Medically Reviewed by
Jason Crawford

Article Last Updated on December 30, 2022

Light is an essential source of energy for all plants in nature, so every marijuana grower needs to be well-versed in how it affects the plant growth cycle. Whether you’re planning on cultivating weed and are looking to learn something about it before you start or you’re an experienced cannabis grower in need of more info, knowing how much light your budding crops need can make or break your harvest.

So, if you want to learn more, stick around, because below we’ll cover some basics about the light cycle of marijuana plants, how much light they need to thrive, and break down some of the most commonly used indoor-growing lights.

The Light Cycle of Marijuana Plants

Cannabis plants are annual – they love light and warm temperatures. They are planted in the spring, grow throughout the summer, and finally, flower in the fall. Unless you use any techniques to manipulate the cycle, like the light dep technique, the natural life cycle should start again the following year.

Since they love light, maximizing their exposure to the light source will enhance photosynthesis, resulting in more bountiful yields. Photosynthesis is the process of transforming light into energy, where the plants harness light to transform carbon dioxide and water into oxygen and glucose. Glucose is essential because it’s their fuel and it’s necessary for their survival. In other words, the more light the plants get, the more fuel they produce, and the faster and bigger they grow.

Ideally, sunlight is the best source of light because it’s natural and provides them with the needed amount of light, but most growers don’t have access to natural sunlight most of the year, so they use indoor grow lights. There are a lot of different choices of lighting systems nowadays, and there are some pretty good benefits that come.

One of the most important is that these lights can be easily controlled, and with that, you can extend the growing season and get a bigger yield from your flowering plants.

How Much Light Do Your Crops Need?

As we mentioned earlier, marijuana is planted in the spring and it flowers in the late summer going into fall. This means that when grown outside it follows the natural cycle of the light depending on the season.

In general, toward the end of summer, the hours of light are getting shorter while the hours of darkness are getting longer. These changes in the environment signal your crops that they need to prepare for flowering by directing their energy towards producing buds instead of vegetative growth.

When you’re an indoor grower, you’re the one providing the light, so you can either follow the natural photoperiod and adjust the light intensity or you can manipulate it and speed up the flowering.

To achieve this, you need to provide your plants with sufficient hours of darkness, in fact, no less than 12 hours. Otherwise, you risk prolonging the maturation of the plant and you’ll get smaller buds with weak THC and CBD content.

If you’re not into following light schedules, though, autoflowering weed doesn’t depend on any lights.

How Much Light During Vegetative Stage?

The purpose of the vegetative phase is to allow the plants to develop a solid root system, strengthen the stems, and start growing out the fan leaves through which it absorbs the light it will use for photosynthesis. This means that, during this stage, it needs a lot of light, at least 18 hours to a maximum of 24 hours per day.

The difference between more hours and fewer hours is just the time it will take for the plant to fully develop. If you provide it with 18 hours of light it could take a few more weeks before the plant is strong enough, and if it’s 24 hours, it’ll need less time.

How Much Light During the Flowering Stage?

In nature, the flowering stage happens when roughly the hours of darkness and light are nearly the same, but the plants are exposed to the unpredictability of the elements. In indoor grow spaces, they can be protected from damage and still provided with sufficient light.

As a general rule, you should provide your crops with 12 hours of light and 12 hours of uninterrupted darkness per day. Note the emphasis on “uninterrupted”. This is very important because any disruption of the process and you risk confusing the plant that will result in delays or lower quality buds. So, keep up with this light schedule diligently and you’ll have flowers in a few weeks.

Common Types of Grow Lights

For indoor cannabis cultivation, there is one most important thing to know about light, and that is the color spectrum in relation to the vegetative and flowering stage.

In nature, during the spring/summer months, when marijuana is naturally in the veg stage, the sun radiates energy on the blue spectrum. This type of light is very much needed for the proper growth of all plants. However, as fall approaches and the nights get longer, the energy rays fall more into the yellow/red spectrum, which encourages flowering.

Therefore, when you buy artificial lights, you must pay attention to the color spectrum to help your crops produce healthy buds.

Fluorescent Grow Lights

Fluorescent grow lights have been used for a long time to simulate sunlight, not only for cultivating cannabis but other indoor plants, especially for germination in places with cold climates and not a lot of sun.

Fluorescent lights are perfect for the early stages of growth when the plant is very sensitive to light and heat, however, for the later stages, they’re too high-maintenance. For starters, they are more expensive, and they require a lot of different fluorescents to mimic natural sunlight.

High-Intensity Discharge – HID Grow Lights

HID lights are quite powerful, and they were first invented with the purpose of being used in large scale areas, like stadiums. However, it was soon discovered that they could also be very useful for indoor growing. These huge light bulbs are more efficient than fluorescent lights, and they can get very hot, so they’re best hooked up to an exhaust fan so that overheating could be prevented.

One type of HID lights are metal halide lights which are perfect to use during the veg stage up until flowering because they produce a blue light that the cannabis plants love during this stage.

High-Pressure Sodium Vapor – HPS Grow Lights

HPS grow lights are one of the most popular among marijuana cultivators today. They are ideal for the flowering stage of cannabis because their yellow tint stimulates bud production.

These lights are known to induce better yields per watt of electricity than other types of grow lights and they don’t get overheated as much. It’s best to get two HPS bulbs with a wattage of 400w or 600w for the best light diffusion. It’s also recommended that you use a compact fluorescent light together with HPS in your grow room.

Light Emitting Diodes – LED Grow Lights

LED lights have a wide spectrum of use, including indoor cannabis cultivation, and they’re a great alternative for HPS. They may not be the best of the best choice for your grow tents, but they have benefits that are hard to pass.

For starters, they are low voltage, meaning they don’t produce too much heat, so the risk of light burn will be minimal. Additionally, they are cost-efficient and don’t waste as much energy, plus they’re long-lasting.

We would recommend you pay attention to the spectrum of color when you buy LED lights and use full-spectrum lights, so you can provide your plants with enough light for photosynthesis.

Please also know that incandescent lights are unsuitable for growing marijuana as they tend to overheat and can cause a light burn.


Light is an essential element for cannabis growing, or any plant for that matter because it plays a major role in photosynthesis. This process is crucial for the plant’s survival because they use light as a means of producing their building blocks for survival.

Sunlight is naturally the best source of energy, but since not many growers have enough access to it, they use artificial lights and indoor growing. To achieve optimal results, it’s important to provide the plants with sufficient light during the vegetative stage, and sufficient darkness during the flowering stage.

A passionate advocate for the benefits of cannabis. Fraser Horton, who has a background in botany and a strong love of nature, has spent years researching how cannabis affects the body and mind. He established Leaf Nation in 2020, where he has devoted himself to educating people about the legalisation of marijuana and its safe and responsible use. Fraser is committed to highlighting cannabis’ potential for improving wellness and working to dispel the stigma associated with its use.


The information presented on this page is provided as a public service to aid in education and is derived from sources believed to be reliable. Readers are responsible for making their own assessment of the topics discussed here. In no event shall Leaf Nation be held reliable for any injury, loss or damage that could happen if using or abusing drugs.