How Much Light Does A Weed Seed Need

Cannabis Light Periods – What do I need to know about marijuana light cycles? (length of sunlight hours each day) If you’re growing a cannabis plant grown from a random seed (“bagseed”), unless How much light does a weed plant need? Discover the answers plus cannabis light schedule tips, how to choose the best lights, helpful FAQs, and more. Ensuring your cannabis plant has enough light (as well as enough hours of darkness) in the grow tent during a 24-hour period will enable it to efficiently

Cannabis Light Periods – What do I need to know about marijuana light cycles? (length of sunlight hours each day)

If you’re growing a cannabis plant grown from a random seed (“bagseed”), unless you somehow have an auto-flowering seed, you will need to understand about cannabis life stages and how they are affected by light periods.

If you don’t understand light periods, your plant may never start making buds! The light schedule experienced by your plant will actually change its life stage. Learn more…

Cannbis plants have two life stages:

1.) Vegetative – Seedling or clone leads to Vegetative Stage

  • Give 18-24 hours of light a day (indoors)
  • Cannabis grows only stems and leaves

2.) Flowering – Flowering (Budding) Stage leads to Harvest

  • Give at least 12 hours of uninterrupted dark each day (short days)
  • Cannabis starts growing flowers/buds

The first stage, “Vegetative” begins when marijuana plants first sprout, at the beginning of their life.

Most indoor growers give their cannabis plants 18-24 hours of light a day during the vegetative stage. The exact number of hours needed to keep a plant in the vegetative stage is dependent on the strain, but 18+ hours/day will keep basically all cannabis plants in the vegetative stage.

Outdoor growers plant their seeds in Spring when the days are naturally longer. In the wild, cannabis seeds naturally germinate in the Spring.

For an indoor grower, when a plant is about half the final size you want it to be, you should change it over to the “Flowering” stage. This is the stage when your plant starts growing buds.

You do this by changing your light so that it only shines for 12 hours a day, and the other 12 hours a day your marijuana plants are kept in TOTAL darkness.

After 2-3 weeks of the 12-12 light schedule, most cannabis plants will show the first signs of their gender (they either are a female plant which starts growing buds, YAY! or they are a male plant which start growing balls/pollen sacs, NO!).

Boy cannabis plants don’t give you any usable amounts of THC, so most growers toss them on sight. These male plants can also impregnate (pollinate) your female plants, which causes your female plants to produce seeds and less buds.

So unless you’re planning on breeding, it’s important that most growers destroy male plants as soon as you notice them growing grape-like balls where their buds would normally be.

Unfortunately, about 50% of all regular (unfeminized) cannabis seeds are male (though this varies from strain to strain, and from environment to environment). Fortunately for small growers, you can purchase feminized (all-female) seeds so you don’t have to worry about male plants if you don’t want to. Learn more about buying seeds.

When does a cannabis plant start budding?

Marijuana plants have an internal process that allows them to detect how long they receive darkness each night. This is because they are a “photo-period” plant, specifically a “short-day” plant which means these plants start making flowers/buds when days start getting short.

In the wild, as the days get shorter and nights grow longer, a marijuana plant “realizes” that winter is coming and will start budding/flowering. It “knows” it’s approaching the end of its life cycle so it frantically starts making buds in time before winter.

When growing marijuana outdoors, a grower doesn’t need to do anything to induce flowering because the sun will take care of things on its own. It’s just important to make sure that there are no lights shining on your plants during their night period (which will disrupt their dark cyle).

However, when growing weed indoors, a marijuana gardener will have to fool their plants into “thinking” winter is coming to induce flowering and kickstart the creation of buds.

This is done by changing the plant’s light schedule to 12-12, where the weed plants gets 12 hours of light a day and 12 hours of total darkness.

You’ll get the best results if the start and end time for the light is exactly the same each day, which is why most growers end up getting a timer to flip their lights on and off, like an automatice light switch.

I tend to set my timer to shine line from 8pm-8am. This gives me time to check on my plants at night when the lights first come on, and I can also check them quickly in the morning before I go to work. It also keeps things cooler since the lights go on at night.

But ANY 12 hour period will work, as long as you remain consistent.

Check out my cannabis grow light guide for more info about picking out suitable lights!

Photoperiod dependent strains vs. auto-flowering strains

So all strains of cannabis that respond to light in this way (where the light period effects what stage they’re in) are called “Photoperiod dependent” strains.

