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The Ideal Light Cycle for Cannabis At Different Growth Stages

Light cycle for cannabis

Growing great weed isn’t the easiest thing to do, especially for cultivators with little to no experience. What seems like a simple task – grabbing some seeds, chucking them into some healthy soil, and watering – can quickly become an overly complicated headache, but it doesn’t need to be.

While the lighting requirements for weed plants are relatively simple, understanding and implementing the optimal light cycle for the different growth stages is crucial if you want to squeeze every last drop of potential (and resin) from your crop.

In today’s write-up, we are going to explain in the simplest terms possible, what light schedule (and spectrum) is needed throughout the entire lifecycle of a cannabis plant and how you can fine-tune it to get the best out of your green girls.

The majority of this article will be dealing with the lighting schedule for photoperiodic plants, but there’s a section lower down that also goes over autoflowering lighting requirements.

Understanding light cycles

Cannabis, like all plants, has evolved to require specific amounts of light and darkness for optimal growth. This natural process is known as photoperiodism and has a direct effect on the plant’s biological processes, such as how large it grows, the time it takes to flower, and the amount of resin it produces on those sticky, dank flowers.

When it comes to weed plants, there are two primary stages where different light cycles need to be implemented. It’s that simple, but it also kind of isn’t. Because yes, as cultivators, we do need to control the amount of light our plants receive, but we also need to understand how to use light cycles effectively, and what spectrum of light is most beneficial during the two main growth stages.

Ideal light cycle for cannabis

Then, we have the little seedlings, which can be treated almost the same as plants that are in full vegetative growth but with a few caveats.

How cannabis plants use light

Ganga is no different from all the other gorgeous flora that covers our pale blue dot. Without light, there would be no life, and that is no more obvious than when looking at plants.

Plants produce their own food through photosynthesis, and this process needs light to occur.

Photosynthesis explained

Photosynthesis is the process by which plants turn light energy from the sun (or indoor grow lights) into chemical energy stored in glucose.

How?

Plants have a pigment in their leaves called chlorophyll, which is what makes them green. When light hits the chlorophyll molecules, they absorb it and use it to split water molecules and combine carbon dioxide to create glucose. This process also releases oxygen as a by-product, which is great, as otherwise we would be, for lack of a better word, screwed. A plant’s stem and leaf structure are also critical to photosynthesis, as they help to transport the glucose to where it’s needed for growth and development.

Light spectrum

One commonly overlooked, or at least misunderstood, factor of weed lighting is the specific spectrum required during the various growth stages. If you’re growing outdoors, then this isn’t an issue you need to worry about, as the sun provides the entire spectrum of light needed for optimal growth. However, when growing indoors, it’s crucial to understand that not all grow lights are created equally and that some will only offer a small subset of the light spectrum.

This is less of an issue in the current age of LED lighting, but many growers till prefer to use high-intensity discharge (HID) lights. These used to be the gold standard in grow lighting, but they do come with a few disadvantages in terms of heat, energy usage and the light spectrum provided.

When considering the optimal light spectrum for growing big, healthy cannabis plants, we need to focus on two specific wavelengths: red and blue light. Both are essential during different growth stages.

Vegetative stage

The ideal spectrum for this growth phase is predominantly blue light, which is found in the cooler white temperatures (6500K) and with added some UV light. These wavelengths mimic the sun’s natural spectrum during the spring months, which is when plants experience the most growth in their vegetative period.

Flowering stage

Once you are ready for your crop to start producing flowers, you will want to switch the spectrum to a more red/orange hue, closer to the 2700K mark. This light spectrum imitates the natural sun during the summer months when plants prepare for reproduction.

How much light does a weed plant need?

Ok, with that out of the way, let’s get down into the real guts of the article – how much light does a cannabis plant need during the various growth stages?

Light schedule for seedlings and clones

Seedlings (as well as clones) are fragile in nature, with just a tiny root system, minimal nutritional requirements, and modest lighting needs. Giving them too much light can be just as disastrous as not enough, so it can be thought of as the real ‘Goldilocks’ stage of lighting.

Indoors

Indoor growers will need to either turn down the power of their light (unless you are using a low-power fluro), or move the light further away from the plants. Alternatively, if you’re running multiple tents or sections of a larger grow, you could have your seedlings in a separate, smaller tent that can be turned off while the main tent is on. Whichever option you choose, the vital consideration is not to expose your seedlings to more than 50% of the light’s output if using a high-powered grow light.

You will know if the seedlings are receiving too little light as they will begin to stretch and search for it. If they are getting too much light, the leaves will begin to look ‘bleached’ or yellow in color.

In terms of timing, seedlings and clones will usually perform best under 18 hours of light per day (18/6) or 20 hours of light per day (20/4). If you’re a novice grower, we recommend sticking to 18/6.

Outdoors

If you are going to be growing outdoors, timing is not such a big factor, as the plants will begin to naturally transition to flowering when daylight hours start to decrease. However, we recommend germinating your seeds indoors using the ‘paper towel’ method around the 1st of March, as long as the weather is getting warm enough.

