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. Cannabis seedlings require the right amount of water, light, and nutrients. Learn how to handle them to get the strongest marijuana plants. 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 seedling stage
Keeping your marijuana happy and healthy comes down to how carefully you care for them through each stage of a marijuana plant’s life. Factors such as how much light should seedlings get should be carefully considered as these are especially important in the younger stages of your plant’s life when they are at their most fragile condition.
A marijuana seed that sprouts will split along the seam that joins the halves of its husk. Driven by gravity , the tail grows longer rapidly, screwing its way down into the soil until the root can supply sufficient leverage to raise the husk containing the two seed halves upright.
From that position, the two halves fold out to act as biological solar panels that gather energy and begin a marijuana plant’s first chlorophyll production even as the tiny taproot sprouts hairlike feeder roots that stretch outward to strengthen its grip in the soil.
In this article, we take a look at the ways to protect your seedlings through their most delicate stage of life.
Our guide to the seedling stage for marijuana plants:
What are seedlings?
The key thing to do during this stage is simply to pay attention and keep tabs on every development or change that occurs in your marijuana seeds. When they sprout, the seeds’ seams will split and allow a white tendril to poke through within several hours of this split. This tendril will grow very quickly, moving downward until it’s deep enough for it to hold up the rest of the plant (namely the stalk and the seed husk).
The husk, now split into two, emerges from the ground and functions as a sort of makeshift leaf – in other words, it absorbs energy that can be used to produce chlorophyll within the plant. While you see the stalk growing upwards, you can also be sure that more roots are sprouting and growing downwards at the same time.
Grab my free Grow Bible for more on marijuana seedlings here
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The first two leaves, completely unique from any future leaves that will grow, pop out of the middle of the seed’s split. These leaves are called sucker leaves and mark the beginning of more leaf growth.
Two-lobed leaves will then grow from the middle of the sucker leaves. These two-lobed leaves usually resemble chicken feet, since they have three lobes. After the three-lobed leaves come two leaves with five lobes, and then two more with seven. Seven-lobed leaves are the ones everyone recognizes as the marijuana leaves.
Two lobed leaves will then grow from the middle of the sucker leaves. These two-lobed leaves usually resemble chicken feet, since they have three lobes. After the three-lobed leaves come two leaves with five lobes, and then two more with seven. Seven-lobed leaves are the ones everyone recognizes as the marijuana leaves.
If you were able to keep your marijuana plants healthy throughout its entire sprouting stage, they would most likely go through a very productive vegetative stage. Protecting your young plants is about more than just survival: it’s about investment in your future harvest.
Also read the article How to germinate marijuana seeds for more about germinating tips
This article covers the various elements you will need to keep a constant eye on to ensure the well being of your seedlings. These elements are protection, water, nutrients, heat, and sunlight.
How to protect seedlings
You will need to protect your young plants from more than just discovery by the authorities. Even if you live in a location where growing marijuana is legal, you have plenty to worry about with protecting your seedlings.
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You should keep your plants in some sort of protected area, such as a rooftop, and shelter it using some sort of a transparent, plastic dome or bubble. These domes can keep your seedlings from being eaten by insects, rodents, or other pests, and they are easy and cheap to make yourself.
Simply slice a clear plastic bottle in half, then recycle the top half and keep the bottom half. You should cut small slots around the edges of this half so that your young plant will have some airflow. Place the dome over your little plant, completely covering it. This will have the additional bonus of insulating your plant in case an unexpected frost occurs. In my free Grow Bible, you will find more DIY hacks for your grow.
In addition to insulation and protection, your seedlings will need the most important elements on Earth: water and sunlight. You have to constantly keep the soil and the seedling moist around the clock, and you need to make sure they are receiving plenty of sunlight.
If you live further north, you could have some more trouble with this since your spring days will be shorter than latitudes further south. One big problem that can happen to outdoor growers is their seedlings not receiving enough sunlight. When this happens, the plant stretches taller and taller in an attempt to reach more sunlight, and then it is too tall to hold itself up any longer.
If you want to keep your grow small I can advise a solution like a Pot for Pot. They offer an all-in-one box with everything your plant will need.
If you have space and resources, you can try sprouting your cannabis seeds inside of a closet to begin with. Using the proper equipment in this environment can give your young plants plenty of sunlight, giving them enough energy to sprout large, healthy leaves right from the beginning.
This type of growing would also ensure that their stems won’t get too long and weak; rather, they will be short and strong. You will be able to avoid the hassle of stabilizing tall, weak stems with sticks. If they don’t do this while the plant has leaned to the ground, it could begin to rot.
That being said, if you start to grow your seedlings indoors, transplanting them later will be necessary, which presents its own difficulties and safety concerns for your young plants. Ultimately, it depends on which option is best for you.