Auto-flowering” marijuana strains pretty much ignore how much light they get each day. Generally you don’t run into these unless you buy them particularly from a cannabis seed bank.

How much light does a weed plant need?

How much light does a weed plant need? A difficult question for many new growers. Light is one of the vital elements of all life forms, including weed plants, so you must get it right for your plants to thrive.

Keep reading as we unpack everything you need to know about cannabis lighting, from picking the best lights to optimum schedules. Let’s get started.

How much light does a weed plant need?

How you use grow lights determines the success of your marijuana plants. Answering the question, how much light does a weed plant need is complex. There are four lighting basics to master for the best results.

  • Light intensity
  • Type of light and placement
  • Light spectrum
  • Light schedule

Outdoors, the sun showers your plants in natural light, but indoors you’re in control of the cannabis light cycle.

Let’s take a closer look at each factor to consider.

Light intensity

Higher light intensity is generally associated with better growth. If you continue to increase the intensity of light that a plant receives, what happens? You might over-saturate your cannabis, which causes burns. However, your crops also risk stretching and stunted growth if you dim the lights too much.

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Two common ways to measure light intensity are:

  • Lumen — measures the light flow that a source emits. The higher the lumen, the brighter the light.
  • Lux — measures the light intensity that reaches a plant’s surface.
Measurements of intensity

Growers typically use lux to measure intensity in their cannabis light schedule since plants only use the light that reaches its surface.

Here’s a simple table showing the best lux levels for two essential life stages:

Life stage Minimum Good Maximum
Vegetative ~15,000 lux ~40,000 lux ~70,000 lux
Flowering ~35,000 lux ~60,000 lux ~85,000 lux

Choosing the right lights

Using specific marijuana grow lights for your weed allows you to maintain the plant’s health and progress to the next growing season.

The more plants you have, the more lights you’ll need for a successful marijuana light schedule. Average home growers use around one or two lights since most states permit no more than 12 plants.

To improve the growth and flowering of your plants, you may invest in an infrared grow light. It’s not necessary but works well with HID and LED lights.

Before you figure out how much light a weed plant needs, you must choose the right lights. HID (High-Intensity Discharge) lamps, such as MH and HPS bulbs, have a hood that reflects light.

Metal-halide lamps (MH) are ideal for the vegetative stage of the cannabis light schedule, while high-pressure sodium (HPS) lamps are perfect for flowering. Using both bulbs allows you to reap double the benefits.

Most HID lights display a particular hue compared to the best LED grow lights, which may show many colors. Using LED lights is a fairly new practice compared to traditional HID lamps. LEDs use lower wattage but provide the same quality light spectrum as HID’s.

Before starting your cannabis light schedule, pick the best lighting options for you. Here are some pros and cons of HID and LED bulbs to help you decide:


Pro’s Con’s
Extremely bright, emitting up to 130,000 lumens Need extra equipment like an electronic ballast and reflector
Efficacy rates of 150 lumens per watt Emit an intense heat that may burn plants or spike room temperature
Relatively low maintenance Degrade over time needing routine replacement
Lower outright cost Power-hungry so higher electricity bills
Easy set up/beginner-friendly
Options for different marijuana lighting cycles


Pro’s Con’s
Energy-efficient/saves money in the long run No industry standard for LED lights
Runs cooler than HID’s, so low risk of burn Cheap models on the market may give inferior results
Mostly plug and go, no extra equipment Potentially lower yields than HID
Streamlined—supports veg and flower phase
Can last up to ten years

Distance from plants

Distance from the light can make or break your plants. Too close, and there’s a risk of light burn on weed, too far, and they won’t get the light they need. The optimum distance during your marijuana light schedule depends on the type of light and growing space.

During the seedling phase of the cannabis light schedule, grow lights should be kept around 24–36 inches away. Keeping an adequate distance prevents the seeds from drying out.

For the vegetative stage, lights should be 12–24 inches away. This phase of the cannabis light cycle requires more light for photosynthesis, so keeping them closer helps.

The light times for growing weed increase when flowering, and they should be kept around 16–36 inches away.

Here’s a handy guide for recommended distance depending on light wattage:

Grow light wattage Closest distance Furthest distance
150W 5 inches 11 inches
250W 6 inches 13 inches
400W 8 inches 19 inches
600W 9 inches 25 inches
1000W 11 inches 31 inches

Light spectrum for cannabis

Did you know the color of the lights influences your plants’ development? Different tones display certain hues based on the length of their waves. These varying shades suit the diverse stages, including a particular light spectrum for vegetative growth or the flowering phase.

