve been doing something similar for some time, I call it "forced flowering," 12hrs sunlight and 12 hrs inside the growing tent with no light (and no odor as I live in an apartment). By law I can have up to 6 plants in flowering stage (no limit for veg plants) so I maximize harvests, when the first set of plants gets to the 4th week of flowering I put 2nd set to 12/12. This southern hemisphere season I would have 3 harvests. I couldn´t get 4 harvests because the seeds failed germination. Keep on growing!