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. 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.
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.
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.
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:
|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:
|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|
|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.
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
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.
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.
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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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? https://www.growweedeasy.com/cloning
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.
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.
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.
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.