The light spectrum for cannabis is the wavelengths between 380-750 nm. The colors represent the light wavelength. For example, if a light has a 400 nm wavelength, it appears purple to the human eye.

Light spectrum for seedling weed

During the seedling phase of your cannabis light schedule, use low-intensity light. Aim for 4000 lux—15% red, 30% blue, and white light.

Once your seedlings sprout their first leaves, you can double the intensity. When you spot more than two sets of leaves, it’s time for the vegetative stage.

Light spectrum for vegetative growth

For the vegetative stage of your marijuana light schedule, the main goals are root growth and tight internodes, so blue light is best. This shade stops your plants from growing too fast and developing long internodes, which causes light-blocking during flowering.

What is the best color spectrum for vegetative growth? For best results, use 27000 lux—100% blue light and less than 60% red.

Best light spectrum for flowering

In the flowering phase of the cannabis light cycle, your plants need more photons, so turn up the lux to 107,500—100% red while maintaining blue light at a lower level.

Lighting schedule

Excessive light increases your electricity bill and burns your plants.

Having a cannabis light schedule gives your flora a break. In the dark, your plants produce hormones that help them form buds.

Light and dark work together like yin and yang to form healthy greenery. How much light does a weed plant need? The answer depends on what stage the herb is in. Here’s a handy guide for the ideal light cycle for weed in different growth stages:

How many weeks does this stage last Lighting schedule
Seedlings 1–2 weeks 24 hours
Vegetative 3–5 weeks 18 hours on/ 6 hours off
Flowering 7–10 weeks 12 hours on/ 12 hours off
Light cycle for seedling marijuana

Seedlings are babies—they need all the care and nourishment you can give. In this phase of the cannabis light schedule, feed them 24 hours of light. After 1–2 weeks, your seedlings will sprout leaves and be ready for vegetation.

Light cycle for vegetative weed

The vegetative phase is the stage where plants grow bigger and taller. They need long days and short nights. To prevent early flowering, ensure they get at least 13 hours of light in your marijuana light schedule. 18 light hours and 6 dark hours will encourage healthy and steady growth.

Light cycle for flowering cannabis

In the flowering phase, weed plants start forming buds. If they don’t get at least 12 hours of darkness, they may revert to the vegetative phase. During this stage of the cannabis light schedule, you must ensure that plants get absolutely no light during the dark times.

Optimizing lighting for maximum yield and minimal cost

Powering the grow room brightness can pull copious amounts of energy and cost you money. There are some ways to ensure you use lighting properly without going bankrupt. Here are some techniques to maintain the marijuana lighting cycles efficiently:

  • Use lower wattage, LED, or energy-efficient bulbs
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Energy-efficient bulbs tend to be pricey upfront but save you money in the long term. HID bulbs can use lots of power, whereas LEDs use less.

Reflective walls bounce light allowing you to make better use of your grow lights. To make walls reflective, you can use materials like mylar and plastic.

Nighttime tariffs are lower in many states, so you can save money by using grow room lights at night. Indoor growing gives you control of the cannabis light cycle, and using lights at night won’t make a difference. As long as you follow the cannabis light schedule, your plants will thrive.

Providing ventilation

Ventilation is just as important as marijuana lighting cycles, water, and nutrients. Adequate airflow helps maintain the proper humidity levels, temperature, and CO2 levels in the room.

The most efficient method is to have two ventilators opposite each other and the exhaust system on a different side of the room. This way, you’ll have a balanced atmosphere with stable humidity levels and temperature.

FAQ related to how much light does a weed plant need

To help you a little more, we’ve put together the most frequently asked questions on marijuana light schedules.

What types of bulbs are best during vegetative and flowering stages?

Metal-halide lamps (MH) are the best option for the vegetative stage, and high-pressure sodium (HPS) lamps are ideal for flowering. During different phases of the cannabis light schedule, bulb settings with different lux are used. Bulbs with lux 27,000 work for vegetative and lux 107,500 for flowering.

How many hours of light do marijuana plants need?

It depends on the stage that the plants are in. In the vegetative stage, they need 18 hours of light and 6 hours of darkness. During the flowering phase of the cannabis light cycle, weed plants need 12 hours of light and 12 hours of darkness.

Can you leave grow lights on 24 hours a day?