How much water do seedlings need
One common problem that occurs when trying to care for a seedling is giving them too much water. It is actually less of an issue having to do with too much water, but actually has more to do with too little oxygen reaching your plant’s roots. This happens most commonly with seedlings being grown in containers since water can only go so far as the walls of the container.
If your plants are being overwatered, you will most likely see symptoms such as drooping leaves. There are plenty of ways to avoid this, however, as long as you keep reading for a better understanding. Your planting situation is the most important factor that comes into play when considering how you have managed to overwater your plants, and how you can fix or avoid the issue.
Overwatering your seeds
Your pot is too big for your seedling
Since a young plant’s tiny roots absorb much less water than their more mature counterparts. If you water as much as the container can hold, these tender roots will not be capable of taking it all in. This situation occasionally referred to as “overpotting,” leads to overwatering. So how do you prevent this situation from happening?
The best thing to do is to begin your seedling’s life in a smaller container to begin with. Once they have grown a fair amount, you can move them to a bigger container. If it’s too late and you need to fix the issue, simply direct your watering to a specific area: a little circle right around your plant. Once the topmost inch of soil is dry again, you can do another round of watering. You can only begin watering normally.
If you plan well enough in advance, you can have a separate container for each of your marijuana plants’ stages of life. If your plants will be living in containers for the entire growing season, you will need to know in advance the size of your last container.
In other words, you will have to have a good idea of how large you would like your plants to become. Because roots grow more rapidly when they are left in one container for a long time, fewer transplants mean a larger size plant (and therefore container). Take that into account when planning your container sizes.
Your pot is too small for your seedling
You can plant seedlings in very small individual containers, such as a solo cup. You can’t keep them cooped up in a solo cup forever, though since their roots will quickly outgrow the small space. If your plants are left in a small container for too long, they will become “root bound.” This means that the roots have wrapped themselves around the outside of the cup, keeping water on the inside from escaping.
Why is being root bound such a bad thing? It can lead to some pretty big problems, including overwatering, nutrient deficiencies, wilting, and more. You can avoid this problem simply by changing the size of their containers as frequently as necessary. (Use these containers)
If your plants’ container does not have an efficient drainage system (i.e. holes punched in the bottom of the container, plus the right kind of soil), overwatering will quickly become a problem that could be life-threatening to your plants.
One way to keep this from happening is to start growing your plants in a soil that drains well from the very beginning. Soils that are clay-based, for instance, retain water and, therefore, should be avoided.
You can also begin with a smaller container, which would help prevent the issue that was mentioned above. Make sure your container has lots of holes where water can drain from.
If you ever notice that water isn’t draining as fast as it should, you can add perlite to your soil to increase the oxygen content. Don’t water your plants quite as often until you notice them drinking more, or try using a Smart Pot instead of a normal pot.
If you follow these tips and are careful about watering your plants, you should be able to avoid overwatering them altogether.
If you’re sure you haven’t been watering your plants too often or too much and they are drooping or wilting, the culprit could actually be a lack of water. Sometimes people who know about the common occurrence of overwatering seedlings have overcompensated, therefore actually watering their plants less than is required.
The roots of your plants need to constantly be able to access water. Plants lose the water they have absorbed through their leaves in a process called transpiration. They do this by sucking up the water from the roots like a straw. If this process keeps going and the roots down below are not receiving enough water, the plant will undergo some serious problems.
If you are able to visibly notice your soil separating away from the container it’s in, you probably have an underwatering problem on your hands.
If your soil is enriched with nutrients and you underwater your plants, the effects will be even more devastating. Your plants will turn a dark green color and will have twisted new growths of a strange color. In this case, the only thing you can do is give your plants more water to re-establish their roots and begin growing once again. If they receive enough water to fight these effects, they will probably be able to combat this situation.
What kind of nutrients
Giving your plants nutrients can come with its own issues. If you give your plants too high a dosage of nutrients, for example, it will turn into nutrient toxicity. Your plant’s leaves will have tip burn and turn darker in color.
Nutrient toxicity can be caused by using a “hot” soil or a type of soil with a lot of nutrients. As long as you are watering your seedlings enough, they should be able to grow out of nutrient toxicity that comes from using hot soil.
Some soils are “slow-release,” such as Miracle-Gro. Avoid these soils at all costs, as they will not help your plant be healthier – in fact, it makes them even more susceptible to nutrient toxicity.
If you provide your seedlings with nutrients when they are too young, they might have a sort of nutrient overdose. As long as your initial potting mix is high quality, you shouldn’t need to worry about adding any more until after a minimum of a few weeks.
If you feed your plants with a large amount of nutrients all at once, you could end up with nutrient toxicity in less than a day. If you’re using a nutrient schedule that comes with the store-bought nutrients, half the amount it says and see how your plants react before adding any more.
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.