How Much Light Does A Weed Seed Need


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The right light schedule is an important part of growing cannabis. Here you can learn creating perfect lighting conditions for seedling, vegetative and flowering stages What do I need to know about light cycles and flowering my marijuana plants? Plants keep getting bigger and bigger with long days, and start making buds when you give them long nights. When cannabis seedlings have grown their second or third set of serrated leaves they are usually hardy enough to flourish in direct sunlight. Learn when that is and how to do that now.

Cannabis Light Schedule For Seedlings, Vegging And Flowering Photoperiod Weed, And Autoflowers

Getting your light schedule right is an important part of growing cannabis. Actually, the size and the quality of your harvest depends largely on the amount of light your cannabis plants receive and its timing. The seedling, vegetative and flowering stages each require unique lighting conditions. Every grower does things in their own way, and we have tried to capture a wide range of successful strategies for implementing marijuana light schedules. Let’s check them out!

Photoperiod Strains And Their Light Needs

Cannabis is an “annual” plant, meaning it lives for just a single growing season. The natural growth cycle for cannabis is to begin life in spring as a seedling, then to “veg” all spring and most of the summer, after which it “flowers” and grows buds in order to propagate the next generation. Because of this heritage, photoperiod cannabis will “flower” when exposed to the right darkness conditions. Give cannabis 12 hours of uninterrupted darkness a day consistently and it will go into flower (thinking that it’s fall when the nights grow longer). Each stage of a cannabis plant’s lifecycle has its own unique lighting requirements.

Autoflowering Vs. Photoperiod: Definition, Grow Info And More

Light Schedule For Photoperiod Cannabis Seedlings

Once your seed has germinated (or your clones rooted), your plant is at the seedling stage. This will last for only 2-3 weeks indoors but up to 6 weeks outside. During this stage, your cannabis plant will be exerting a lot of its energy to develop its root system. As a seedling, your plant will transition from growing single blade leaves to growing leaves with multiple blades (also known as “fingers”) – this is an indication that the seedling stage is over.

Light Schedule Options For The Seedling Stage

The optimal marijuana light schedule for seedlings is 18/6, meaning lights are on 18 hours per day and off (giving the plant darkness) for 6. However, some growers experiment with cannabis light schedules of 20/4 and even 24/0. Running lights on all day with a 24/0 light schedule will max out the limit of your equipment, especially fans, and will run up a high electricity bill. Opting for a 20/4 schedule gives your plants a small dark period to respire while still maximizing the amount of light your plants get, and also giving your equipment a short breather. It’s okay to experiment and find out what cannabis seedling light schedule works best for you.

Seedlings do not require bright light to grow well. Contrary to what many may think, light that’s overpowering can sometimes harm seedlings. However, if you dim your lights or use a weaker bulb, make sure to check the distance between the bulb and your plant. If the light is too far away from your plant, it will stretch, and if the light is too close, it may burn (especially if it’s an HID). With an LED, for example, you’ll want to keep your light between 60-76cm (24-30 inches) away from your seedlings.

Are You Using Cannabis LED Lights Wrong?

Light Schedule For Photoperiods In The Vegetative Stage

During the vegetative stage, cannabis undertakes most of its growth. By now, the plant should have a solid root structure on which to grow. The length of the vegetative stage can differ widely among growers – some prefer to take longer before switching the plant to 12 hours of darkness to make it flower. However, they will generally have an indoor cannabis light schedule during the vegetative stage for at least 4 weeks depending on the phenotype and genetics of the plant.

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Marijuana Light Schedule Options For The Vegetative Stage

During the vegetative stage, you don’t want to let your plants go into flower. This means you should have your cannabis plants under consistent light for a minimum of 16 hours a day. There are growers who opt to go even further and provide a period of 20 hours of light. Finding the right amount of light for the genetics of your plant can take some tweaking. That being said, a pattern of 18/6 (18 hours of light followed by 6 hours of darkness) is the standard marijuana light cycle for indoor vegetative growth.