Minimum temps of around 12C (55F) are required to keep the plants alive and growing, but the warmer, the better, maxing out at around 30C (86F). If that’s where the local climate is at, get germinating, and once that tap root pops out, it’s time to plant outdoors.

Best light cycle for the vegetative growth stage

Indoors 

Again, just like the seedlings or clones, plants that are in the vegetative stage need a lighting schedule of 18 hours of light per day, with 6 hours of darkness.

Cannabis under LED lights

Some growers like the 20/4 schedule, but this not only increases energy costs but also shortens the lifespan of your lighting setup.

Outdoors 

As spring starts to wane, and summer kicks in, the sun will naturally provide enough light for your plants. If you are not experiencing at least 18 hours of natural daylight when growing outdoors, it may be a good idea to add some supplemental lighting during the vegetation cycle to ensure the crop doesn’t switch over to flowering growth early.

Best light cycle for the flowering growth stage

Indoors 

Once you are happy with the size of your plants contained within your indoor gardening setup, it’s time to switch that light cycle to force the plants to start flowering. Keep in mind that the plants will still put on a fair bit of size during the first 10 or so days after you switch the light cycle over, so plan accordingly in terms of spacing.

So, whenever you’re ready, switch the light timers over to 12 hours light per day (with 12 hours of darkness). This will ensure that your photoperiod plants will start to flower correctly.

Outdoors 

After the summer solstice (June 21st), the sun will begin to offer less than 18 hours of daylight, and this is what triggers photoperiod plants to transition from vegetative growth into flowering. This period will fluctuate depending on your location, but as a rough guide, you can use the summer solstice as a reference point for when to expect this natural decrease in daylight hours.

Light cycles for autoflowers

If you are planning on growing a crop with all autoflowering strains, then you can pretty much disregard all the above advice in terms of lighting schedules (although the light spectrum information is still relevant).

Most autoflowering strains will begin to flower after approximately 3-4 weeks of vegetative growth, and will do so regardless of the light cycle they are exposed to.

In terms of lighting schedules for autoflowering strains, growers typically use 18-24 hours of light per day, with 6-0 hours of darkness. We recommend sticking to whichever schedule you choose and keeping it consistent throughout the plant’s entire life cycle.

Although there are growers who do get amazing results with leaving the lights on 24 hours a day when growing autos, but it does come with certain risks and downsides. Without the lights off at all, the crop gets no time to rest and can become stressed. This stress is not a good thing and can lead to poorer yield numbers than what can be achieved with a more balanced lighting schedule. Make sure to do some research on the particular strain if you are considering leaving the lights on 24 hours a day.

Or, alternatively, just keep the schedule to 18/6 or 20/4. This will reduce the stress on the crop and save you money in the long run.

Managing light cycles indoors

This one is simple. all you have to do is run your lighting rig through a timer, and set it to the time schedule needed for the stage of growth the plants are at. There are a variety of timers available, and you can even use your smartphone to automate the lighting schedule if you’re not a fan of manual timers.

Manual timers are a cheaper option, but if you have a few dollars to spare, it’s pretty easy to find a low-budget smart timer that you can connect to your cell phone allowing you to control the lighting remotely (they are actually not much more expensive, but some growers prefer the manual timers as they’re less likely to be accidentally altered).

How to maintain light cycles outdoors

Outdoor growers will have to rely on the sun’s natural lighting cycle to trigger the growth stages. However, if you are growing in an area that experiences a lot of cloudy days or shorter daylight, you may want to use some supplemental lighting. This is easily done, especially if you have a greenhouse set up. You can simply add some lighting during the darker hours to ensure your plants get enough light during either the veg or flowering stage.

Or, if you live somewhere that is getting too many hours of daylight, like areas in Northern Europe, you may need to use a light deprivation greenhouse or tent to force the plants into flowering mode. These are becoming increasingly popular, even in areas where the natural lighting cycle is perfect for outdoor cannabis cultivation, as it allows outdoor growers to stagger their harvests and manage crop sizes.

The Wrap Up

Growing weed can easily become a slightly overwhelming endeavor, especially if you’re starting your first indoor garden. But there’s no need to stress. It’s definitely not rocket science, as much as some blogs and growers may like to overcomplicate things.

Take your time, do some light reading, and you’ll easily be able to get the type of harvest results that we all hope for. Just remember that different stages of growth require different amounts of light, and try to keep the rest of the process as uncomplicated as possible.

If you would like to learn more, head over to our dedicated growing section, and you’ll be ready to kick things off in no time.

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Sam North

Sam North is a content writer with a passion for everything cannabis. After working multiple seasons on weed farms absorbing the ins and outs of cannabis cultivation and culture, he decided to transition into a role that would allow him to work from anywhere, anytime. Sam now writes for multiple weed publications. He has extensive experience with a wide range of canna-agriculture styles, from smaller artisanal farms to large-scale commercial operations, and is here to share his knowledge to give you all the best chance of cultivation success. About this Author

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