You can only leave grow lights on for 24 hours a day during the seedling stage. Seedlings need all the nurturing they can get, and more light helps them sprout faster. During this phase of the marijuana light schedule, low-intensity light helps seedlings grow.

How much sunlight does a weed plant need?

How much sunlight does a weed plant need? The more, the merrier. Cannabis plants need at least 10–12 hours of direct sunlight to thrive. They can still grow healthy with a minimum of 6 hours of daily sunlight, but you’ll get a smaller yield.

What is the best color spectrum for vegetative growth?

Blue light bulbs with 27,000 lux—100% blue light and less than 60% red are best during the vegetative stage. It works best because blue light produces chlorophyll—a chemical that helps plants grow stronger and move to the next stage.

If you continue to increase the intensity of light that a plant receives, what happens?

If you increase light gradually following the recommendations for the cannabis light schedule, then your plant will flourish. Increasing intensity outside of the guidelines can cause your weed to burn. Cannabis plants need the most intense light when flowering, and too much before that can be detrimental.

Key Takeaways

How much light does a weed plant need? Cannabis plants have different light needs depending on their stage. Seedlings need 24 hours, vegetative stage weed needs 18 hours, and flowering plants need 12 hours of light.
There are many factors to consider, such as color spectrum, light type, marijuana lighting cycles, and intensity. Remember, healthy plants start with quality seeds. Shop our selection at i49 of the finest weed seeds now.

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    How Much Light Does a Weed Plant Need?

    Ensuring your cannabis plant has enough light (as well as enough hours of darkness) in the grow tent during a 24-hour period will enable it to efficiently conduct photosynthesis and produce healthy buds. For that matter, full spectrum grow lights are an integral part of your grow room if you’re planning to become an indoor grower.

    While there’s no right answer to how much light weed plants need, since marijuana plants need different amounts of light during each growth (or growing) stage, there are some general guidelines as to how much light they need in each stage. In order to help you grow healthy plants, we’ll go over how you need to take care of them, and what light schedule you need to maintain.

    Optimal Light Schedule for Growing Marijuana Plants (Amount of Light vs Hours of Light)

    From germinating to growing flowers and buds, your plant needs to have the ideal growing conditions in order to thrive, and one of the most essential is light. Marijuana plants go through the following four stages on their journey to growing resinous buds:

    • Germination stage (3-10 days);
    • Seedling stage (2-3 weeks);
    • Vegetative stage (3-16 weeks);
    • Flowering stage (8-11 weeks).

    Each stage needs specific light and nutrient conditions which marijuana growers tailor to the specific strain they’re cultivating. Whether it’s a Sativa or an Indica, an autoflowering plant or a photoperiod plant, a high in CBD medical marijuana strain or a strain high in THC, each one requires specific growing conditions that will determine the quality of the buds, as well as the yield.

    Seed to Harvest Light Guide for Indoor Growers of Cannabis Plants

    Germination Stage

    Germination can happen anywhere between 24 hours and 72 hours, and during this stage the cannabis plant develops a taproot. During this period, seeds should be kept in a cool dark place, away from direct sunlight or artificial light, until they reach the seedling stage.

    Seedling Stage

    The seedling stage lasts between 2 and 3 weeks, and this is when the plants start to develop leaves and new fan blades. The plant should have 16 hours of light per day during this stage, and 8 hours of darkness. Seedlings might benefit from fluorescent CFL blue lights since they don’t produce a lot of heat and have a lower wattage.

    Vegetative Stage

    The vegetative stage lasts between 3 and 16 weeks during which the cannabis grows stems and leaves, but no buds. This is when your cannabis plants should have between 12 to 18 hours of light.

    Flowering Stage

    The flowering stage can last from 5 to 16 weeks and the duration of this stage depends on the type of strain you’re cultivating. This is when flowers start to form, which then produce the resinous buds that give us our favorite cannabis products. During this stage, the marijuana plant needs 12 hours of light and 12 hours of uninterrupted darkness in order to induce flowering.

    Outdoor Plants (Photoperiod vs Autoflowers) Light Cycle

    While indoor plants need light changes in order to induce flowering, outdoor growers have nothing to worry about since photoperiod cannabis plants have an internal process that allows them to detect when they need to move on to the next growing stage.

    Photoperiodism refers to short-day plants which produce flowers when the days are getting shorter, meaning their life cycle is nearing the end. Therefore, in the northern hemisphere weed plants grow from April/May to September/October, while in the southern hemisphere they grow from September/October to April/May.