Outdoors, growers don’t have the ability to end the vegetative stage by switching the cannabis’ light schedule. Growing outdoors means being at the mercy of mother nature and waiting for dark days in the late summer or early fall to trigger the flip into flower. Planning your outdoor grow ahead of time is a must to ensure a successful result and maximize your yields. Ideally, plants should be placed in the ground as soon as the nightly frost ends, allowing your plant a few months of vegetative growth before it flowers.

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Light Schedule For Photoperiods In The Flowering Stage

The flowering stage is the third and final phase of a cannabis plant’s lifecycle. This stage occurs naturally outside when the plant gets 12 hours of sunlight or less each day, when the days shorten during late summer or early fall. Indoors cannabis can be tricked into thinking it’s time to flower by shortening the light cycle to 12 hours of light and 12 hours of darkness.

Light Schedule Options For The Flowering Stage

Indoors, once you’re ready to begin the flowering stage, you’ll want to set up a light schedule of 12 hours of light followed by 12 hours of darkness (12/12). This stage lasts for 8 to 12 weeks. Your lights should be on a constant cycle of 12/12, as any interruption can cause havoc on your grow. In fact, too much light can cause your plant to revert to the vegetative stage (“re-veg”) or even turn into a hermaphrodite. For best results indoors, use a color spectrum between yellow and red.

Outdoors, much is beyond the grower’s control. You want to plant early enough so that when flowering hits, you have a large enough plant to pack on bud weight. You also want to choose a location with full sun to maximize the amount of light your plant receives during the day.

Dialing-In Light Schedule For Autoflowering Strains

Autoflowering cannabis plants have been crossbred with cannabis Ruderalis to create cannabis plants that flower based on time rather than a light/dark schedule. Autoflowers naturally have a short vegetative phase and don’t require the switch to 12/12 to start flowering. Some growers opt for an 18/6 light schedule throughout the lifecycle of their autos, believing that the plant needs a short recovery period of darkness. Other growers opt for a marijuana light schedule of 24/0, arguing that it maximizes the productivity of the plant’s short vegetative growth period. There are even growers who opt to provide their autos with a 12/12 schedule during flowering – however, note, this will reduce the size of your buds and is not actually necessary, as autoflowering plants flower based on time, not light cycles.

Top 9 High Yielding Autoflower Strains to Grow in 2022

Cannabis Light Schedule: Short Overview

Properly lighting cannabis takes some work but is fully worthwhile. Each phase of a cannabis plant’s growth requires unique lighting needs. Seedlings don’t want too much light, vegetative plants need at least 16-18 hours of light, and flowering plants need 12 hours of darkness every night. Knowing what stage your plant is in and how to give it the light it needs is crucial to growing healthy cannabis plants. Every plant’s genetics are unique, so don’t be afraid of experimenting with different light schedules to find out what works best for your needs.

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Cannabis Light Schedules: Vegetative Stage vs Flowering Stage

Cannabis plants keep getting bigger and bigger with long days, and start making buds when you give them long nights.

Cannabis is a “photoperiod” plant, which means the amount of light received each day decides when the plant starts flowering or making buds. This article explains how much light a day your photoperiod cannabis plants need to grow and start budding, so you get to a happy harvest day. What about auto-flowering strains?

Vegetative – Seedling or clone leads to Vegetative Stage –
Give 18-24 hours of light a day

Flowering – Flowering (Budding) Stage leads to Harvest –
Give 12 hours light & 12 hours dark each day

Seedling or Clone

While not technically a “stage,” all grows start with cannabis seeds or clones.


Plant your seeds or clones when you’re ready to start growing! What are clones?


Some outdoor growers start their plants indoors to give them a headstart before putting plants outside.

If you’re growing cannabis outdoors with seeds, you should wait until a few weeks after the spring equinox to put your seeds outside. In the northern hemisphere this means seeds go outside in-or-after April, In the southern hemisphere seeds go outside in-or-after October.

For growers starting with cannabis clones, generally you should wait a few weeks longer than with seeds. Cannabis clones are more prone to flowering early outdoors than seeds, so you might want to put your clones out in late Spring or early Summer. (What are clones?)

If you live in a cold climate, you must also wait until after the last frost before putting your plants outside. Freezing temps will kill cannabis plants. Strain choice is very important. Some strains flower earlier than others. For outdoor growers in cold climates, it’s important to make sure you grow a strain that is matched up with your local weather, so that plants are ready for harvest before temperatures drop.

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Vegetative Stage

The vegetative stage is one of the most important parts of the life of your cannabis plant.

The vegetative stage is the growing stage of the plant. When in veg, cannabis plants grow bigger and taller, growing only stems and leaves. As a grower, you are able to control the size and shape of your plants in the vegetative stage using simple training methods.

During the entire vegetative stage the plant does not produce buds at all. It only grows stems and leaves. During the vegetative stage plants tend to grow very fast, especially when conditions are right.

What keeps cannabis in the vegetative stage?

Short nights keep cannabis plants in the vegetative stage. You can keep a cannabis plant in the vegetative stage for basically forever as long as the plant continues to get short nights (shorter than 1s-12 hours, depending on the strain).

Cannabis will stay in the vegetative stage as long as the plant gets short nights (less than 11-12 hours of darkness each day)

Whether you’re growing indoors or outdoors, you must make sure your cannabis plants get at least 13 hours of light each day to stay in the vegetative stage. If your plant gets a few long nights, it may start budding before you want.

The plant can receive as much as 24 hours of light a day while in the vegetative stage. Many indoor growers provide 18-24 hours of light a day (known as 18-6 or 24-0 light schedules) during the vegetative stage to encourage faster vegetative growth.

Don’t want to worry about light schedules? For growers that don’t want to pay attention to light schedules, there are auto-flowering strains of cannabis, which will automatically go through their whole life in about 3 months no matter what light schedule is provided. For some growers, an auto-flowering strain may be more simple than a traditional (photoperiod) strain.


Most indoor growers provide 18-24 hours of light a day (known as 18-6 or 24-0 light schedules). Giving your cannabis plants more hours of light each day in the flowering stage will encourage faster growth.

Lingo: When a grower provides 18 hours of light a day and 6 hours of darkness, this is commonly known as the 18/6 light schedule. For 24 hours a day, this is referred to as the 24-0 light schedule.


As long as your plant is getting plenty of light a day, your plant will automatically stay in the vegetative stage from late spring until late summer. Every strain is a bit different.

Flowering Stage

Cannabis starts budding when plants get at least 12 hours of uninterrupted darkness each night. After plants start budding, they must continue to get long dark nights until harvest or they may revert back to the vegetative stage.


Indoors most growers put their plants on a 12-12 schedule to initiate flowering. Outdoors the plant will naturally start budding in late summer when nights are growing longer and longer as winter approaches. Just make sure plants aren’t exposed to light during their dark period!

What is 12-12 Lighting?

The indoor grower will need to artificially induce flowering/budding in plants by changing the light schedule so the plant receives only 12 hours of light a day, and 12 hours of uninterrupted darkness.

Once the plant is changed over to the flowering (12/12) light schedule, there is generally another 6 weeks-5 months (average 2.5 months) before the plant’s buds are ready for harvest.


Outdoor growers wait until their cannabis plants start naturally flowering on their own, usually after mid-summer when days start getting shorter than 12 hours.

It’s important to make sure plants aren’t exposed to light at night during their dark period, even street lights or spotlights, as this can prevent cannabis plants from flowering properly.

Growing Indoors? Not Sure When To Switch To Flowering?

So indoor growers have a choice to flower their plants whenever they want… When is the best t ime to start flowering your cannabis indoors?