    On the other hand, autoflowering strains start to flower automatically instead of waiting for a specific light cycle. Meaning, after the short vegetative stage of about 2 to 4 weeks, they begin to produce flowers.

    Different Types of Grow Lights for Your Marijuana Plants

    When growing marijuana indoors, you’ll need to make sure that you do everything to adjust the light cycles (length of sunlight hours each day) according to the needs of the cannabis plant. There are different types of grow lights that’ll get the job done, so you’ll have no problem finding quality grow lights that are effective but don’t cost a fortune.

    Fluorescent Grow Lights (CFL)

    Compact fluorescent lights, or CFLs, are the most common grow lights that you can see on the market and in grow tents. They’re budget friendly and suited for small grow tents of 1-2 plants, which makes them one of the best grow lights for these conditions (price vs performance). They go well with standard light fixtures, save energy, and are available in various wattages and color temperatures.

    Daylight spectrum CFL lights are suitable for the vegetative growth phase and warmer CFL lights are better for the flowering phase. A standard 40W CFL bulb is expected to last for about a year, and the expected cannabis yield is about 0.3 grams per watt (or a yield of 12 grams per standard light). Keep in mind that if you have a bigger yield, CFLs might not be the proper lighting system for you.

    High Intensity Discharge HID (MH and HPS) Grow Lights

    High-intensity discharge or HID grow lights are the most popular in the cannabis cultivation community. Growers believe that these lights produce the best and largest yields. The two main types of HIDs are MH (metal halide) and HPS (high pressure sodium), with the former producing a cooler, blue light, and the latter a warmer, red light. Therefore, MH lights are best suited for the vegetative phase, while the HPS for the flowering phase, so growers often do a mix of both.

    You can expect a yield of about 0.5–1g+ per watt (300–600 grams per standard HID light). Keep in mind that they can increase the temperature in your grow room or even burn plants, so check your plants regularly. The bulbs are also likely to increase your electricity bills on account of them being power hungry.

    The lifespan of the bulbs is about one year, and you should replace them annually in order to maintain optimal light input. In order to implement these types of lights you’ll need the bulbs, a ballast, and a reflector, that will cost about $180+ (€150+), but this is still cheaper than high-end LED lights.

    LED Grow Lights

    LEDs are the most energy-efficient grow lights on the market. Their light is suitable for both the vegetative and flowering phases, and some even have the function to change the light spectrum according to the growing phase. Even though they’re the most expensive, they can also save you money over time. They’re much cooler compared to HID lights which reduce the risk of burning plants, while also decreasing the cooling bill.

    Commercially available LED grow lights don’t need a special ballast, you can just plug them and start growing your plants. Low-quality LEDs are about $140 (€120) for a single light, while high-quality LEDs can cost up to a few hundred euros. But as we said, this is actually a budget-friendly option, as one light has a lifespan of 5–10 years. Compared to HID grow lights, they may have a lower yield, although the standard yield is about 0.5g–1.8g per watt with LED lights.

    LEC Grow Lights

    LEC lights, ceramic metal halide (CMH), or ceramic discharge metal halide (CDM) lights, use a ceramic arc tube that projects a more natural color compared to the quartz tube in the MH lights. They have a built in ballast which makes setup really easy, more lumens per watt, and a longer lifespan compared to HID lights.

    The downside to LEC lights is that they generate lots of heat and emit UV-B rays that improve trichome production (but are harmful to our skin and eyes), so be careful when going in your grow rooms while they’re on. Standard LEC lights cost around $300 (€250), while high-quality models can cost up to $1200 (€1,000). Their lifespan is approximately two years, and you can expect a yield of 1.5g per watt.

    Final Thoughts on the Amount of Light Weed Plants Need

    Light is integral to the overall growth cycle of the cannabis plant, so the appropriate amounts of full spectrum light, as well as a good amount of uninterrupted darkness, are both essential for proper cannabis growth. No matter if you’re growing indoor or outdoor plants, autoflowering or photoperiod cannabis, you’ll need to adjust the light in order for your plants to thrive.

    And when picking out the lights, don’t just look at the wattage, but also make sure that the amount of light they emit is suitable for your particular plant. In the end, all you need to start cannabis cultivation is to choose your preferred strain, pick a cannabis growing method, whether it’s SOG, SCROG, defoliating, trimming, or something else, and equip your grow space with enough lights.

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