The real answer is that it’s a matter of personal preference and also depends on what end result you’re looking for. There are two major considerations when choosing the right time to switch to 12/12, the age of the plant and the height of the plant:

Age: Some growers feel that a marijuana plant which has been grown from seed will not produce as many buds or have enough resin production if the plant is not given at least 60 days in the vegetative stage to mature before it’s changed over to the flowering stage. This is not true. many growers initiate flowering soon after germinating a seed in order to keep plants small and short. This is often called “12-12 from seed.” Just remember, no matter what you do, a young cannabis plant will not start flowering until it is 2-3 weeks old. Even if you put a seed on a 12-12 schedule from the beginning, it will not start properly budding for about 3 weeks. When growing with cannabis clones, age is not an issue and growers can switch directly to flowering once your clone has established roots. This is because even though a clone may be small, it’s still a ‘mature’ plant since it is made of a piece from a mature plant. Rooted clones tend to grow much faster for the first few weeks than plants grown from seed. In any case, age is not much of an issue, and you should switch your light schedule at the time that best fits your needs.

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Height: A general rule is that your marijuana plant will double or triple in size during the flowering stage from the point where you first change over the light schedule to 12/12. Some plants will grow more, some will grow less, but a good rule of thumb is to change your light schedule over to flowering when your plants have reached half of their final desired height. Bending, known as “LST” or “low stress training” can be used to control colas that get too tall. Simply bend too-tall colas down and away from the center of the plant. Some growers will even slightly break or “supercrop” branches to get them to bend at a 90 degree angle. For those growing in a small space, height may be the primary concern. However, there are many techniques available to grow a short, bushy weed plant or basically train your cannabis plant to grow into any shape you want.

Here’s an example of LST to keep a plant short:

In optimal conditions if height and space is not an issue, you would probably want to vegetate your cannabis plant for 60 days or more before switching it over to flowering. This gives your plant plenty of time to grow big (so you get bigger yields), and allows new growers to dial in their grow before plants enter the sensitive flowering stage. In the vegetative stage, it is easy to recover from problems, but problems are a lot more serious in the flowering stage, where mistakes can dramatically hurt your final yields.

Giving cannabis plants more time in the vegetative stage, and taking time to train them to fit your space, will give you the best final yields. However, if space is tight, then it’s better to switch when the plant is half the final desired height, or even to just attempt to flower your cannabis plant straight from seed.


After the vegetative and flowering stage are over, it is time to harvest your plants!

When can seedlings be put under lights or in the sun?

Not sure whether to grow your cannabis seeds in the sun or under lights? This article explains how much light you need, how to prevent your seedlings from falling over, and when to plant them outside.

“When can cannabis seedlings be put under lights or in the sun?” is a common and sensible question that is often asked by novice cannabis enthusiasts after germinating seeds indoors under lights.

For those who are growing indoors, cannabis seedlings may be put under lights as soon as they emerge from the soil or growing medium.

How much light for cannabis seedlings?

When using HID lighting (usually a metal halide lamp), young seedlings should be kept at least 50cm from the bulb.

With compact fluorescent lamps (usually 100w or more), a distance of around 15cm should be maintained between the top of the young seedlings and the bulb.

If using normal fluorescent tubes (18-36w), seedlings can be kept within a few centimetres of the light source.

Cannabis seedlings growing under lights should always be given a gentle breeze from an oscillating fan (a small household fan on the lowest setting is fine), as constant gentle movement will strengthen their stems significantly.

Are your cannabis seedlings falling over?

If cannabis seedlings grow tall and then fall over, this is almost always a result of growing in an environment with static air.

Seedlings intended for outdoor growing should be kept by a sunny window for the first week or two after emerging from the soil.

When they have grown their second or third set of serrated leaves (after the the round cotyledons that initially emerge from the seed) seedlings are usually hardy enough to flourish in direct sunlight.

When to plant your cannabis seedlings outside?

If outdoor temperatures are suitable, seedlings may be acclimatised to direct sunlight by giving them progressively longer daily exposure to outdoor conditions.

Starting with about three hours outside, at the sunniest time of day, seedlings can be given an extra hour of outside exposure each day, so that within about two weeks they can be left outdoors permanently.

Laws and regulations regarding cannabis cultivation differ from country to country. Sensi Seeds therefore strongly advises you to check your local laws and regulations. Do not act in conflict with the law.

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